8726eeefb3
* cranelift-isle: Add "partial" flag for constructors Instead of tying fallibility of constructors to whether they're either internal or pure, this commit assumes all constructors are infallible unless tagged otherwise with a "partial" flag. Internal constructors without the "partial" flag are not allowed to use constructors which have the "partial" flag on the right-hand side of any rules, because they have no way to report last-minute match failures. Multi-constructors should never be "partial"; they report match failures with an empty iterator instead. In turn this means you can't use partial constructors on the right-hand side of internal multi-constructor rules. However, you can use the same constructors on the left-hand side with `if` or `if-let` instead. In many cases, ISLE can already trivially prove that an internal constructor always returns `Some`. With this commit, those cases are largely unchanged, except for removing all the `Option`s and `Some`s from the generated code for those terms. However, for internal non-partial constructors where ISLE could not prove that, it now emits an `unreachable!` panic as the last-resort, instead of returning `None` like it used to do. Among the existing backends, here's how many constructors have these panic cases: - x64: 14% (53/374) - aarch64: 15% (41/277) - riscv64: 23% (26/114) - s390x: 47% (268/567) It's often possible to rewrite rules so that ISLE can tell the panic can never be hit. Just ensure that there's a lowest-priority rule which has no constraints on the left-hand side. But in many of these constructors, it's difficult to statically prove the unhandled cases are unreachable because that's only down to knowledge about how they're called or other preconditions. So this commit does not try to enforce that all terms have a last-resort fallback rule. * Check term flags while translating expressions Instead of doing it in a separate pass afterward. This involved threading all the term flags (pure, multi, partial) through the recursive `translate_expr` calls, so I extracted the flags to a new struct so they can all be passed together. * Validate multi-term usage Now that I've threaded the flags through `translate_expr`, it's easy to check this case too, so let's just do it. * Extract `ReturnKind` to use in `ExternalSig` There are only three legal states for the combination of `multi` and `infallible`, so replace those fields of `ExternalSig` with a three-state enum. * Remove `Option` wrapper from multi-extractors too If we'd had any external multi-constructors this would correct their signatures as well. * Update ISLE tests * Tag prelude constructors as pure where appropriate I believe the only reason these weren't marked `pure` before was because that would have implied that they're also partial. Now that those two states are specified separately we apply this flag more places. * Fix my changes to aarch64 `lower_bmask` and `imm` terms
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 and various extensions like SIMD), although it needs to be used within an external WebAssembly embedding such as Wasmtime to be part of a complete WebAssembly implementation. It is also usable as a backend for non-WebAssembly use cases: for example, there is an effort to build a Rust compiler backend using Cranelift.
Cranelift is production-ready, and is used in production in several places, all within the context of Wasmtime. It is carefully fuzzed as part of Wasmtime with differential comparison against V8 and the executable Wasm spec, and the register allocator is separately fuzzed with symbolic verification. There is an active effort to formally verify Cranelift's instruction-selection backends. We take security seriously and have a security policy as a part of Bytecode Alliance.
Cranelift has three backends: x86-64, aarch64 (aka ARM64), and s390x (aka IBM Z). All three backends fully support enough functionality for Wasm MVP, and x86-64 and aarch64 fully support SIMD as well. On x86-64, Cranelift supports both the System V AMD64 ABI calling convention used on many platforms and the Windows x64 calling convention. On aarch64, Cranelift supports the standard Linux calling convention and also has specific support for macOS (i.e., M1 / Apple Silicon).
Cranelift's code quality is within range of competitiveness to browser JIT engines' optimizing tiers. A recent paper includes third-party benchmarks of Cranelift, driven by Wasmtime, against V8 and an LLVM-based Wasm engine, WAVM (Fig 22). The speed of Cranelift's generated code is ~2% slower than that of V8 (TurboFan), and ~14% slower than WAVM (LLVM). Its compilation speed, in the same paper, is measured as approximately an order of magnitude faster than WAVM (LLVM). We continue to work to improve both measures.
The core codegen crates have minimal dependencies and are carefully written to handle malicious or arbitrary compiler input: in particular, they do not use callstack recursion.
Cranelift performs some basic mitigations for Spectre attacks on heap bounds checks, table bounds checks, and indirect branch bounds checks; see #1032 for more.
Cranelift's APIs are not yet considered stable, though we do follow semantic-versioning (semver) with minor-version patch releases.
Cranelift generally requires the latest stable Rust to build as a policy, and is tested as such, but we can incorporate fixes for compilation with older Rust versions on a best-effort basis.
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 were:
- Wasmtime non-Web wasm engine.
- Debug build backend for the Rust compiler.
- WebAssembly compiler for the SpiderMonkey engine in Firefox (currently not planned anymore; SpiderMonkey team may re-assess in the future).
- Backend for the IonMonkey JavaScript JIT compiler in Firefox (currently not planned anymore; SpiderMonkey team may re-assess in the future).
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.
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: