72e6be9342
This patch includes:
- A complete rework of the way that CLIF blocks and edge blocks are
lowered into VCode blocks. The new mechanism in `BlockLoweringOrder`
computes RPO over the CFG, but with a twist: it merges edge blocks intto
heads or tails of original CLIF blocks wherever possible, and it does
this without ever actually materializing the full nodes-plus-edges
graph first. The backend driver lowers blocks in final order so
there's no need to reshuffle later.
- A new `MachBuffer` that replaces the `MachSection`. This is a special
version of a code-sink that is far more than a humble `Vec<u8>`. In
particular, it keeps a record of label definitions and label uses,
with a machine-pluggable `LabelUse` trait that defines various types
of fixups (basically internal relocations).
Importantly, it implements some simple peephole-style branch rewrites
*inline in the emission pass*, without any separate traversals over
the code to use fallthroughs, swap taken/not-taken arms, etc. It
tracks branches at the tail of the buffer and can (i) remove blocks
that are just unconditional branches (by redirecting the label), (ii)
understand a conditional/unconditional pair and swap the conditional
polarity when it's helpful; and (iii) remove branches that branch to
the fallthrough PC.
The `MachBuffer` also implements branch-island support. On
architectures like AArch64, this is needed to allow conditional
branches within plausibly-attainable ranges (+/- 1MB on AArch64
specifically). It also does this inline while streaming through the
emission, without any sort of fixpoint algorithm or later moving of
code, by simply tracking outstanding references and "deadlines" and
emitting an island just-in-time when we're in danger of going out of
range.
- A rework of the instruction selector driver. This is largely following
the same algorithm as before, but is cleaned up significantly, in
particular in the API: the machine backend can ask for an input arg
and get any of three forms (constant, register, producing
instruction), indicating it needs the register or can merge the
constant or producing instruction as appropriate. This new driver
takes special care to emit constants right at use-sites (and at phi
inputs), minimizing their live-ranges, and also special-cases the
"pinned register" to avoid superfluous moves.
Overall, on `bz2.wasm`, the results are:
wasmtime full run (compile + runtime) of bz2:
baseline: 9774M insns, 9742M cycles, 3.918s
w/ changes: 7012M insns, 6888M cycles, 2.958s (24.5% faster, 28.3% fewer insns)
clif-util wasm compile bz2:
baseline: 2633M insns, 3278M cycles, 1.034s
w/ changes: 2366M insns, 2920M cycles, 0.923s (10.7% faster, 10.1% fewer insns)
All numbers are averages of two runs on an Ampere eMAG.
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 SimpleJIT 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: