Clash Language User Guide

Welcome to the Clash Language User Guide, the official documentation of the Clash Compiler. Clash is an open-source functional hardware description language (HDL) that borrows syntax and semantics from the Haskell programming language. To learn more, we suggest reading the introduction to Clash.

The table of contents below (and in the sidebar) allows easy access to different pages in the documentation. You can also use the search function in the top left corner.

Note

The Clash Compiler and Clash Language User Guide are open-source efforts developed by QBayLogic B.V. and other volunteers. The Clash Team always appreciates feedback and contributions to the project to help improve the development experience.

If you don’t understand something, or think something is missing or incorrect in the documentation you can open an issue or pull request in the GitHub repository.

General

Introduction to Clash

Functional Hardware

Clash is an open-source functional hardware description language (HDL) that closely mirrors the syntax and semantics of the Haskell programming language. It is used for creating hardware designs, typically for running on field programmable gate arrays (FPGAs) or application-specific integrated circuits (ASICs).

Clash is both a compiler, and a set of libraries for circuit design, that transform high level Haskell descriptions of synchronous, sequential logic into low-level VHDL, Verilog, or SystemVerilog. It provides a unique approach to design of sequential circuits, but with a high amount of abstraction power that blurs the line between strictly behavioral or structural synthesis approaches.

Clash aims to modernize the hardware development experience, making it easier to quickly and correctly develop complex circuit designs. This is achieved by making Clash:

Expressive

Clash uses the Haskell type system to its full potential – including modern extensions and techniques – to being a high level of type safety and expressiveness to hardware design.

This expressive typing makes it easier to develop safe, maintainable hardware. Combinatorial and sequential logic is separated by type, and global safety invariants such as separating incompatible clock domains are enforced in the type system.

Intuitive

Clash makes it easy to express circuit designs in an intuitive manner, allowing high level structural components to be easily connected in designs. Moreover, unlike most “high level synthesis” tools, this extends to precise control over register placement and pipelining.

Interactive

Unlike traditional HDL tools, Clash has a fully interactive read-eval-print loop (REPL), allowing circuits to be interactively designed and tested.

Performant

Clash reuses parts of the Glasgow Haskell Compiler to provide fast simulation of circuits for development and testing.

Efficient

Clash uses a “whole program synthesis” approach in order to view the entire circuit at once, and optimizes this design before translating to a specific target. This allows meaningful optimizations to be performed on the entire design.

Extensible

Additional primitives and black boxes can be added to Clash in the language of your choice, allowing you to use your own vendor or IP library within projects.

Clash allows seamless interoperability with libraries written in Haskell, including mtl, lens and QuickCheck. This makes it even easier to quickly prototype complex designs.

Intended Audience

Clash is ideal for developers from different backgrounds, although the main intended audiences are

Hardware Engineers

You are a hardware engineer, used to using tools like VHDL and Verilog to implement circuit designs. Clash offers the familar mixed simulation / synthesis capabilities of these tools, while providing a langauge with powerful abstractions.

Haskell Programmers

You are a Haskell programmer, looking to start developing hardware. Clash offers the ability to start prototyping and simulating designs in a familiar environment – lowering the learning curve significantly.

Maturity and Support

Clash is a continually evolving tool, having been actively developed since 2009. With the release of Clash 1.0 there has been an increased focus on maintaining API stability between releases, meaning circuit designs written in Clash should continue to work between minor releases. Today, the Clash Compiler is actively developed by QBayLogic B.V. and volunteers.

Several companies and enthusiasts are already using Clash to develop circuit designs, ranging from small designs on hobbyist boards to larger designs on modern FPGA and ASIC architectures.

While care is taken to thoroughly test the Clash compiler, some bugs may exist. We encourage users to file issues, or contribute pull requests on our GitHub repository.

Meta-information: Web Sites, Mailing Lists, etc.

Mailing list: for updates and questions join the mailing list clash-language+subscribe@googlegroups.com or read the forum

Slack: Invite yourself at fpchat-invite.herokuapp.com. To join #clash, click on “Channels” and search for “clash”.

IRC: freenode#clash-lang

Clash Version Numbering Policy

Clash follows the Haskell PVP Specification for its version numbers, for all packages. The main libraries that make up the Clash compiler maintain the same version numbers, making it easy to identify which versions are compatible.

Note

Due to the Clash’s tight integration with GHC, updates to the GHC version that Clash uses result in changes to the Clash version. As GHC’s internals change frequently, even for minor bumps, it cannot be guaranteed that these changes will not result in Clash changes.

It is recommended (but not required) that downstream Clash packages and published Clash code also follow the PVP specification.

Frequently Asked Questions

Basic Questions

  • Q: How do I install Clash?

    A: Check out the Installing Clash page in the Getting Started section of the manual.

  • Q: Is the name “Clash”, “CLaSH”, or “CλaSH”?

    A: It’s Clash.

    In its research stages Clash was called “CλaSH”, an acronym for the CAES Language for Synchronous Hardware. CAES is a group of the Faculty of Electrical Engineering, Mathematics and Computer Science at the University of Twente. Clash was originally developed by Christaan Baaij and supervisor Jan Kuper. The stylization “CλaSH” is an homage to Haskell, whose official logo has long been the venerable Greek lambda character.

  • Q: Is Clash a “high level synthesis” tool?

    A: While clash provides a high level language features, hardware descriptions written in Clash are not decoupled from clock-level timing. Clash does therefore not offer what is generally understood as “high level synthesis”. Compared to the big three hardware description languages, VHDL, Verilog, and SystemVerilog, Clash arguably is high-level. It offers many of the powerful abstractions that modern software programming languages offer. In fact, it inherits many of the software’s industry bleeding-edge features by virtue of basing its implemenation on Haskell.

Clash Support

  • Q: Is Clash production ready?

    A: Clash is constantly evolving, and since the 1.0 release there is a focus on maintaining API backwards compatibility. Clash is used successfully in real-world scenarios, and QBayLogic Clash support can help with education and implementation of Clash projects.

  • Q: Will Clash work with my EDA tools?

    A: In general, Clash should work well with Xilinx and Intel FPAGs and their EDA tools – as development typically focuses on these vendors. Clash has also been successfully used on Microsemi (formerly Actel) SmartFusion 2 and Lattice Semiconductor iCE40 FPGAs, and some basic IP for these exist.

    For most toolchains, the default primitives supplied by Clash should work with minimal effort. If not, it is possible to call your vendor’s library manually, or use a tool like Yosys to do mapping. It is also possible to consult QBayLogic Clash support for more assistance.

  • Q: Does Clash support Project IceStorm?

    A: The Verilog backend for Clash emits Verilog 2001, which is supported by Yosys. This means it can be placed and packed with arachne-pnr and icestorm. Additionally, Clash has some support for the Lattice Semiconductor iCE40 FPGA.

  • Q: Can Clash be used for ASIC designs, as well as FPGA designs?

    Clash can be used for ASIC designs, however the RTL produced by Clash may not be immediately suitable as it is largely platform agnostic. While this is not a problem for FPGAs, it can make developing ASICs more complicated as many ASIC vendors have different proprietary tool flows, with limited information available about their workings.

    If you are using Clash to develop for ASIC, and need assitance with getting your toolchain to work, you can contact QBayLogic Clash support for assistance.

Clash and Haskell

  • Q: Is Clash its own programming language, or is it “Haskell”?

    A: Clash is a programming language in its own right, complete with its own executable and standard library. Clash is also related to the Haskell programming language, and may be thought of as a dialect of Haskell for developing hardware. While the surface syntax and typing rules are the same, the semantics change as code progresses through the compilation pipeline.

    Do to the shared behavior in the early stages of the compiler, components from GHC (the most common Haskell compiler) are reused in the Clash compiler. This is how Clash achieves such high interoperability with existing Haskell projects.

  • Q: Clash has better inference for type level natural numbers than GHC. How is this possible?

    A: Clash’s enhanced type checking functionality is due to the use of GHC compiler plugins, which can be used in any Haskell project. To enable these plugins, pass the following compiler flags to GHC:

    {-# OPTIONS_GHC -fplugin GHC.TypeLits.Normalise       #-}
{-# OPTIONS_GHC -fplugin GHC.TypeLits.Extra.Solver    #-}
{-# OPTIONS_GHC -fplugin GHC.TypeLits.KnownNat.Solver #-}

    These plugins come from the ghc-typelits-natnormalise, ghc-typelits-extra, and ghc-typelits-knownnat packages respectively, which are all available from Hackage and Stackage.

  • Q: Do I need to know Haskell in order to use Clash?

    A: As Clash is deeply integrated with Haskell, it is recommended that users have some familiarity with Haskell, or functional programming in general. Clash uses some advanced features of Haskell, and real-world designs will often want to leverage the existing Haskell ecosystem.

    For developers who are particularly familiar with either Haskell or hardware design, Clash should be relatively intuitive to use. Additionally, obvious mistakes with designs will be identified and reported due to the strong type system identifying mistakes at compile-time.

Clash and other HDLs

  • Q: Do I need to know existing RTL/HDL languages in order to use Clash?

    A: Clash currently outputs VHDL, Verilog, and SystemVerilog. While it’s not necessary to understand these descriptions, you will need to some understanding of vendor tools to actually deploy it.

  • Q: What’s the difference between Clash and “Lava”?

    A: Lava dialects (including the modern variant Blarney) are all embedded domain specific languages (EDSLs) inside Haskell. On top of that they use a so-called deep embedding to be able to transform a circuit description into a netlist (to subsequently output that as a VHDL/Verilog file). Clash on the other hand uses “standard” compiler techniques to create a netlist from the Haskell abstract syntax tree (AST). This “standard” compiler technique enables the following features not available in (Haskell-based) EDSLs:

    1. Clash allows the use of normal Haskell operations such as (==) on both the meta-level (how the program is structured/generated), and the object-level (the functionality of the program).

    2. Clash allows the use of regular Haskell syntax to model the concept of ‘choice’ at the object-level (the functionality of the program): if-expressions, guards, case, etc.

    3. Clash allows programmers to use native Haskell pattern matching.

Basically, with Clash you can use regular Haskell to describe the behavior of the circuit, most importantly all of it’s choice-constructs (case-expressions, guards, etc.). With an EDSL you are “limited” by the constructs of the DSL, making your circuit descriptions look less like regular Haskell functions.

  • Q: What’s the difference between Clash and Chisel/Spinal/Migen/Hardcaml?

    A: The biggest difference between these toolchains and Clash is that Clash exists as a Haskell derivative, with a full synthesizing compiler to RTL – while Chisel/Spinal/Migen/Hardcaml exists as an embedding of hardware semantics inside Scala/Scala/Python/OCaml. Aside from the “host language” differences, this means that Chisel/Spinal/Migen/Hardcaml are conceptually closer to something like Lava/Blarney than Clash. So within these languages you can only use the host language constructs to structure and compose the constructs of the EDSL, and you can’t use host language constructs to describe the behavior of the circuit; i.e. you cannot use the host language’s regular if-expression to model the concept of choice, but you have to use e.g. Chisel’s when-function.

    Aside from the above, there is also a varying degree of native simulation and interactivity. In Clash you can evaluate/simulate any (sub-)component in the interactive interpreter for an immediate and localized design feedback loop. The only EDSLs that have a similar interactive interpreter for fast design feedback are the older variants of Lava. They used a so-called dual-embedding, where the EDSL primitives also contained a normal Haskell function which described their behavior, and so the composition of these primitives could be evaluated as a regular Haskell function.

    The other EDSLs all offer simulation, but there is a higher latency to get from a design to a simulation of a design, and they are not as interactive. Blarney emits Verilog, and you can then use a Verilog simulator to simulate the Blarney design. Spinal also emits Verilog, but it then uses Verilator to compile it to an object-file which is loaded back into Scala, allowing you to interact with your Spinal design from within Scala. Chisel is also not interpreted directly, instead, a Chisel description is “lowered” to FIRRTL where that FIRRTL description is then executed inside Scala by the FIRRTL interpreter. Migen works similarly to Chisel as far as the approach to simulation goes, although perhaps more direct: it directly interprets its own deep embedding data structure (its IR) to enable native simulation.

    All of this influences the style in which you write circuits and the creative process by which you come to a solution; the effects of this on the quality of results (QoR) and development time are, however, both hard to qualify and hard to quantify. That is, although all of these languages, both the EDSLs and Clash, enable full control over the QoR (i.e. you can get as many registers and as much logic as you intended), the way in which you get there can vary from problem domain to problem domain and person to person. If you have enough time, we encourage to try several of them and see which style is the most natural fit for you; if you’re limited on time, we of course recommend that you just go with Clash ;-)

License

Copyright (c) 2012-2016, University of Twente,
              2016-2019, Myrtle Software Ltd,
              2017-2019, QBayLogic B.V., Google Inc.
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are met:

1. Redistributions of source code must retain the above copyright notice, this
   list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright notice,
   this list of conditions and the following disclaimer in the documentation
   and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

Getting Started

Installing Clash

Check out clash-lang.org/install to install the latest stable release of Clash, or to setup a Clash project.

Get Clash from source

Get the source code using Git and enter the cloned directory:

git clone git@github.com:clash-lang/clash-compiler.git

# Alternatively, if you haven't setup SSH keys with GitHub:
# git clone https://github.com/clash-lang/clash-compiler.git

cd clash-compiler

To check out a released version, use:

git checkout v1.2.3

To checkout a release branch use:

git checkout 1.2

Note that release branches might contain non-released patches.

Cabal

To use Cabal you need both Cabal and GHC installed on your system. For Linux and MacOS users we recommend using ghcup. Windows users are recommended to use the Haskell Platform.

To run clash use:

cabal v2-run --write-ghc-environment-files=always -- clash

If this fails, make sure you’ve got an up-to-date package index:

cabal update
Stack

Install Stack and run:

stack run -- clash
Nix

Or use Nix to get a shell with the clash and clashi binaries on your PATH:

nix-shell

Example: Multiply and Accumulate

Combinatorial MAC

With Clash installed, it is now possible to begin creating hardware designs. To give a brief overview of Clash, we will define a simple multiply-and-accumulate circuit. Make a new file called MAC.hs, and enter the following preamble:

module MAC where

import Clash.Prelude
import Clash.Explicit.Testbench

This declares the module and imports some useful modules from the Clash standard library. The standard library contains necessary functions and data types for writing circuit descriptions. As with Haskell, module identifiers in Clash must always start with a capital letter and correspond to the name of the file.

The logic of our circuit is expressed as a function which takes an accumulator and two extra inputs, and outputs the new value of the accumulator – which is the old value plus the product of the two other inputs.

mac :: (Num a) => a -> (a, a) -> a
mac acc (x, y) = acc + x * y

The type of the function is given after the ::, and says that the type a is some numeric type (e.g. Int, Signed 8, Double), the first argument is a number, the second value is a pair of numbers, and the result is a number.

Synchronous MAC

By adding another output parameter to this function, with the previous value of the accumulator, we can define the function as a Mealy machine. This allows us to use our combinatorial definition of mac to create a synchronous circuit (which we call macS).

mac :: (Num a) => a -> (a, a) -> (a, a)
mac acc (x, y) = (acc + x * y, acc)

macS :: (HiddenClockResetEnable dom, Num a, NFDataX a) => Signal dom (a, a) -> Signal dom a
macS = mealy mac 0

The input and output of macS are values of the Signal type. This type represents synchronous values (functions without signals are combinatorial). There is also an additional dom type, for synthesis domain, and a constraint HiddenClockResetEnable – which says the synthesis domain has a clock, reset and enable line. These are implicit, although they can be exposed using the exposeClockResetEnable funcion.

HDL Generation and Testing

To generate HDL from a synchronous circuit, a function needs to be marked as a topEntity. The simplest way to achieve this is to create a function with this name, as Clash will use this definition automatically (similar to how main is a special function in other languages).

topEntity
  :: Clock System
  -> Reset System
  -> Enable System
  -> Signal System (Int, Int)
  -> Signal System Int
topEntity = exposeClockResetEnable macS

It is now possible to generate HDL for this circuit description, by running either clash --HDL from the command line, or running :HDL in clashi (where HDL is either vhdl, verilog or systemverilog). This will generate the HDL in a subdirectory named after the HDL being output.

Warning

Any function used to generate HDL from must have a monomorphic type. This means there can be no type variables in the type signature (i.e. for the circuit defined so far you need to specify both dom and a.

We can test that this circuit works as expected by defining a test bench. This allows an input to be used and the actual output to be compared against an expected output.

testBench :: Signal System Bool
testBench = done
 where
  testInput    = stimuliGenerator clk rst ((1,1) :> (2,2) :> (3,3) :> (4,4) :> Nil)
  expectOutput = outputVerifier' clk rst (0 :> 1 :> 5 :> 14 :> 30 :> 46 :> 62 :> Nil)
  done         = expectOutput (topEntity clk rst en testInput)
  en           = enableGen
  clk          = tbSystemClockGen (fmap not done)
  rst          = systemResetGen

From clashi it is possible to sample this test bench, using the sampleN function, which takes in the number of samples to draw and the signal which generates samples.

>>> sampleN 8 testBench
[False, False, False, False, False, False, False, False]

Developing Hardware with Clash

Clash as a Language

As Clash reuses parts of the GHC compiler for its front-end, the syntax and semantics should be familiar to Haskell programmers. For people unfamiliar with Haskell, there are many resources to learn the language, such as

Clash does make some use of more advances features of GHC Haskell, which are exposed by GHC as language extensions. The extensions used by Clash are

Warning

Since GHC 8.6, the StarIsType extension is defined. This extension is explicitly turned off by Clash, meaning Data.Kind.Type must be used to refer to Haskell types.

Clash also enables some GHC plugins by default which improve the type inference for type level numbers. The plugins enabled by default are

Users are free to control the language extensions and GHC options with the normal OPTIONS_GHC and LANGUAGE pragmas in source files. For more information, see the GHC User’s Guide.

Clash Prelude

Basic Types

The Clash prelude includes many different numeric types, which are used to safely define other types / functions. These include, but may not be limited to

  • Type level natural numbers (Nat), which allow numbers to be used in types. Conceptually, this is similar to const generics in C++.

    It is possible to have term level values which refer to a type level number. This is called SNat n (for singleton natural number). These are defined up to 1024 with the prefix “d” (e.g. d256).

  • Unsigned n and Signed n numbers with an arbitrary width (given as a type level natural number). These allow fixed-width arithmetic to be used on arbitrary numbers.

  • Index n provides natural numbers up to an arbitrary value (given as a type level natural number). These allow indexing into fixed width structures like Vec n a.

Another commonly used type is BitVector n. This provides a fixed size vector of Bit values which can be indexed, and used to perform unsigned integer arithmetic. Any type that can be marshalled to / from a BitVector n implements the BitPack class, which defines the conversion.

Note

It is also possible to derive instances of BitPack using Generic, by writing deriving (Generic, BitPack) in the type definition. This automatically determines how to do the conversion at compile-time.

More generally, there is a Vec n a type which allows collections of arbitrary values to be used. These vectors are tagged with their length, to prevent out of bounds access at compile-time.

Warning

The Vec n a type exports pattern synonyms for inserting at the left and right of a vector. The types of the Cons constructor and (:>) pattern are slightly different, and may behave differently in practice.

The Cons constructor has a more general type, allowing it to be used in some cases where the pattern cannot be used. However, this additional power comes at the cost of type inference. It is recommended that users use the (:>) pattern by default, and only use Cons when necessary.

Synthesis Domains

Synchronous circuits have a synthesis domain, which determines the behaviour of things which can affect signals in the domain. Domains consist of

  • a name, which uniquely refers to the domain

  • the clock period in ps

  • the active edge of the clock

  • whether resets are synchronous (edge-sensitive) or not

  • whether the initial (power up) behaviour is defined

  • whether resets are high or low polarity

The prelude provides some common domains, namely XilinxSystem and IntelSystem for the standard configurations of each vendor. There is also a generic domain, System, which can be used for vendor-agnostic purposes (i.e. writing a generic test bench). It is possible to define new synthesis domains for custom hardware using the createDomain function, which also defines the necessary instances for domains.

A value in a synchronous circuit is wrapped in the Signal dom a type, which specifies the synthesis domain and the type of value. Any function which needs access to a domain can use the constraints HasDomain (to find it’s domain) or KnownDomain (to extract configuration).

The default API exposed by the prelude is implicit with regards to clocks, reset lines and enable lines – as these can be determined at compile time. However, if they are needed the Clash.Explicit module contains explicit versions of the API which expose these directly in function arguments. It is also possible to use functions like exposeClockResetEnable to turn an implicitly defined function to an explicitly defined function.

State Machines

The Clash prelude contains combinators for two classical finite state machines which can be used to define synchronous circuits. The first of these is mealy, which encodes a Mealy machine. This is a machine specified by

  • A transfer function of type state -> input -> (state, output)

  • An initial state

  • An input signal which can change at each cycle

Note

The Mealy machine is similar to the State monad, which Haskell programmers may already be familar with. Practically speaking, the only difference is that this machine also has an input signal which is changed externally to the definition of the machine.

It is also possible to define a Moore machine using the moore function in the Clash prelude. This differs to the Mealy machine by providing output based on the previous state (as oppoesd to the newly calculated state), and is specified by

  • A transfer function of type state -> input -> state

  • An output function of type state -> output

  • An initial state

  • An input signal which can change at each cycle

Sometimes, there may be multiple inputs / outputs needed for a machine. As machines only input and output a single signal, there is a way to combine and separate multiple signals. The Bundle class specifies how to convert between some type which is a signal of a product, and some type which is a product of signals, e.g.

bundle   :: (Signal dom a, Signal dom b) -> Signal dom (a, b)
unbundle :: Signal dom (a, b) -> (Signal dom a, Signal dom b)

There are combinators which can automatically perform this bundling and unbundling for you as required, called mealyB and mooreB. The Bundle class is already defined for many types, including tuples (up to 62 elements), Maybe a, Either a b and Vec n a.

RAM and ROM

The Clash prelude provides the ability to work with synchronous and asynchronous ROM, asynchronous RAM and synchronous Block RAM. The simplest of these are ROM, which only allow indexing into a Vec n a of elements. ROM is defined using the functions in Clash.Prelude.ROM.

RAM is more complex, as it allows both reading and writing. The function to define a RAM takes in a signal for the address to read, and a signal for an optional address to update (bundled with the new value). At each cycle it outputs the value of the memory address read in the previous cycle. Asynchronous RAM is defined in Clash.Prelude.RAM.

An FPGA may include a block RAM, which is a larger memory structure and more suitable for some applications. Block RAM also has a synchronous read port, allowing memory access to be synchronized to a clock. Block RAM is used the same way as async RAM, allowing the two to be compared quickly. Block RAM is defined in Clash.Prelude.BlockRam.

Undefined Values

When working with hardware designs, there are times when undefined values may be encountered in simulation. Clash provides a custom exception type, XException, for cases when an undefined value is encountered. There are also many utility functions for working with exceptions, such as

  • errorX, which throws an XException

  • isX and hasX, which check for XExceptions when evaluating

  • maybeIsX and maybeHasX, which discard inforamtion about exceptions

There are also implementations of typical classes in Haskell which have been changed to work with undefined values. Currently these are

  • ShowX, which works like the Show class in Haskell. When an undefined value is encountered an “X” is printed. Show can still be used, but will throw an exception if an undefined value is encountered.

  • NFDataX, which works like the NFData class in the deepseq library. This allows evaluating values to normal form in code when undefined may be present. NFData can still be used, but will bubble up exceptions if undefined is encountered.

Clash Compiler Flags

--vhdl

Use the VHDL backend for code generation. This currently emits VHDL 1993 source which can be consumed by other tools.

--verilog

Use the Verilog backend for code generation. This currently emits Verilog 2001 source which can be consumed by other tools.

--systemverilog

Use the SystemVerilog backend for code generation. This currently emits SystemVerilog 2012 source which can be consumed by other tools.

-fclash-debug

Set the debugging mode for the compiler, exposing additional output. The available options are

  • DebugNone to show no debug messages

  • DebugSilent to test invariants and error if any are violated. This is implicitly enabled by any debug flag

  • DebugFinal to show expressions after they have been completely normalized

  • DebugCount to count how often each transformation is applied

  • DebugName to show the names of transformations as they are applied

  • DebugTry to show names of tried and applied transformations

  • DebugApplied to show sub-expressions after they are rewritten

  • DebugAll to show all sub-expressions when a rewrite is attempted

Default: DebugNone

Note

This flag exists for backwards compatibility. It is now possible to set debugging flags individually with -fclash-debug-invariants, -fclash-debug-info and -fclash-debug-count-transformations.

-fclash-debug-invariants

Check invariants while debugging and print warnings / errors which may be useful, such as alterting when unexpected changes occur or when a transformation introduces free variables / shadowing.

-fclash-debug-info

Specify the information to show about individual transformations while debugging. From least to most information, these are

  • None to show no information

  • FinalTerm to show the final result of normalization

  • AppliedName to show the names of applied transformations

  • AppliedTerm to show the result of applied transformations

  • TryName to show the names of attempted transforamtions, as well as the result of any transformations which are applied

  • TryTerm to show the names and results of all transformations attempted whether they were applied or not

    Default: None

-fclash-debug-count-transformations

Count the transformations that are applied and print a summary at the end of the normalization phase.

-fclash-debug-history[=FILENAME]

Saves all applied rewrites into FILENAME, for later analysis with the clash-term tool. When no filename is given it defaults to history.dat.

-fclash-debug-transformations

List the transformations that are to be debugged. This is given as a comma-separated list of transformations, e.g.

clash -fclash-debug-transformations inlineNonRep,topLet,appProp

Default: []

-fclash-debug-transformations-from=N

Only print debug output from applied transformation N and onwards.

clash -fclash-debug-transformations-from=21570

Default: 0

-fclash-debug-transformations-limit=N

Only print debug output for N applied transformations.

clash -fclash-debug-transformations-limit=12

Default: MAX_INT

-fclash-hdldir

Specify the directory that generated HDL is written into. For example

clash -fclash-hdldir build/hdl

will create a directory build/hdl

Default: Either vhdl, verilog, or systemverilog depending on the synthesis target.

-fclash-hdlsyn

Specify the HDL synthesis tool which will be used. Available options are Vivado, Quartus and Other, but some synonyms for these exist (Xilinx and ISE are synonyms for Vivado, Altera and Intel are synyonyms for Quartus).

Default: Other

-fclash-no-cache

Don’t reuse previously generated output from Clash, instead generating HDL from a clean state. While this leads to longer builds, it can be useful in development.

Warning

Previously this flag was called -fclash-nocache, however this is now deprecated.

Default: Cache generated HDL

-fclash-no-check-inaccessible-idirs

Check that all include directories (containing primitives) exist when running Clash. If any directory does not exist, an error is thrown.

Default: Check directories

-fclash-clear

Remove HDL directories before writing to them (if cache can’t be used). By default, Clash will only write to non-empty directories if it can prove all files in it are generated by a previous run. This option applies to directories of the various top entities, i.e., the subdirectories made in the directory passed in with -fclash-hdldir.

Default: Clean before build

-fclash-no-prim-warn

Disable warnings for primitives that are annotated with warnAlways. This means warnings from annotations like

{-# ANN f (warnAlways "This primitive is dangerous") #-}

will not be shown when compiling.

Default: Show warnings

-fclash-spec-limit

Change the number of times a function can undergo specialization.

Default: 20

-fclash-inline-limit

Change the number of times a function f can undergo inlining inside some other function g. This prevents the size of g growing dramatically.

Default: 20

-fclash-inline-function-limit

Set the threshold for function size. Below this threshold functions are always inlined (if it is not recursive).

Default: 15

-fclash-inline-constant-limit

Set the threshold for constant size. Below this threshold constants are always inlined. A value of 0 inlines all constants.

Default: 0

-fclash-evaluator-fuel-limit

Set the threshold for unfolding potentially non-terminating bindings in the evaluator. A value of 0 only unfolds terminating bindings.

Default: 20

-fclash-intwidth

Set the bit width for the Int/Word/Integer types. The only allowed values are 32 or 64.

Default: Machine word size (WORD_SIZE_IN_BITS)

-fclash-error-extra

Print additional information with compiler errors if it as available. If there is extra information and this flag is not enabled, a message will be printed suggesting this flag.

Default: False

-fclash-float-support

Enable support for floating point numbers. If this is disabled, Clash will not attempt to convert Float and Double values for hardware.

Default: False

-fclash-component-prefix

Prefix the names of generated HDl components with a string. For example a component foo would be called xcorp_foo if run with

clash -fclash-component-prefix "xcorp"

Default: “”

-fclash-old-inline-strategy

The new inlining strategy for Clash inlines all functions which are not marked with NOINLINE or a synthesize attribute. The old inlining strategy differed, attempting only to inline functions which were deemed “cheap”. The old inlining strategy may be quicker in practice for some circuits.

Default: False

-fclash-no-escaped-identifiers

Disable extended identifiers, as used in some HDLs like VHDL to allow more flexibility with names. Clash will only generate basic identifiers if this is used.

Default: Escaped identifiers are allowed

-fclash-lower-case-basic-identifiers

Clash will only generate lower case basic identifiers if this is used. This affects places where the various HDLs only allow basic identifiers to be used, most notably module and file names.

Default: Disabled

-fclash-compile-ultra

Aggressively run the normalizer, potentially gaining much better runtime performance at the expense of compile time.

Default: False

-fclash-force-undefined{,0,1}

Set the value to use when an undefined value is inserted into generated HDL. This flag can be suffixed with either 0 or 1 to force use of that bit, or left without a suffix to use a HDL-specific default (e.g. x in Verilog).

Default: Disabled

-fclash-aggressive-x-optimization

Remove all undefined branches from case expressions, replacing them with another defined value in the expression. If only one branch is defined, the case expression is elided completely. If no branches are defined the entire expression is replaced with a call to errorX.

Implies: -fclash-aggressive-x-optimization-blackboxes

Default: False

-fclash-aggressive-x-optimization-blackboxes

Allow blackboxes to detect undefined values and change their behavior accordingly. For example, if register is used in combination with an undefined reset value, it will leave out the reset logic entirely. This flag is enabled when using -fclash-aggressive-x-optimization.

Default: False

-fclash-edalize

Generate metadata for use with Edalize. This generates edam.py files in all top entities with the configuration for building that entity. Users still need to edit this file to specify the EDA tool to use, and if necessary the device to target (for Quartus, Vivado etc.)

Default: False

-main-is

When using one of --vhdl, --verilog, or --systemverilog, this flag refers to synthesis target. For example, running Clash with clash My.Module -main-is top --vhdl would synthesize My.Module.top.

Hacking on Clash

Clash/Haskell Style Guide

This is a short document describing the preferred coding style for this project. When something isn’t covered by this guide you should stay consistent with the code in the other modules. The code style rules should be considered strong suggestions but shouldn’t be dogmatically applied - if there’s a good reason for breaking them do it. If you can’t or don’t want to apply a guideline or if a guideline is missing, consider:

  • How your style affects future changes. Does changing part of it cause a lot of realignments? Is it easily extendable by copy-pasting lines?

  • Whether whitespace is effectively used. Do new indent-blocks start 2 spaces deeper than the previous one? Is it easy to see which block is which?

  • How it scales. Is the style applicable to small examples as well as large ones?

The guidelines formulated below try to balance the points above.

Formatting

Line Length

Try to keep below 80 characters (soft), never exceed 90 (hard).

Indentation

Tabs are illegal. Use spaces for indenting. Indent your code blocks with 2 spaces. Indent the where keyword 1 space to set it apart from the rest of the code and indent the definitions in a where clause 1 space. Some examples:

sayHello :: IO ()
sayHello = do
  name <- getLine
  putStrLn $ greeting name
 where
  greeting name = "Hello, " ++ name ++ "!"

filter
  :: (a -> Bool)
  -> [a]
  -> [a]
filter _ [] = []
filter p (x:xs)
  | p x       = x : filter p xs
  | otherwise = filter p xs
Blank Lines

One blank line between top-level definitions. No blank lines between type signatures and function definitions. Add one blank line between functions in a type class instance declaration if the function bodies are large. Use your judgement.

Whitespace

Surround binary operators with a single space on either side. Use your better judgement for the insertion of spaces around arithmetic operators but always be consistent about whitespace on either side of a binary operator. Don’t insert a space after a lambda. Add a space after each comma in a tuple:

good = (a, b, c)
bad = (a,b,c)

Refuse the temptation to use the latter when almost hitting the line-length limit. Restructure your code or use multiline notation instead. An example of a multiline tuple declaration is:

goodMulti =
  ( a
  , b
  , c )

goodMulti2 =
  ( a
  , b
  , c
  )

Use nested tuples as such:

nested =
  ( ( a1
    , a2 )
  , b
  , c )

Similar to goodMulti2, you can put the trailing ) on a new line. Use your judgement.

Data Declarations

Align the constructors in a data type definition. If a data type has multiple constructors, each constructor will get its own line. Example:

data Tree a
  = Branch !a !(Tree a) !(Tree a)
  | Leaf
  deriving (Eq, Show)

Data types deriving lots of instances may be written like:

data Tree a
  = Branch !a !(Tree a) !(Tree a)
  | Leaf
  deriving
    ( Eq, Show, Ord, Read, Functor, Generic, NFData
    , Undefined, BitPack, ShowX)

Data types with a single constructor may be written on a single line:

data Foo = Foo Int

Format records as follows:

data Person = Person
  { firstName :: !String
  -- ^ First name
  , lastName :: !String
  -- ^ Last name
  , age :: !Int
  -- ^ Age
  } deriving (Eq, Show)
List Declarations

Align the elements in the list. Example:

exceptions =
  [ InvalidStatusCode
  , MissingContentHeader
  , InternalServerError ]

You may put the closing bracket on a new line. Use your judgement.

exceptions =
  [ InvalidStatusCode
  , MissingContentHeader
  , InternalServerError
  ]

You may not skip the first newline.

-- WRONG!
directions = [ North
             , East
             , South
             , West
             ]

unless it fits on a single line:

directions = [North, East, South, West]
Vector Declarations

Small vectors may be written on a single line:

nrs = 1 :> 2 :> 3 :> 4 :> 5 :> Nil

Large vectors should be written like:

exceptions =
     North
  :> East
  :> South
  :> West
  :> Nil

Or:

exceptions =
     North :> East :> South
  :> West :> Middle :> Nil
Language pragmas

Place LANGUAGE pragmas right after a module’s documentation. Do not align the #-}s. Safe, Unsafe, or in some way “special” language pragmas should follow the normal ones separated by a single blank line. Pragmas should be ordered alphabetically. Example:

{-|
  .. docs ..
-}

{-# LANGUAGE CPP #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE QuasiQuotes #-}

{-# LANGUAGE Safe #-}
Pragmas

Put pragmas immediately following the function they apply to. Example:

id :: a -> a
id x = x
{-# NOINLINE id #-}
Hanging Lambdas

You may or may not indent the code following a “hanging” lambda. Use your judgement. Some examples:

bar :: IO ()
bar =
  forM_ [1, 2, 3] $ \n -> do
    putStrLn "Here comes a number!"
    print n

foo :: IO ()
foo =
  alloca 10 $ \a ->
  alloca 20 $ \b ->
  cFunction a b
Export Lists

Format export lists as follows:

module Data.Set
  (
    -- * The @Set@ type
    Set
  , empty
  , singleton

    -- * Querying
  , member
  ) where
If-then-else clauses

Generally, guards and pattern matches should be preferred over if-then-else clauses. Short cases should usually be put on a single line.

When writing non-monadic code (i.e. when not using do) and guards and pattern matches can’t be used, you can align if-then-else clauses like you would normal expressions:

foo =
  if cond0 then
    ...
  else
    ...

When used in monadic contexts, use:

foo =
  if cond0 then do
    ...
  else do
    ...

The same rule applies to nested do blocks:

foo = do
  instruction <- decodeInstruction
  skip <- load Memory.skip
  if skip == 0x0000 then do
    execute instruction
    addCycles $ instructionCycles instruction
  else do
    store Memory.skip 0x0000
    addCycles 1
Case expressions

The alternatives in a case expression can be indented using either of the two following styles:

foobar =
  case something of
    Just j  -> foo
    Nothing -> bar

or as

foobar =
  case something of
    Just j ->
      foo
    Nothing ->
      bar

In monadic contexts, use:

foobar =
  case something of
    Just j -> do
      foo
      bar
    Nothing -> do
      fizz
      buzz

Align the -> arrows when it helps readability, but keep in mind that any changes potentially trigger a lot of realignments. This increases your VCS’s diff sizes and becomes tedious quickly.

Type signatures

Small type signatures can be put on a single line:

f :: a -> a -> b

Longer ones should be put on multiple lines:

toInt
  :: Int
  -- ^ Shift char by /n/
  -> Char
  -- ^ Char to convert to ASCII integer
  -> Int

Multiple constraints can be added with a “tuple”:

toInt
  :: (Num a, Show a)
  => a
  -- ^ Shift char by /n/
  -> Char
  -- ^ Char to convert to ASCII integer
  -> Int

Many constraints need to be split accross multiple lines too:

toInt
  :: ( Num a
     , Show a
     , Foo a
     , Bar a
     , Fizz a
     )
  => a
  -- ^ Shift char by /n/
  -> Char
  -- ^ Char to convert to ASCII integer
  -> Int

forall’s dot must be aligned:

toInt
  :: forall a
   . (Num a , Show a)
  => a
  -- ^ Shift char by /n/
  -> Char
  -- ^ Char to convert to ASCII integer
  -> Int

If you have many type variables, many constraints, and many arguments, your function would end up looking like:

doSomething
  :: forall
       clockDomain
       resetDomain
       resetKind
       domainGatedness
   . ( Undefined a
     , Ord b
     , NFData c
     , Functor f )
  => f a
  -> f b
  -> f c

Imports

Imports should be grouped in the following order:

  1. clash-prelude†

  2. standard library imports

  3. related third party imports

  4. local application/library specific imports

Put a blank line between each group of imports. Create subgroups per your own judgement. The imports in each group should be sorted alphabetically, by module name.

Always use explicit import lists or qualified imports for standard and third party libraries. This makes the code more robust against changes in these libraries. Exception: The Prelude.

† When writing circuit designs. Does not apply when hacking on the compiler itself.

Comments

Language

Use American English. Initialization, synchronization, ..

Punctuation

Write proper sentences; start with a capital letter and use proper punctuation.

Top-Level Definitions

Comment every top level function (particularly exported functions), and provide a type signature; use Haddock syntax in the comments. Comment every exported data type. Function example:

-- | Send a message on a socket. The socket must be in a connected
-- state. Returns the number of bytes sent. Applications are
-- responsible for ensuring that all data has been sent.
send
  :: Socket
  -- ^ Connected socket
  -> ByteString
  -- ^ Data to send
  -> IO Int
  -- ^ Bytes sent

For functions the documentation should give enough information apply the function without looking at the function’s definition.

Record example:

-- | Bla bla bla.
data Person = Person
  { age  :: !Int
  -- ^ Age
  , name :: !String
  -- ^ First name
  }

For fields that require longer comments format them like so:

data Record = Record
  { field1 :: !Text
  -- ^ This is a very very very long comment that is split over
  -- multiple lines.

  , field2 :: !Int
  -- ^ This is a second very very very long comment that is split
  -- over multiple lines.
  }
End-of-Line Comments

Separate end-of-line comments from the code using 2 spaces. Align comments for data type definitions. Some examples:

data Parser =
  Parser
    !Int         -- Current position
    !ByteString  -- Remaining input

foo :: Int -> Int
foo n = salt * 32 + 9
  where
    salt = 453645243  -- Magic hash salt.

Naming

Use camel case (e.g. functionName) when naming functions and upper camel case (e.g. DataType) when naming data types.

For readability reasons, don’t capitalize all letters when using an abbreviation. For example, write HttpServer instead of HTTPServer. Exception: Two letter abbreviations, e.g. IO.

Use American English. That is, synchronizer, not synchroniser.

Modules

Use singular when naming modules e.g. use Data.Map and Data.ByteString.Internal instead of Data.Maps and Data.ByteString.Internals.

Dealing with laziness

By default, use strict data types and lazy functions.

Data types

Constructor fields should be strict, unless there’s an explicit reason to make them lazy. This avoids many common pitfalls caused by too much laziness and reduces the number of brain cycles the programmer has to spend thinking about evaluation order.

-- Good
data Point = Point
  { pointX :: !Double
  , pointY :: !Double
  }

-- Bad
data Point = Point
  { pointX :: Double
  , pointY :: Double
  }
Functions

Have function arguments be lazy unless you explicitly need them to be strict.

The most common case when you need strict function arguments is in recursion with an accumulator:

mysum :: [Int] -> Int
mysum = go 0
  where
    go !acc []    = acc
    go acc (x:xs) = go (acc + x) xs

Misc

Point-free style

Avoid over-using point-free style. For example, this is hard to read:

-- Bad:
f = (g .) . h
Warnings

Code should be compilable with -Wall -Werror. There should be no warnings.

The Clash Compiler

Prerequisites

Hacking on Clash requires more dependencies than simply running Clash. The test suite requires having a tool available to synthesize any backend being tested. This means you need

  • ghdl installed to test VHDL

  • iverilog installed to test Verilog

  • ModelSim installed to test System Verilog

  • SymbiYosys and Z3 installed to test Verilog and System Verilog

Subprojects

The Clash compiler consists of different cabal libraries, which together provide a complete compiler. Primarily, this consists of

clash-ghc

The front-end of the compiler, using parts of the GHC front-end. This provides the ability to load modules, translate GHC Core to Clash Core, and implements the clash and clashi executables.

A lot of the code in this library is separated by the version of GHC it works with. For example, src-bin-9.0 is specific to GHC 9.0.x.

clash-lib

The back-end of the compiler, exposed as a library. This is the largest library in the project, and includes the various ASTs (e.g. Core, Netlist), normalization, code generation, and primitives / black boxes.

clash-prelude

The standard library for Clash as a language. This includes anything that is used to develop hardware in Clash, such as Signals, Clocks and combinators for common forms of state machine.

The clash-prelude library also re-exports parts of the Haskell base library, allowing circuit designs to re-use common functions and definitions.

The repository also contains other libraries. These either provide additional functionality which is not required, or are not yet production-ready. These are

clash-cores

A collection of IP cores for use in Clash designs. Currently, this includes Lattice Ice IO cores, SPI (with slaves implemented with the Lattice SBIO found on Lattice FPGAs), a UART and support for Xilinx floating point IP.

Note

This library is optional, and is not required to use Clash. In the future it may be extended with additional IP cores.

clash-cosim

Co-simulation for Clash, allowing Verilog to be run inline as though it were a normal Haskell function. This provides a QuasiQuoter for use in Haskell.

Warning

This library is very experimental, and is not guaranteed to work with the most recent development version of Clash.

clash-term

A development tool for analysing how the normalizer in clash-lib affects the core of a particular design. It allows the result of each different optimizer pass to be seen for debugging purposes.

clash-lib-hedgehog

Hedgehog Generators for clash-lib.

clash-prelude-hedgehog

Hedgehog Generators for clash-prelude.

Changelog for the Clash project

1.6.4 Aug 30th 2022

Fixed:

  • Input validation of the used arguments in blackboxes is now complete. #2184

  • Clash.Annotations.BitRepresentation.Deriving.deriveAnnotation no longer has quadratic complexity in the size of the constructors and fields. #2209

  • Fully resolve type synonyms when deriving bit representations. #2209

  • Disregard ticks when determining whether terms are shared. Fixes #2233.

  • The blackbox parser will make sure it fully parses its input, and report an error when it can’t. #2237

  • Wrap ~ARG[n] in parentheses. Fixes #2213

  • The VHDL shift primitives no longer generate bound check failures. Fixes #2215

  • Evaluator fails impredicative type instantiation of error values #2272

  • Fix out of bound errors in toEnum/fromSLV for sum types #2220

  • Netlist generation fails for certain uses of GADTs #2289

  • The documentation for ANN TestBench had it backwards; it now correctly indicates the annotation is on the test bench, not the device under test. #1750

Fixes with minor changes:

  • reduceXor now produces a result if the argument has undefined bits instead of throwing an XException (the result is an undefined bit). reduceAnd and reduceOr already always produced a result. #2244

Added:

  • Support for symbols in types while deriving bit representations. #2209

  • Support for promoted data types while deriving bit representations. #2209

  • scanlPar and scanrPar in Clash’s Prelude, as well as the RTree versions tscanl and tscanr. These variants of scanl1 and scanr1 compile to a binary tree of operations, with a depth of O(log(n)) (n being the length of the vector) rather than a depth of n for scanl1 and scanr1. #2177

  • The GADT constructors for RTree (RLeaf and RBranch) are now exported directly in addition to the patterns LR and BR. #2177

  • Added the ~ISSCALAR template which can be used to check if an argument is rendered to a scalar in HDL. #2184

  • Added support for records and infix constructors when using Clash.Annotations.BitRepresentation.Deriving.deriveAnnotation. #2191

  • Clash now contains instances for ShowX, NFDataX and BitPack on the newtypes from the Data.Functor modules (Identity, Const, Compose, Product and Sum). #2218

1.6.3 Apr 7th 2022

Fixed:

  • Handle ~ISUNDEFINED hole in black boxes for BitVector and for product types. This means that with -fclash-aggressive-x-optimization-blackboxes, resets are now omitted for undefined reset values of such types as well. #2117

  • The alteraPll primitive was unusable since commit d325557750 (release v1.4.0), it now works again. #2136

  • Simulation/Synthesis mismatch for X-exception to undefined bitvector conversion #2154

  • The VHDL blackbox for Signed.fromInteger can now handle any Netlist Expr as input #2149

  • Clash no longer escapes extended identifiers when rendering SDC files. #2142

  • The types defined in clash-prelude-hedgehog now come with Show instances #2133

  • Extreme values are now generated from the input range instead of the type’s bounds #2138

Internal change:

  • Clash now always generates non-extended identifiers for port names, so that generated names play nicer with different vendor tools. #2142

  • Top entity name available in netlist context. Top entity name used in generated name for include files. #2146

1.6.2 Feb 25th 2022

Fixed:

  • Clash now compiles for users of Clang - i.e., all macOS users.

  • The trueDualPortBlockRam model did not accurately simulate concurrent active ports, thus causing a Haskell/HDL simulation mismatch for asyncFIFOSynchronizer.

  • trueDualPortBlockRam Haskell/HDL simulation mismatch for port enable.

  • Sometimes trueDualPortBlockRam swapped the names of the ports in exception messages. #2102

  • The evaluator rule for unpack{Float,Double}# are now corrected to return boxed float and double instead of unboxed literals. #2097

Changed:

  • The trueDualPortBlockRam model now only models read/write conflicts for concurrent active ports

  • The trueDualPortBlockRam model now models write/write conflicts for concurrent active ports

1.6.1 Feb 11th 2022

Changed:

  • We accidentally released v1.6.0 with the Cabal flag multiple-hidden enabled. This is an experimental feature, supposed to be disabled by default for releases. v1.6.1 disables it again.

Added:

  • Clash.Class.HasDomain.TryDomain instances for Clash sized types

1.6.0 Feb 10th 2022

Added:

  • Clash.Class.Counter: a class that defines a odometer-style supercounter. #1763

  • isLike function for BitPack types. #1774

  • ‘seqErrorX’ for catching both XException and ErrorCall. #1774

  • Clash.Explicit.BlockRam.File.memFile, a function for creating the contents of the data files this blockRAM uses. Can also be imported from Clash.Prelude.BlockRam.File, Clash.Prelude.ROM.File and Clash.Explicit.ROM.File. #1840

  • Support for Yosys compatible SVA to Clash.Verification. This enables formal verification using SymbiYosis for Verilog and SystemVerilog. #1798

  • Clash.Explicit.Signal.Delayed.forward, a function that can be used to retime a DSignal into the future without applying any logic. #1882

  • Clash.Signal.andEnable is the HiddenEnable version of Clash.Explicit.Signal.andEnable (formerly known as enable) #1849

  • runUntil, a function to sample a signal until it returns a value that satisfies the user-given test. It is a convenience function that, among others, allow easy running of a testBench style function in Haskell simulation, logging assertion failures to stderr. #1940

  • Support for true dual ported block ram through Clash.Prelude.BlockRam.trueDualPortBlockRam and Clash.Explicit.BlockRam.trueDualPortBlockRam. #1726 #1975

  • clash-{prelude,lib}-hedgehog packages which provide generators for types in clash-prelude and clash-lib. The former is published on Hackage. #1976

  • Clash now contains black boxes which are verilator compatible. When running with --verilog or --systemverilog a C++ shim is automatically produced which can be used to quickly generate a verilated executable. Users who wish to interact with verilator simulations are recommended to use clashilator. #2019

  • Support for YAML blackboxes. Clash will now pickup on files with a .primitives.yaml extension. While we recommend upgrading your primitive files to the new format, old style primitives are still supported. We’ve included a tool to automatically upgrade your JSON files, see #2037

  • MemBlob: a datastructure for efficient constants, typically used for initializing memories. #2041

Fixed:

  • BlockRam simulation is now less strict. #1458

  • Don’t overflow VHDL’s integer type when addressing RAM/ROM in simulation.Addresses are masked to 32 bits to be sure to keep it within the simulator’s range. #1875

  • show on BitVector 0 no longer results in an empty string. #1785

  • Clash now preserves transfinite floating numbers (NaN, Infinity) when packing/unpacking #1803

  • SynthesisAnnotations can now be defined in type synoynms without being excluded from the generated HDL #1771

  • Manifest files now correctly list bidirectional ports as “inout” rather than “in” #1843

  • div/rem/mod now avoid division by zero during VHDL simulation. Due to the use of concurrent statements, even unreachable code would previously result in simulation error #1873

  • Don’t overflow the range of VHDL’s natural type in shift/rotate, leading to simulation issues. Shift now saturates to a 31-bit shift amount. For rotate, in simulation only, the rotate amount is modulo the word width of the rotated value #1874

  • shiftL for Clash datatypes does not cause a crash anymore when running Clash code with a really large shift amount #1874

  • VHDL generated for Signed.fromInteger now truncates, like the Clash simulation, when the result is smaller than the argument #1874

  • Clash now preserves boolean combinatorial logic better when generating HDL #1881

  • valid field of TemplateFunction is now checked for includes #1945

  • Clash now generates clock generators that ensure that the amount of time between simulation start and the first active edge of the clock is equal to (/or longer than/) the period of the clock. The first active edges of the clocks do still occur simultaneously. #2001

  • Expected values in assert become undefined when using -fclash-compile-ultra #2040

  • toEnum/fromEnum on sized types is now less eager to report warnings about integer functions being used #2046

Changed:

  • Clash.Verification.PrettyPrinters has been moved from clash-prelude to to Clash.Verification.Pretty in clash-lib. #1798

  • RAM/ROM functions: They now throw XExeception for out-of-bounds address inputs, so this condition no longer aborts simulation. #1875

  • Vec’s show instance now generates valid Haskell. #1776

  • ShowX and its functions now produce valid Haskell #1782

  • bLit now infers the size of the generated BitVector from the string given to it. This means you don’t have to give it an explicit type signature anymore. This does slightly modify the syntax needed to invoke bLit. E.g., $$(bLit "00..1") :: BitVector 5 should be rewritten as $(bLit "00..1"). If you relied on the size inference, wrap the new invocation in resize. For example: resize $(bLit "00..1"). #1784

  • NumericUnderscores is now enabled by default in clash, clashi, and starter projects using Clash >=1.6. #1785

  • Show instance of BitVector now includes a 0b prefix, making it a copyable expression for fully defined vectors. #1785

  • blockRam uses STArray as the underlying representation to improve simulation performance #1878

  • asyncRom now throws XException for out-of-bounds addressing, no longer aborting simulation #1878

  • Clash now renders ADTs with all zero-width fields as enumerations in VHDL #1879

  • A warning about possible hard-to-debug issues has been added to the Clash.Signal documentation on hidden clocks, resets, and enables, in the form of the section named “Monomorphism restriction leads to surprising behavior” #1960

  • Clash.Explicit.Testbench.outputVerifier and outputVerifierBitVector now emit a warning if they are used improperly. This situation only arises when they are used in synthesized code rather than a test bench context. When the clock domains circuitDom and testDom are two different domains, the clock crossing inside outputVerifier is only suitable inside a test bench, not inside a synthesized circuit. Clash now emits a warning for this case. #1931

  • resetSynchronizer now no longer takes an Enable argument. The argument was already marked for removal and was ignored. #1964

  • Clash can now compile multiple entities concurrently, providing speedups to designs with multiple entities to build #2034

  • All asyncRam variants and asyncFIFOSynchronizer now require that the data has an NFDataX instance. #2055

  • Clash now respects the -Werror option from GHC #2066

  • asyncFIFOSynchronizer now uses the synchronous dual-ported RAM trueDualPortBlockRam, where it previously used a dual-ported RAM with an asynchronous read port asyncRam. With this change it’s nearly guaranteed that asyncFIFOSynchronizer actually synthesizes to a circuit that uses the dual-ported RAMs found on most FPGAs. #2083

Deprecated:

  • The function Clash.Explicit.Signal.enable is renamed to andEnable and the existing name deprecated #1849

  • ‘-fclash-float-support’: it is now on by default and can’t be turned off. #2048

Removed:

  • GHC 8.4 is no longer supported. Users should upgrade to at least GHC 8.6. #1762

Internal changes:

  • clash-lib now uses Data.Monoid.Ap instead of Data.Semigroup.Monad.Mon. This means users defining primitives with TemplateFunction will need to replace Mon/getMon with Ap/getAp. #1835

  • Clash now supports more expressive debug options at the command line #1800.

  • Added zeroWidthSpec transformation #1891

  • Added collapseRHSNoops inlining stage and WorkIdentity constructor #1896

  • Added HasType and InferType classes for getting / inferring core types from data representing some typed “thing” #1915

  • Added HasFreeVars class for getting free variables from data “containing” variables #1917

  • Added the primitive equality type (~#) to Clash.Core.TysPrim. In order to make this change, undefinedTy and unsafeCoerceTy were moved from Clash.Core.Type to Clash.Core.Util. #1955

  • Clash now keeps information about which let bindings are recursive from GHC core. This can be used to avoid performing free variable calculations, or sorting bindings in normalization. #1980 #2000

  • Manifest files now use SHA256 for a cache invalidation digest #1985

1.4.7 Jan 30th 2022

Fixed:

  • Clash now shows days in time strings for compile runs which take longer than a day #1989.

  • Types defined in the package head are no longer qualified in the package body when rendering VHDL #1996.

  • asyncRam with different read and write clocks no longer produce the wrong results in Haskell simulation. #2031

  • Clash.Explicit.RAM.asyncRam# Haskell simulation incorrectly treated an undefined write enable as asserted. It now causes an undefined value to be written instead. This problem did not propagate to the other asyncRam functions, where the same condition would simultaneously lead to an undefined write address, which would be handled correctly. This problem also only affects Haskell simulation, not the generated HDL. #2031

  • Clash.Explicit.BlockRam.blockRam# and Clash.Explicit.BlockRam.File.blockRamFile# Haskell simulation incorrectly treated an undefined write enable as asserted. It now causes an undefined value to be written instead. This problem did not propagate to the other blockRam functions, where the same condition would simultaneously lead to an undefined write address, which would be handled correctly. This problem also only affects Haskell simulation, not the generated HDL.(#2054)

Internal changes:

  • Removed instances of Hashable Term and Hashable Type #1986

  • Added structural equality on Term (Clash.Core.Subst.eqTerm) and Type (Clash.Core.Subst.eqType)

Internal fixes:

  • Enable used to be a Bool in the Blackbox DSL, so we could use boolToBit. However it now has its own type in the DSL (Enable domainName), so we’ve added a new conversion function in order to convert it to a Bool.

1.4.6 Oct 26th 2021

Fixed:

  • Clash tries to cast-specialize non-“global binders” resulting in “specialisation of non-work-free cast” warning #1933

  • More consistently render bare untyped and unsized literals for ~LIT tags. This fixes #1934

1.4.5 Oct 13th 2021

Changed:

  • clash-lib now supports prettyprinter 1.7

Documentation:

  • The documentation on hidden clocks, resets, and enables has been corrected and extended in Clash.Signal.

1.4.4 Oct 11th 2021

Changed:

  • clash-lib now supports aeson >= 2.0

Fixed:

  • Dont’ loop on recursive data types hiding behind type families #1921

  • Recognize enableGen as workfree and don’t duplicate registers #1935

1.4.3 Aug 8th 2021

Fixed:

  • Clash no longer generates calls to {shift,rotate}_{left,right} in VHDL where the count is a negative number #1810.

  • Clash no longer incurs unnecessary compile-time complexity while compiling Vector indexing operator #1557

1.4.2 May 18th 2021

Fixed:

  • Erroneous examples in Clash.Annotation.TopEntity documentation #646 and #654

  • unconcat cannot be used as initial/reset value for a register #1756

  • showX now doesn’t crash if a spine of a Vec is undefined

  • ~ISACTIVEENABLE in blackboxes works again, and now acts on Signal dom Bool in addition to Enable dom. Since #1368, enable lines were always generated even if they were known to be always enabled. Fixes #1786.

  • clash –show-options now shows -fclash-* options in GHC 9.0 #1787

  • makeRecursiveGroups now correctly identifies mutual recursion between global binders (#1796).

1.4.1 April 6th 2021

Fixed:

  • Broken VHDL primitive template for setSlice# #1715

  • Unable to reduce nested type families #1721

  • DEC transformation fails for functions applied to more than 62 arguments #1669

  • Erroneous examples in BlockRam.File and ROM.File documentation #1608

  • Blackboxes of Clash.Sized.Vector functions error on vectors containing Clocks, Reset, or Enable #1606

  • Clash.Signal.Delayed.delayI cannot be reset, the HiddenReset constraint was unintentional. Asserting its reset has never worked. Removed the constraint #1739.

  • Annotate attributes cannot use type families #1742

Changed:

  • Clash.Prelude.ROM.File.romFile now takes an Enum addr => addr as address argument, making it actually useful. #407

1.4.0 March 12th 2021

Highlighted changes (repeated in other categories):

  • Clash no longer disables the monomorphism restriction. See #1270, and mentioned issues, as to why. This can cause, among other things, certain eta-reduced descriptions of sequential circuits to no longer type-check. See #1349 for code hints on what kind of changes to make to your own code in case it no longer type-checks due to this change.

  • Type arguments of Clash.Sized.Vector.fold swapped: before forall a n . (a -> a -> a) -> Vec (n+1) a -> a, after forall n a . (a -> a -> a) -> Vec (n+1) a. This makes it easier to use fold in a 1 <= n context so you can “simply” do fold @(n-1)

  • Fixed now obeys the laws for Enum as set out in the Haskell Report, and it is now consistent with the documentation for the Enum class on Hackage. As Fixed is also Bounded, the rule in the Report that succ maxBound and pred minBound should result in a runtime error is interpreted as meaning that succ and pred result in a runtime error whenever the result cannot be represented, not merely for minBound and maxBound alone.

  • Primitives should now be stored in *.primitives files instead of *.json. While primitive files very much look like JSON files, they’re not actually spec complaint as they use newlines in strings. This has recently been brought to our attention by Aeson fixing an oversight in their parser implementation. We’ve therefore decided to rename the extension to prevent confusion.

Fixed:

  • Result of Clash.Class.Exp.(^) has enough bits in order to deal with x^0.

  • Resizes to Signed 0 (e.g., resize @(Signed n) @(Signed 0)) don’t throw an error anymore

  • satMul now correctly handles arguments of type Index 2

  • Clash.Explicit.Reset.resetSynchronizer now synchronizes on synchronous domains too #1567.

  • Clash.Explicit.Reset.convertReset: now converts synchronous domains too, if necessary #1567.

  • inlineWorkFree now never inlines a topentity. It previously only respected this invariant in one of the two cases #1587.

  • Clash now reduces recursive type families #1591

  • Primitive template warning is now retained when a PrimitiveGuard annotation is present #1625

  • signum and RealFrac for Fixed now give the correct results.

  • Fixed a memory leak in register when used on asynchronous domains. Although the memory leak has always been there, it was only triggered on asserted resets. These periods are typically short, hence typically unnoticable.

  • createDomain will not override user definitions of types, helping users who strive for complete documentation coverage [#1674] https://github.com/clash-lang/clash-compiler/issues/1674

  • fromSNat is now properly constrained #1692

  • As part of an internal overhaul on netlist identifier generation #1265:

    • Clash no longer produces “name conflicts” between basic and extended identifiers. I.e., \x\ and x are now considered the same variable in VHDL (likewise for other HDLs). Although the VHDL spec considers them distinct variables, some HDL tools - like Quartus - don’t.

    • Capitalization of Haskell names are now preserved in VHDL. Note that VHDL is a case insensitive languages, so there are measures in place to prevent Clash from generating both Foo and fOO. This used to be handled by promoting every capitalized identifier to an extended one and wasn’t handled for basic ones.

    • Names generated for testbenches can no longer cause collisions with previously generated entities.

    • Names generated for components can no longer cause collisions with user specified top entity names.

    • For (System)Verilog, variables can no longer cause collisions with (to be) generated entity names.

    • HO blackboxes can no longer cause collisions with identifiers declared in their surrounding architecture block.

Changed:

  • Treat enable lines specially in generated HDL #1171

  • Signed, Unsigned, SFixed, and UFixed now correctly implement the Enum law specifying that the predecessor of minBound and the successor of maxBound should result in an error #1495.

  • Fixed now obeys the laws for Enum as set out in the Haskell Report, and it is now consistent with the documentation for the Enum class on Hackage. As Fixed is also Bounded, the rule in the Report that succ maxBound and pred minBound should result in a runtime error is interpreted as meaning that succ and pred result in a runtime error whenever the result cannot be represented, not merely for minBound and maxBound alone.

  • Type arguments of Clash.Sized.Vector.fold swapped: before forall a n . (a -> a -> a) -> Vec (n+1) a -> a, after forall n a . (a -> a -> a) -> Vec (n+1) a. This makes it easier to use fold in a 1 <= n context so you can “simply” do fold @(n-1)

  • Moved Clash.Core.Evaluator into Clash.GHC and provided generic interface in Clash.Core.Evalautor.Types. This removes all GHC specific code from the evaluator in clash-lib.

  • Clash no longer disables the monomorphism restriction. See #1270, and mentioned issues, as to why. This can cause, among other things, certain eta-reduced descriptions of sequential circuits to no longer type-check. See #1349 for code hints on what kind of changes to make to your own code in case it no longer type-checks due to this change.

  • Clash now generates SDC files for each topentity with clock inputs

  • deepErrorX is now equal to undefined#, which means that instead of the whole BitVector being undefined, its individual bits are. This makes sure bit operations are possible on it. #1532

  • From GHC 9.0.1 onwards the following types: BiSignalOut, Index, Signed, Unsigned, File, Ref, and SimIO are all encoded as data instead of newtype to work around an issue where the Clash compiler can no longer recognize primitives over these types. This means you can no longer use Data.Coerce.coerce to coerce between these types and their underlying representation.

  • Signals on different domains used to be coercable because the domain had a type role “phantom”. This has been changed to “nominal” to prevent accidental, unsafe coercions. #1640

  • Size parameters on types in Clash.Sized.Internal.* are now nominal to prevent unsafe coercions. #1640

  • hzToPeriod now takes a Ratio Natural rather than a Double. It rounds slightly differently, leading to more intuitive results and satisfying the requested change in #1253. Clash expresses clock rate as the clock period in picoseconds. If picosecond precision is required for your design, please use the exact method of specifying a clock period rather than a clock frequency.

  • periodToHz now results in a Ratio Natural

  • createDomain doesn’t override existing definitions anymore, fixing #1674

  • Manifest files are now stored as clash-manifest.json

  • Manifest files now store hashes of the files Clash generated. This allows Clash to detect user changes on a next run, preventing accidental data loss.

  • Primitives should now be stored in *.primitives files. While primitive files very much look like JSON files, they’re not actually spec complaint as they use newlines in strings. This has recently been brought to our attention by Aeson fixing an oversight in their parser implementation. We’ve therefore decided to rename the extension to prevent confusion.

  • Each binder marked with a Synthesize or TestBench pragma will be put in its own directory under their fully qualified Haskell name. For example, two binders foo and bar in module A will be synthesized in A.foo and A.bar.

  • Clash will no longer generate vhdl, verilog, or systemverilog subdirectories when using -fclash-hdldir.

  • Data.Kind.Type is now exported from Clash.Prelude #1700

Added:

  • Support for GHC 9.0.1

  • Clash.Signal.sameDomain: Allows user obtain evidence whether two domains are equal.

  • xToErrorCtx: makes it easier to track the origin of XException where pack would hide them #1461

  • Additional field with synthesis attributes added to InstDecl in Clash.Netlist.Types #1482

  • Data.Ix.Ix instances for Signed, Unsigned, and Index #1481 #1631

  • Added nameHint to allow explicitly naming terms, e.g. Signals.

  • Checked versions of resize, truncateB, and fromIntegral. Depending on the type resize, truncateB, and fromIntegral either yield an XException or silently perform wrap-around if its argument does not fit in the resulting type’s bounds. The added functions check the bound condition and fail with an error call if the condition is violated. They do not affect HDL generation. #1491

  • HasBiSignalDefault: constraint to Clash.Signal.BiSignal, pullUpMode gives access to the pull-up mode. #1498

  • Match patterns to bitPattern #1545

  • Non TH fromList and unsafeFromList for Vec. These functions allow Vectors to be created from a list without needing to use template haskell, which is not always desirable. The unsafe version of the function does not compare the length of the list to the desired length of the vector, either truncating or padding with undefined if the lengths differ.

  • Clash.Explicit.Reset.resetGlitchFilter: filters glitchy reset signals. Useful when your reset signal is connected to sensitive actuators.

  • Clash can now generate EDAM for using Edalize. This generates edam.py files in all top entities with the configuration for building that entity. Users still need to edit this file to specify the EDA tool to use, and if necessary the device to target (for Quartus, Vivado etc.). #1386

  • -fclash-aggressive-x-optimization-blackboxes: when enabled primitives can detect undefined values and change their behavior accordingly. For example, if register is used in combination with an undefined reset value, it will leave out the reset logic entirely. Related issue: #1506.

  • Automaton-based interface to simulation, to allow interleaving of cyle-by-cycle simulation and external effects #1261

New internal features:

  • constructProduct and deconstructProduct in Clash.Primitives.DSL. Like tuple and untuple, but on arbitrary product types.

  • Support for multi result primitives. Primitives can now assign their results to multiple variables. This can help to work around synthesis tools limits in some cases. See #1560.

  • Added a rule for missing Int comparisons in GHC.Classes in the compile time evaluator. #1648

  • Clash now creates a mapping from domain names to configurations in LoadModules. #1405

  • The convenience functions in Clash.Primitives.DSL now take a list of HDLs, instead of just one.

  • Clash.Netlist.Id overhauls the way identifiers are generated in the Netlist part of Clash.

  • Added defaultWithAction to Clash-as-a-library API to work around/fix issues such as #1686

  • Manifest files now list files and components in an reverse topological order. This means it can be used when calling EDA tooling without causing compilation issues.

Deprecated:

  • Clash.Prelude.DataFlow: see #1490. In time, its functionality will be replaced by clash-protocols.

Removed:

  • The deprecated function freqCalc has been removed.

1.2.5 November 9th 2020

Fixed:

  • The normalizeType function now fully normalizes types which require calls to reduceTypeFamily #1469

  • flogBaseSNat, clogBaseSNat and logBaseSNat primitives are now implemented correctly.Previously these primitives would be left unevaluated causing issues as demonstrated in #1479

  • Specializing on functions with type family arguments no longer fails #1477

  • satSucc, satPred correctly handle “small types” such as Index 1.

  • msb no longer fails on values larger than 64 bits

  • undefined can now be used as a reset value of autoReg@Maybe #1507

  • Signal’s fmap is now less strict, preventing infinite loops in very specific situations. See #1521

  • Clash now uses correct function names in manifest and sdc files #1533

  • Clash no longer produces erroneous HDL in very specific cases #1536

  • Usage of fold inside other HO primitives (e.g., map) no longer fails #1524

Changed:

  • Due to difficulties using resetSynchronizer we’ve decided to make this function always insert a synchronizer. See: #1528.

1.2.4 July 28th 2020

  • Changed:

    • Relaxed upper bound versions of aeson and dlist, in preparation for the new Stack LTS.

    • Reverted changes to primitive definitions for ‘zipWith’, ‘map’, ‘foldr’, and ‘init’ introduced in 1.2.2. They have shown to cause problems in very specific circumstances.

1.2.3 July 11th 2020

  • Changed:

    • Upgrade to nixos 20.03. Nix and snap users will now use packages present in 20.03.

  • Added:

    • instance Monoid a => Monoid (Vec n a)

    • instance Text.Printf(Index)

    • instance Text.Printf(Signed)

    • instance Text.Printf(Unsigned)

  • Fixed:

    • Clash renders incorrect VHDL when GHCs Worker/Wrapper transformation is enabled #1402

    • Minor faults in generated HDL when using annotations from Clash.Annotations.SynthesisAttributes

    • Cabal installed through Snap (clash.cabal) can now access the internet to fetch pacakges. [#1411]https://github.com/clash-lang/clash-compiler/issues/1411

    • Generated QSys file for altpll incompatible with Quartus CLI (did work in Quartus GUI)

    • Clash no longer uses component names that clash with identifiers imported from:

      • IEEE.STD_LOGIC_1164.all

      • IEEE.NUMERIC_STD.all

      • IEEE.MATH_REAL.all

      • std.textio.all when generating VHDL. See https://github.com/clash-lang/clash-compiler/issues/1439.

1.2.2 June 12th 2020

  • Changed:

    • The hardwired functions to unroll primitive definitions for ‘zipWith’, ‘map’, ‘foldr’, and ‘init’ have been changed to only unroll a single step, whereas they would previously unroll the whole definition in one step. This allows Clash to take advantage of the lazy nature of these functions, in turn speeding up compilation speeds significantly in some cases. Part of PR 1354.

  • Added:

    • Support for GHC 8.10

    • Ability to load designs from precompiled modules (i.e., stored in a package database). See #1172

    • Support for ‘-main-is’ when used with --vhdl, --verilog, or --systemverilog

    • A partial instance for NFDataX (Signal domain a)

  • Fixed:

    • Clash’s evaluator now inlines work free definitions, preventing situations where it would otherwise get stuck in an infinite loop

    • caseCon doesn’t apply type-substitution correctly #1340

    • Clash generates illegal SystemVerilog slice #1313

    • Fix result type of head and tail Verilog blackboxes #1351

    • Certain recursive let-expressions in side a alternatives of a case-expression throw the Clash compiler into an infinite loop #1316

    • Fixes issue with one of Clash’s transformations, inlineCleanup, introducing free variables #1337

    • Fails to propagate type information of existential type #1310

    • Certain case-expressions throw the Clash compiler into an infinite loop #1320

    • Added blackbox implementation for ‘Clash.Sized.Vector.iterateI’, hence making it usable as a register reset value #1240

    • iterate and iterateI can now be used in reset values #1240

    • Prim evaluation fails on undefined arguments #1297

    • Missing re-indexing in (Un)Signed fromSLV conversion #1292

    • VHDL: generate a type qualification inside ~TOBV, fixes #1360

1.2.1 April 23rd 2020

  • Changed:

    • Treat Signed 0, Unsigned 0, Index 1, BitVector 0 as unit. In effect this means that ‘minBound’ and ‘maxBound’ return 0, whereas previously they might crash #1183

    • Infix use of deepseqX is now right-associative

  • Added:

    • Add ‘natToInteger’, ‘natToNatural’, and ‘natToNum’. Similar to ‘snatTo*’, but works solely on a type argument instead of an SNat.

    • Clash.Sized.Vector.unfoldr and Clash.Sized.Vector.unfoldrI to construct vectors from a seed value

    • Added NFDataX instances for Data.Monoid.{First,Last}

  • Fixed:

    • The Verilog backend can now deal with non-contiguous ranges in custom bit-representations.

    • Synthesizing BitPack instances for type with phantom parameter fails #1242

    • Synthesis of fromBNat (toBNat d5) failed due to unsafeCoerce coercing from Any

    • Memory leak in register primitives #1256

    • Illegal VHDL slice when projecting nested SOP type #1254

    • Vivado VHDL code path (-fclash-hdlsyn Vivado) generates illegal VHDL #1264

1.2.0 March 5th 2020

As promised when releasing 1.0, we’ve tried our best to keep the API stable. We think most designs will continue to compile with this new version, although special care needs to be taken when using:

  • Use inline blackboxes. Instead of taking a single HDL, inline primitives now take multiple. For example, InlinePrimitive VHDL ".." must now be written as InlinePrimitive [VHDL] "..".

  • Use the Enum instance for BitVector, Index, Signed, or Unsigned, as they now respect their maxBound. See #1089.

On top of that, we’ve added a number of new features:

  • makeTopEntity: Template Haskell function for generating TopEntity annotations. See the documentation on Haddock for more information.

  • Clash.Explicit.SimIO: ((System)Verilog only) I/O actions that can be translated to HDL I/O. See the documentation on Haddock for more information.

  • Clash.Class.AutoReg: A smart register that improves the chances of synthesis tools inferring clock-gated registers, when used. See the documentation on Haddock for more information.

The full list of changes follows. Happy hacking!

  • New features (API):

    • Clash.Class.Parity type class replaces Prelude odd and even functions due to assumptions that don’t hold for Clash specific numerical types, see #970.

    • NFDataX.ensureSpine, see #748

    • makeTopEntity Template Haskell function for generating TopEntity annotations intended to cover the majority of use cases. Generation failures should either result in an explicit error, or a valid annotation of an empty PortProduct. Any discrepancy between the shape of generated annotations and the shape of the Clash compiler is a bug. See #795. Known limitations:

      • Type application (excluding Signals and :::) is best effort:

      • Data types with type parameters will work if the generator can discover a single relevant constructor after attempting type application.

      • Arbitrary explicit clock/reset/enables are supported, but only a single HiddenClockResetEnable constraint is supported.

      • Data/type family support is best effort.

    • Added Bundle ((f :*: g) a) instance

    • Added NFDataX CUShort instance

    • Clash’s internal type family solver now recognizes AppendSymbol and CmpSymbol

    • Added Clash.Magic.suffixNameFromNat: can be used in cases where suffixName is too slow

    • Added Clash.Class.AutoReg. Improves the chances of synthesis tools inferring clock-gated registers, when used. See #873.

    • Clash.Magic.suffixNameP, Clash.Magic.suffixNameFromNatP: enable prefixing of name suffixes

    • Added Clash.Magic.noDeDup: can be used to instruct Clash to /not/ share a function between multiple branches

    • A BitPack a constraint now implies a KnownNat (BitSize a) constraint, so you won’t have to add it manually anymore. See #942.

    • Clash.Explicit.SimIO: ((System)Verilog only) I/O actions that can be translated to HDL I/O; useful for generated test benches.

    • Export Clash.Explicit.Testbench.assertBitVector #888

    • Add Clash.Prelude.Testbench.assertBitVector to achieve feature parity with Clash.Explicit.Testbench. #891

    • Add Clash.XException.NFDataX.ensureSpine #803

    • Add Clash.Class.BitPack.bitCoerceMap #798

    • Add Clash.Magic.deDup: instruct Clash to force sharing an operator between multiple branches of a case-expression

    • InlinePrimitive can now support multiple backends simultaneously #425

    • Add Clash.XException.hwSeqX: render declarations of an argument, but don’t assign it to a result signal

    • Add Clash.Signal.Bundle.TaggedEmptyTuple: allows users to emulate the pre-1.0 behavior of “Bundle ()”. See #1100

  • New features (Compiler):

    • #961: Show -fclash-* Options in clash --show-options

  • New internal features:

    • #918: Add X-Optimization to normalization passes (-fclash-aggressive-x-optimization)

    • #821: Add DebugTry: print name of all tried transformations, even if they didn’t succeed

    • #856: Add -fclash-debug-transformations: only print debug info for specific transformations

    • #911: Add ‘RenderVoid’ option to blackboxes

    • #958: Prefix names of inlined functions

    • #947: Add “Clash.Core.TermLiteral”

    • #887: Show nicer error messages when failing in TH code

    • #884: Teach reduceTypeFamily about AppendSymbol and CmpSymbol

    • #784: Print whether Id is global or local in ppr output

    • #781: Use naming contexts in register names

    • #1061: Add ‘usedArguments’ to BlackBoxHaskell blackboxes

  • Fixes issues:

    • #974: Fix indirect shadowing in reduceNonRepPrim

    • #964: SaturatingNum instance of Index now behaves correctly when the size of the index overflows an Int.

    • #810: Verilog backend now correctly specifies type of BitVector 1

    • #811: Improve module load behavior in clashi

    • #439: Template Haskell splices and TopEntity annotations can now be used in clashi

    • #662: Clash will now constant specialize partially constant constructs

    • #700: Check work content of expression in cast before warning users. Should eliminate a lot of (superfluous) warnings about “specializing on non work-free cast”s.

    • #837: Blackboxes will now report clearer error messages if they’re given unexpected arguments.

    • #869: PLL is no longer duplicated in Blinker.hs example

    • #749: Clash’s dependencies now all work with GHC 8.8, allowing clash-{prelude,lib,ghc} to be compiled from Hackage soon.

    • #871: RTree Bundle instance is now properly lazy

    • #895: VHDL type error when generating Maybe (Vec 2 (Signed 8), Index 1)

    • #880: Custom bit representations can now be used on product types too

    • #976: Prevent shadowing in Clash’s core evaluator

    • #1007: Can’t translate domain tagType.Errors.IfStuck…

    • #967: Naming registers disconnects their output

    • #990: Internal shadowing bug results in incorrect HDL

    • #945: Rewrite rules for Vec Applicative Functor

    • #919: Clash generating invalid Verilog after Vec operations #919

    • #996: Ambiguous clock when using ClearOnReset and resetGen together

    • #701: Unexpected behaviour with the Synthesize annotation

    • #694: Custom bit representation error only with VHDL

    • #347: topEntity synthesis fails due to insufficient type-level normalisation

    • #626: Missing Clash.Explicit.Prelude definitions

    • #960: Blackbox Error Caused by Simple map

    • #1012: Case-let doesn’t look through ticks

    • #430: Issue warning when not compiled with executable-dynamic: True

    • #374: Clash.Sized.Fixed: fromInteger and fromRational don’t saturate correctly

    • #836: Generate warning when toInteger blackbox drops MSBs

    • #1019: Clash breaks on constants defined in terms of GHC.Natural.gcdNatural

    • #1025: inlineCleanupwill not produce empty letrecs anymore

    • #1030: bindConstantVar will bind (workfree) constructs

    • #1034: Error (10137): object “pllLock” on lhs must have a variable data type

    • #1046: Don’t confuse term/type namespaces in ‘lookupIdSubst’

    • #1041: Nested product types incorrectly decomposed into ports

    • #1058: Prevent substitution warning when using type equalities in top entities

    • #1033: Fix issue where Clash breaks when using Clock/Reset/Enable in product types in combination with Synthesize annotations

    • #1075: Removed superfluous constraints on ‘maybeX’ and ‘maybeIsX’

    • #1085: Suggest exporting topentities if they can’t be found in a module

    • #1065: Report polymorphic topEntities as errors

    • #1089: Respect maxBound in Enum instances for BitVector,Index,Signed,Unsigned

  • Fixes without issue reports:

    • Fix bug in rnfX defined for Down (baef30e)

    • Render numbers inside gensym (bc76f0f)

    • Report blackbox name when encountering an error in ‘setSym’ (#858)

    • Fix blackbox issues causing Clash to generate invalid HDL (#865)

    • Treat types with a zero-width custom bit representation like other zero-width constructs (#874)

    • TH code for auto deriving bit representations now produces nicer error messages (7190793)

    • Adds ‘–enable-shared-executables’ for nix builds; this should make Clash run much faster (#894)

    • Custom bit representations can now mark fields as zero-width without crashing the compiler (#898)

    • Throw an error if there’s data left to parse after successfully parsing a valid JSON construct (#904)

    • Data.gfoldl is now manually implemented, in turn fixing issues with gshow (#933)

    • Fix a number of issues with blackbox implementations (#934)

    • Don’t inline registers with non-constant clock and reset (#998)

    • Inline let-binders called [dsN | N <- [1..]] (#992)

    • ClockGens use their name at the Haskell level #827

    • Render numbers inside gensym #809

    • Don’t overwrite existing binders when specializing #790

    • Deshadow in ‘caseCase’ #1067

    • Deshadow in ‘caseLet’ and ‘nonRepANF’ #1071

  • Deprecations & removals:

    • Removed support for GHC 8.2 (#842)

    • Removed support for older cabal versions, only Cabal >=2.2 supported (#851)

    • Reset and Enable constructors are now only exported from Clash.Signal.Internal

    • #986 Remove -fclash-allow-zero-width flag

1.0.0 September 3rd 2019

  • 10x - 50x faster compile times

  • New features:

    • API changes: check the migration guide at the end of Clash.Tutorial

    • All memory elements now have an (implicit) enable line; “Gated” clocks have been removed as the clock wasn’t actually gated, but implemented as an enable line.

    • Circuit domains are now configurable in:

      • (old) The clock period

      • (new) Clock edge on which memory elements latch their inputs (rising edge or falling edge)

      • (new) Whether the reset port of a memory element is level sensitive asynchronous reset) or edge sensitive (synchronous reset)

      • (new) Whether the reset port of a memory element is active-high or active-low (negated reset)

      • (new) Whether memory element power on in a configurable/defined state (common on FPGAs) or in an undefined state (ASICs)

      • See the blog post on this new feature

    • Data types can now be given custom bit-representations: http://hackage.haskell.org/package/clash-prelude/docs/Clash-Annotations-BitRepresentation.html

    • Annotate expressions with attributes that persist in the generated HDL, e.g. synthesis directives: http://hackage.haskell.org/package/clash-prelude/docs/Clash-Annotations-SynthesisAttributes.html

    • Control (System)Verilog module instance, and VHDL entity instantiation names in generated code: http://hackage.haskell.org/package/clash-prelude/docs/Clash-Magic.html

    • Much improved infrastructure for handling of unknown values: defined spine, but unknown leafs: http://hackage.haskell.org/package/clash-prelude/docs/Clash-XException.html#t:NFDataX

    • Experimental: Multiple hidden clocks. Can be enabled by compiling clash-prelude with -fmultiple-hidden

    • Experimental: Limited GADT support (pattern matching on vectors, or custom GADTs as longs as their usage can be statically removed; no support of recursive GADTs)

    • Experimental: Use regular Haskell functions to generate HDL black boxes for primitives (in an addition to existing string templates for HDL black boxes) See for example: http://hackage.haskell.org/package/clash-lib/docs/Clash-Primitives-Intel-ClockGen.html

  • Fixes issues:

0.99.3 July 28th 2018

  • Fixes bugs:

    • Evaluator recognizes Bit literals #329

    • Use existential type-variables in context of GADT pattern match

    • Do not create zero-bit temporary variables in generated HDL

    • Use correct arguments in nested primitives #323

    • Zero-constructor data type needs 0 bits #238

    • Create empty component when result needs 0 bits

    • Evaluator performs BigNat arithmetic

  • Features:

    • Bundle and BitPack instances up to and including 62-tuples

    • Handle undefined writes to RAM properly

    • Handle undefined clock enables properly

0.99.1 May 12th 2018

  • Allow ~NAME[N] tag inside ~GENSYM[X]

  • Support HDL record selector generation #313

  • InlinePrimitive support: specify HDL primitives inline with Haskell code

  • Support for ghc-typelits-natnormalise-0.6.1

  • Lift instances for TopEntity and PortName

  • InlinePrimitive support: specify HDL primitives inline with Haskell code

0.99 March 31st 2018

  • New features:

    • Major API overhaul: check the migration guide at the end of Clash.Tutorial

    • New features:

      • Explicit clock and reset arguments

      • Rename CLaSH to Clash

      • Implicit/Hidden clock and reset arguments using a combination of reflection and ImplicitParams.

      • Large overhaul of TopEntity annotations

      • PLL and other clock sources can now be instantiated using regular functions: Clash.Intel.ClockGen and Clash.Xilinx.ClockGen.

      • DDR registers:

        • Generic/ASIC: Clash.Explicit.DDR

        • Intel: Clash.Intel.DDR

        • Xilinx: Clash.Intel.Xilinx

    • Bit is now a newtype instead of a type synonym and will be mapped to a HDL scalar instead of an array of one (e.g std_logic instead of std_logic_vector(0 downto 0))

    • Hierarchies with multiple synthesisable boundaries by allowing more than one function in scope to have a Synthesize annotation.

      • Local caching of functions with a Synthesize annotation

    • Bit type is mapped to a HDL scalar type (e.g. std_logic in VHDL)

    • Improved name preservation

    • Zero-bit values are filtered out of the generated HDL

    • Improved compile-time computation

  • Many bug fixes

Older versions

Check out:

  • https://github.com/clash-lang/clash-compiler/blob/3649a2962415ea8ca2d6f7f5e673b4c14de26b4f/clash-prelude/CHANGELOG.md

  • https://github.com/clash-lang/clash-compiler/blob/3649a2962415ea8ca2d6f7f5e673b4c14de26b4f/clash-lib/CHANGELOG.md

  • https://github.com/clash-lang/clash-compiler/blob/3649a2962415ea8ca2d6f7f5e673b4c14de26b4f/clash-ghc/CHANGELOG.md

References