Improving GHC's configuration logic and cross-compilation support with ghc-toolchain

Rodrigo worked on an internship with the GHC team at Well-Typed this summer. In this post he reports on his progress improving GHC’s configuration tooling as a step towards better cross-compiler support.

GHC, like most high-level language compilers, depends upon a set of tools like assemblers, linkers, and archivers for the production of machine code. Collectively these tools are known as a toolchain and capture a great deal of platform-dependent knowledge.

Traditionally, developers generate a ./configure script using the venerable autoconf tool, then users execute this script when they install a GHC binary distribution. The ./configure script determines the location of programs (such as the C compiler) and which options GHC will need to pass them.

While this autoconf-centric model of toolchain configuration has served GHC well, it has two key issues:

• For cross-compiling to a different platform, it would be highly valuable to users if GHC would become a runtime-retargetable compiler (like rustc and go). That is, the user should be able to download a single GHC binary distribution and use it to compile not only for their local machine, but also any other targets that GHC supports.

• The ten-thousand-line sh file that is GHC’s ./configure script has historically been challenging to maintain and test. Modifications to the ./configure script are among the most risky changes routinely made to the compiler, because it is easy to introduce a bug on some specific toolchain configuration, and infeasible to test all possible configurations in CI.

To address these issues, we are introducing ghc-toolchain, a new way to configure the toolchain for GHC, which will eventually replace the existing toolchain configuration logic in the ./configure script. Its main goal is to allow new compilation toolchains to be configured for GHC at any point in time, notably after the compiler has been installed. For example, calling ghc-toolchain --triple=x86_64-w64-mingw32 will configure a compilation toolchain on the host machine capable of producing code for an x86_64 machine running Windows using MinGW. This is an important step towards making GHC runtime-retargetable, and since ghc-toolchain is implemented in Haskell, it will be much easier to modify and test than the ./configure script.

In this post we explain in more detail how GHC interacts with the system toolchain and how ghc-toolchain facilitates our future goal of making GHC a runtime-retargetable compiler.

Compiler Toolchains

GHC cannot produce executables from Haskell programs in isolation – it requires a correctly configured toolchain to which it can delegate some responsibilities. For example, GHC’s native code generator backend is capable of generating assembly code from a Haskell program, however, producing object code from that assembly, and linking the objects into an executable, are all tasks done by the compilation toolchain, which is invoked by GHC using the flags that were configured for it.

Configuring a compiler toolchain is about locating the set of tools required for compilation, the base set of flags required to invoke each tool, and properties of these tools. For example, this might include:

• determining various characteristics of the platform (e.g. the word size).
• probing to find which tools are available (the C compiler, linker, archiver, object merging tool, etc.),
• identifying which flags GHC should pass to each of these tools,
• determining whether the tools support response files to work around command-line length limits, and
• checking for and working around bugs in the toolchain.

At the moment, when a GHC binary distribution is installed, the ./configure script will perform the above steps and store the results in a settings file. GHC will then read this file so it can correctly invoke the toolchain programs when compiling Haskell executables.

To cross-compile a Haskell program, a user must build GHC from source as a cross-compiler (see the GHC wiki). This requires configuring a cross-compilation toolchain, that is, a toolchain that runs on the machine compiling the Haskell program but that produces executables to run on a different system. It is currently a rather involved process.

The runtime-retargetable future of GHC

A key long-term goal of this work is to allow GHC to become runtime-retargetable. This means being able to call ghc --target=aarch64-apple-darwin and have GHC output code for an AArch64 machine, or call ghc --target=javascript-ghcjs to generate Javascript code, regardless of the platform ghc is being invoked on.

Crucially, this requires the configuration step to be repeated at a later point, rather than only when the GHC binary distribution is installed. Once GHC is fully runtime-retargetable, this will allow you to use multiple different toolchains, potentially targeting different platforms, with the same installed compiler.

• At the simplest level, you might just have two different toolchains for your host platform (for example, a gcc-based toolchain and a clang-based toolchain), or you might just configure a toolchain which uses the new mold linker rather than ld.gold.

• In a more complex scenario, you may have a normal compiler toolchain as well as several different cross-compiler toolchains. For example, a toolchain which produces Javascript, a toolchain which produces WebAssembly, a toolchain which produces AArch64 object code and so on.

The idea is that the brand new ghc-toolchain will be called once to configure the toolchain that GHC will use when compiling for a target, then ghc --target=<triple> can be called as many times as needed. For example, if you have an x86 Linux machine and wish to produce code for AArch64 devices, the workflow could look something like:

# Configure the aarch64-apple-darwin target first
# (We only need to do this once!)
ghc-toolchain --triple=aarch64-apple-darwin --cc-opt="-I/some/include/dir" --cc-linker-opt="-L/some/library/dir"

# Now we can target aarch64-apple-darwin (as many times as we'd like!)
ghc --target=aarch64-apple-darwin -o MyAwesomeTool MyAwesomeTool.hs
ghc --target=aarch64-apple-darwin -o CoolProgram CoolProgram.hs

Introducing ghc-toolchain

ghc-toolchain is a standalone tool for configuring a toolchain. It receives as input a target triplet (e.g. x86_64-deb10-linux) and user options, discovers the configuration, and outputs a “target description” (.target file) containing the configured toolchain.

At the moment, .target files generated by ghc-toolchain can be used by GHC’s build system (Hadrian) by invoking ./configure with --enable-ghc-toolchain. Otherwise, Hadrian reads the configuration from a .target file generated by ./configure itself.

In the future, ghc-toolchain will be shipped in binary distributions to allow new toolchains to be added after the compiler is installed (generating new .target files). GHC will then be able to choose the .target file for the particular target requested by the user.

From a developer standpoint, ghc-toolchain being written in Haskell makes it easier to modify in future, especially when compared to the notoriously difficult to write and debug ./configure scripts.

Migration to ghc-toolchain

We are migrating to ghc-toolchain in a staged manner, since toolchain configuration logic is amongst the most sensitive things to change in the compiler. We want to ensure that the configuration logic in ghc-toolchain is correct and agrees with the logic in ./configure. Therefore, in GHC 9.10 ghc-toolchain will be shipped and validated but not enabled by default.

To validate ghc-toolchain, GHC will generate .target files with both ./configure and ghc-toolchain and compare the outputs against each other, emitting a warning if they differ. This means we will be able to catch mistakes in ghc-toolchain (and in ./configure too!) before we make ghc-toolchain the default method for configuring toolchains in a subsequent release. This mechanism has already identified plenty of issues to resolve.

Future work

Despite ghc-toolchain bringing us closer to a runtime-retargetable GHC, there is still much work left to be done (see #11470). The next step is to instruct GHC to choose between multiple available .target files at runtime, instead of reading the usual settings file (tracked in #23682).

Beyond that, however, there are many open questions still to resolve:

• How will the runtime system, and core libraries such as base, be provided for the multiple selected targets?
• How will this fit into ghcup’s installation story?
• How will cabal handle multiple targets?

At the moment, binary distributions include the RTS/libraries already compiled for a single target. Instead, we are likely to need some mechanism for users to recompile the RTS/libraries when they configure a new target, or to download ready-built versions from upstream.

Moreover, accommodating TemplateHaskell under runtime retargetability is particularly nontrivial, and needs more design work.

Conclusion

ghc-toolchain is a new tool for configuring toolchains and targets. It improves on GHC’s existing ./configure-based configuration workflow by allowing multiple targets’ toolchains to be configured at any time, and by making maintenance and future updates to the toolchain configuration logic much easier. However, toolchain configuration is a challenging part of the compiler, so we’re being conservative in migrating to ghc-toolchain, and carefully validating it before making it the default.

Moreover, ghc-toolchain is an important step towards making a runtime-retargetable GHC a reality, though there is still much work left to do. We are grateful to all the GHC developers involved in working towards runtime-retargetability.

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