195bf0e29a
* Fully support multiple returns in Wasmtime For quite some time now Wasmtime has "supported" multiple return values, but only in the mose bare bones ways. Up until recently you couldn't get a typed version of functions with multiple return values, and never have you been able to use `Func::wrap` with functions that return multiple values. Even recently where `Func::typed` can call functions that return multiple values it uses a double-indirection by calling a trampoline which calls the real function. The underlying reason for this lack of support is that cranelift's ABI for returning multiple values is not possible to write in Rust. For example if a wasm function returns two `i32` values there is no Rust (or C!) function you can write to correspond to that. This commit, however fixes that. This commit adds two new ABIs to Cranelift: `WasmtimeSystemV` and `WasmtimeFastcall`. The intention is that these Wasmtime-specific ABIs match their corresponding ABI (e.g. `SystemV` or `WindowsFastcall`) for everything *except* how multiple values are returned. For multiple return values we simply define our own version of the ABI which Wasmtime implements, which is that for N return values the first is returned as if the function only returned that and the latter N-1 return values are returned via an out-ptr that's the last parameter to the function. These custom ABIs provides the ability for Wasmtime to bind these in Rust meaning that `Func::wrap` can now wrap functions that return multiple values and `Func::typed` no longer uses trampolines when calling functions that return multiple values. Although there's lots of internal changes there's no actual changes in the API surface area of Wasmtime, just a few more impls of more public traits which means that more types are supported in more places! Another change made with this PR is a consolidation of how the ABI of each function in a wasm module is selected. The native `SystemV` ABI, for example, is more efficient at returning multiple values than the wasmtime version of the ABI (since more things are in more registers). To continue to take advantage of this Wasmtime will now classify some functions in a wasm module with the "fast" ABI. Only functions that are not reachable externally from the module are classified with the fast ABI (e.g. those not exported, used in tables, or used with `ref.func`). This should enable purely internal functions of modules to have a faster calling convention than those which might be exposed to Wasmtime itself. Closes #1178 * Tweak some names and add docs * "fix" lightbeam compile * Fix TODO with dummy environ * Unwind info is a property of the target, not the ABI * Remove lightbeam unused imports * Attempt to fix arm64 * Document new ABIs aren't stable * Fix filetests to use the right target * Don't always do 64-bit stores with cranelift This was overwriting upper bits when 32-bit registers were being stored into return values, so fix the code inline to do a sized store instead of one-size-fits-all store. * At least get tests passing on the old backend * Fix a typo * Add some filetests with mixed abi calls * Get `multi` example working * Fix doctests on old x86 backend * Add a mixture of wasmtime/system_v tests
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: