a2b9664bed
Currently, the `build.rs` script that generates Rust source from the ISLE DSL will only do this generation if the `rebuild-isle` Cargo feature is specified. By default, it is not. This is based on the principle that we (the build script) do not modify the source tree as managed by git; git-managed files are strictly a human-managed and human-edited resource. By adding the opt-in Cargo feature, a developer is requesting the build script to perform an explicit action. (In my understanding at least, this principle comes from the general philosophy of hermetic builds: the output should be a pure function of the input, and part of this is that the input is read-only. If we modify the source tree, then all bets are off.) Unfortunately, requiring the opt-in feature also creates a footgun that is easy to hit: if a developer modifies the ISLE DSL source, but forgets to specify the Cargo feature, then the compiler will silently be built successfully with stale source, and will silently exclude any changes that were made. The generated source is checked into git for a good reason: we want DSL compiler to not affect build times for the overwhelmingly common case that Cranelift is used as a dependency but the backends are not being actively developed. (This overhead comes mainly from building `islec` itself.) So, what to do? This PR implements a middle ground first described in [this conversation](https://github.com/bytecodealliance/wasmtime/pull/3506#discussion_r743113351), in which we: - Generate a hash (SHA-512) of the ISLE DSL source and produce a "manifest" of ISLE inputs alongside the generated source; and - Always read the ISLE DSL source, and see if the manifest is still valid, on builds that do not have the opt-in "rebuild" feature. This allows us to know whether the ISLE compiler output would have been the same (modulo changes to the DSL compiler itself, which are out-of-scope here), without actually building the ISLE compiler and running it. If the compiler-backend developer modifies an ISLE source file and then tries to build `cranelift-codegen` without adding the `rebuild-isle` Cargo feature, they get the following output: ```text Error: the ISLE source files that resulted in the generated Rust source * src/isa/x64/lower/isle/generated_code.rs have changed but the generated source was not rebuilt! These ISLE source files are: * src/clif.isle * src/prelude.isle * src/isa/x64/inst.isle * src/isa/x64/lower.isle Please add `--features rebuild-isle` to your `cargo build` command if you wish to rebuild the generated source, then include these changes in any git commits you make that include the changes to the ISLE. For example: $ cargo build -p cranelift-codegen --features rebuild-isle (This build script cannot do this for you by default because we cannot modify checked-into-git source without your explicit opt-in.) ``` which will tell them exactly what they need to do to fix the problem! Note that I am leaving the "Rebuild ISLE" CI job alone for now, because otherwise, we are trusting whomever submits a PR to generate the correct generated source. In other words, the manifest is a communication from the checked-in tree to the developer, but we still need to verify that the checked-in generated Rust source and the manifest are correct with respect to the checked-in ISLE source.
Cranelift Code Generator
A Bytecode Alliance project
Cranelift is a low-level retargetable code generator. It translates a target-independent intermediate representation into executable machine code.
For more information, see the documentation.
For an example of how to use the JIT, see the JIT Demo, which implements a toy language.
For an example of how to use Cranelift to run WebAssembly code, see Wasmtime, which implements a standalone, embeddable, VM using Cranelift.
Status
Cranelift currently supports enough functionality to run a wide variety of programs, including all the functionality needed to execute WebAssembly MVP functions, although it needs to be used within an external WebAssembly embedding to be part of a complete WebAssembly implementation.
The x86-64 backend is currently the most complete and stable; other architectures are in various stages of development. Cranelift currently supports both the System V AMD64 ABI calling convention used on many platforms and the Windows x64 calling convention. The performance of code produced by Cranelift is not yet impressive, though we have plans to fix that.
The core codegen crates have minimal dependencies, support no_std mode (see below), and do not require any host floating-point support, and do not use callstack recursion.
Cranelift does not yet perform mitigations for Spectre or related security issues, though it may do so in the future. It does not currently make any security-relevant instruction timing guarantees. It has seen a fair amount of testing and fuzzing, although more work is needed before it would be ready for a production use case.
Cranelift's APIs are not yet stable.
Cranelift currently requires Rust 1.37 or later to build.
Contributing
If you're interested in contributing to Cranelift: thank you! We have a contributing guide which will help you getting involved in the Cranelift project.
Planned uses
Cranelift is designed to be a code generator for WebAssembly, but it is general enough to be useful elsewhere too. The initial planned uses that affected its design are:
- WebAssembly compiler for the SpiderMonkey engine in Firefox.
- Backend for the IonMonkey JavaScript JIT compiler in Firefox.
- Debug build backend for the Rust compiler.
- Wasmtime non-Web wasm engine.
Building Cranelift
Cranelift uses a conventional Cargo build process.
Cranelift consists of a collection of crates, and uses a Cargo
Workspace,
so for some cargo commands, such as cargo test, the --all is needed
to tell cargo to visit all of the crates.
test-all.sh at the top level is a script which runs all the cargo
tests and also performs code format, lint, and documentation checks.
Building with no_std
The following crates support `no_std`, although they do depend on liballoc:
- cranelift-entity
- cranelift-bforest
- cranelift-codegen
- cranelift-frontend
- cranelift-native
- cranelift-wasm
- cranelift-module
- cranelift-preopt
- cranelift
To use no_std mode, disable the std feature and enable the core feature. This currently requires nightly rust.
For example, to build `cranelift-codegen`:
cd cranelift-codegen
cargo build --no-default-features --features core
Or, when using cranelift-codegen as a dependency (in Cargo.toml):
[dependency.cranelift-codegen]
...
default-features = false
features = ["core"]
no_std support is currently "best effort". We won't try to break it, and we'll accept patches fixing problems, however we don't expect all developers to build and test no_std when submitting patches. Accordingly, the ./test-all.sh script does not test no_std.
There is a separate ./test-no_std.sh script that tests the no_std support in packages which support it.
It's important to note that cranelift still needs liballoc to compile. Thus, whatever environment is used must implement an allocator.
Also, to allow the use of HashMaps with no_std, an external crate called hashmap_core is pulled in (via the core feature). This is mostly the same as std::collections::HashMap, except that it doesn't have DOS protection. Just something to think about.
Log configuration
Cranelift uses the log crate to log messages at various levels. It doesn't
specify any maximal logging level, so embedders can choose what it should be;
however, this can have an impact of Cranelift's code size. You can use log
features to reduce the maximum logging level. For instance if you want to limit
the level of logging to warn messages and above in release mode:
[dependency.log]
...
features = ["release_max_level_warn"]
Editor Support
Editor support for working with Cranelift IR (clif) files: