43f1765272
* Cranellift: remove Baldrdash support and related features.
As noted in Mozilla's bugzilla bug 1781425 [1], the SpiderMonkey team
has recently determined that their current form of integration with
Cranelift is too hard to maintain, and they have chosen to remove it
from their codebase. If and when they decide to build updated support
for Cranelift, they will adopt different approaches to several details
of the integration.
In the meantime, after discussion with the SpiderMonkey folks, they
agree that it makes sense to remove the bits of Cranelift that exist
to support the integration ("Baldrdash"), as they will not need
them. Many of these bits are difficult-to-maintain special cases that
are not actually tested in Cranelift proper: for example, the
Baldrdash integration required Cranelift to emit function bodies
without prologues/epilogues, and instead communicate very precise
information about the expected frame size and layout, then stitched
together something post-facto. This was brittle and caused a lot of
incidental complexity ("fallthrough returns", the resulting special
logic in block-ordering); this is just one example. As another
example, one particular Baldrdash ABI variant processed stack args in
reverse order, so our ABI code had to support both traversal
orders. We had a number of other Baldrdash-specific settings as well
that did various special things.
This PR removes Baldrdash ABI support, the `fallthrough_return`
instruction, and pulls some threads to remove now-unused bits as a
result of those two, with the understanding that the SpiderMonkey folks
will build new functionality as needed in the future and we can perhaps
find cleaner abstractions to make it all work.
[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1781425
* Review feedback.
* Fix (?) DWARF debug tests: add `--disable-cache` to wasmtime invocations.
The debugger tests invoke `wasmtime` from within each test case under
the control of a debugger (gdb or lldb). Some of these tests started to
inexplicably fail in CI with unrelated changes, and the failures were
only inconsistently reproducible locally. It seems to be cache related:
if we disable cached compilation on the nested `wasmtime` invocations,
the tests consistently pass.
* Review feedback.
145 lines
6.0 KiB
Markdown
145 lines
6.0 KiB
Markdown
Cranelift Code Generator
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========================
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**A [Bytecode Alliance][BA] project**
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Cranelift is a low-level retargetable code generator. It translates a
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[target-independent intermediate representation](docs/ir.md)
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into executable machine code.
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[BA]: https://bytecodealliance.org/
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[](https://github.com/bytecodealliance/wasmtime/actions)
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[](https://bytecodealliance.zulipchat.com/#narrow/stream/217117-cranelift/topic/general)
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[](https://docs.rs/cranelift)
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For more information, see [the documentation](docs/index.md).
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For an example of how to use the JIT, see the [JIT Demo], which
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implements a toy language.
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[JIT Demo]: https://github.com/bytecodealliance/cranelift-jit-demo
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For an example of how to use Cranelift to run WebAssembly code, see
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[Wasmtime], which implements a standalone, embeddable, VM using Cranelift.
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[Wasmtime]: https://github.com/bytecodealliance/wasmtime
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Status
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------
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Cranelift currently supports enough functionality to run a wide variety
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of programs, including all the functionality needed to execute
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WebAssembly (MVP and various extensions like SIMD), although it needs to be
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used within an external WebAssembly embedding such as Wasmtime to be part of a
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complete WebAssembly implementation. It is also usable as a backend for
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non-WebAssembly use cases: for example, there is an effort to build a [Rust
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compiler backend] using Cranelift.
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Cranelift is production-ready, and is used in production in several places, all
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within the context of Wasmtime. It is carefully fuzzed as part of Wasmtime with
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differential comparison against V8 and the executable Wasm spec, and the
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register allocator is separately fuzzed with symbolic verification. There is an
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active effort to formally verify Cranelift's instruction-selection backends. We
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take security seriously and have a [security policy] as a part of Bytecode
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Alliance.
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Cranelift has three backends: x86-64, aarch64 (aka ARM64), and s390x (aka IBM
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Z). All three backends fully support enough functionality for Wasm MVP, and
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x86-64 and aarch64 fully support SIMD as well. On x86-64, Cranelift supports
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both the System V AMD64 ABI calling convention used on many platforms and the
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Windows x64 calling convention. On aarch64, Cranelift supports the standard
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Linux calling convention and also has specific support for macOS (i.e., M1 /
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Apple Silicon).
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Cranelift's code quality is within range of competitiveness to browser JIT
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engines' optimizing tiers. A [recent paper] includes third-party benchmarks of
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Cranelift, driven by Wasmtime, against V8 and an LLVM-based Wasm engine, WAVM
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(Fig 22). The speed of Cranelift's generated code is ~2% slower than that of
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V8 (TurboFan), and ~14% slower than WAVM (LLVM). Its compilation speed, in the
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same paper, is measured as approximately an order of magnitude faster than WAVM
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(LLVM). We continue to work to improve both measures.
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[Rust compiler backend]: https://github.com/bjorn3/rustc_codegen_cranelift
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[security policy]: https://bytecodealliance.org/security
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[recent paper]: https://arxiv.org/abs/2011.13127
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The core codegen crates have minimal dependencies and are carefully written to
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handle malicious or arbitrary compiler input: in particular, they do not use
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callstack recursion.
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Cranelift performs some basic mitigations for Spectre attacks on heap bounds
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checks, table bounds checks, and indirect branch bounds checks; see
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[#1032] for more.
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[#1032]: https://github.com/bytecodealliance/wasmtime/issues/1032
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Cranelift's APIs are not yet considered stable, though we do follow
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semantic-versioning (semver) with minor-version patch releases.
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Cranelift generally requires the latest stable Rust to build as a policy, and
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is tested as such, but we can incorporate fixes for compilation with older Rust
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versions on a best-effort basis.
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Contributing
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------------
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If you're interested in contributing to Cranelift: thank you! We have a
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[contributing guide] which will help you getting involved in the Cranelift
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project.
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[contributing guide]: https://bytecodealliance.github.io/wasmtime/contributing.html
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Planned uses
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------------
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Cranelift is designed to be a code generator for WebAssembly, but it is
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general enough to be useful elsewhere too. The initial planned uses that
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affected its design were:
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- [Wasmtime non-Web wasm engine](https://github.com/bytecodealliance/wasmtime).
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- [Debug build backend for the Rust compiler](rustc.md).
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- WebAssembly compiler for the SpiderMonkey engine in Firefox
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(currently not planned anymore; SpiderMonkey team may re-assess in
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the future).
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- Backend for the IonMonkey JavaScript JIT compiler in Firefox
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(currently not planned anymore; SpiderMonkey team may re-assess in
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the future).
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Building Cranelift
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------------------
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Cranelift uses a [conventional Cargo build
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process](https://doc.rust-lang.org/cargo/guide/working-on-an-existing-project.html).
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Cranelift consists of a collection of crates, and uses a [Cargo
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Workspace](https://doc.rust-lang.org/book/ch14-03-cargo-workspaces.html),
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so for some cargo commands, such as `cargo test`, the `--all` is needed
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to tell cargo to visit all of the crates.
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`test-all.sh` at the top level is a script which runs all the cargo
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tests and also performs code format, lint, and documentation checks.
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<details>
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<summary>Log configuration</summary>
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Cranelift uses the `log` crate to log messages at various levels. It doesn't
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specify any maximal logging level, so embedders can choose what it should be;
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however, this can have an impact of Cranelift's code size. You can use `log`
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features to reduce the maximum logging level. For instance if you want to limit
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the level of logging to `warn` messages and above in release mode:
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```
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[dependency.log]
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...
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features = ["release_max_level_warn"]
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```
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</details>
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Editor Support
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--------------
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Editor support for working with Cranelift IR (clif) files:
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- Vim: https://github.com/bytecodealliance/cranelift.vim
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