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rustc: Add a new wasm ABI

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This commit implements the idea of a new ABI for the WebAssembly target,
one called `"wasm"`. This ABI is entirely of my own invention
and has no current precedent, but I think that the addition of this ABI
might help solve a number of issues with the WebAssembly targets.

When `wasm32-unknown-unknown` was first added to Rust I naively
"implemented an abi" for the target. I then went to write `wasm-bindgen`
which accidentally relied on details of this ABI. Turns out the ABI
definition didn't match C, which is causing issues for C/Rust interop.
Currently the compiler has a "wasm32 bindgen compat" ABI which is the
original implementation I added, and it's purely there for, well,
`wasm-bindgen`.

Another issue with the WebAssembly target is that it's not clear to me
when and if the default C ABI will change to account for WebAssembly's
multi-value feature (a feature that allows functions to return multiple
values). Even if this does happen, though, it seems like the C ABI will
be guided based on the performance of WebAssembly code and will likely
not match even what the current wasm-bindgen-compat ABI is today. This
leaves a hole in Rust's expressivity in binding WebAssembly where given
a particular import type, Rust may not be able to import that signature
with an updated C ABI for multi-value.

To fix these issues I had the idea of a new ABI for WebAssembly, one
called `wasm`. The definition of this ABI is "what you write
maps straight to wasm". The goal here is that whatever you write down in
the parameter list or in the return values goes straight into the
function's signature in the WebAssembly file. This special ABI is for
intentionally matching the ABI of an imported function from the
environment or exporting a function with the right signature.

With the addition of a new ABI, this enables rustc to:

* Eventually remove the "wasm-bindgen compat hack". Once this
  ABI is stable wasm-bindgen can switch to using it everywhere.
  Afterwards the wasm32-unknown-unknown target can have its default ABI
  updated to match C.

* Expose the ability to precisely match an ABI signature for a
  WebAssembly function, regardless of what the C ABI that clang chooses
  turns out to be.

* Continue to evolve the definition of the default C ABI to match what
  clang does on all targets, since the purpose of that ABI will be
  explicitly matching C rather than generating particular function
  imports/exports.

Naturally this is implemented as an unstable feature initially, but it
would be nice for this to get stabilized (if it works) in the near-ish
future to remove the wasm32-unknown-unknown incompatibility with the C
ABI. Doing this, however, requires the feature to be on stable because
wasm-bindgen works with stable Rust.
This commit is contained in:
Alex Crichton 2021-04-01 16:08:29 -07:00
parent 69e1d22ddb
commit 482a3d06c3
23 changed files with 405 additions and 198 deletions
Download patch file Download diff file Expand all files Collapse all files

44
compiler/rustc_codegen_llvm/src/attributes.rs
View file

@ -13,6 +13,7 @@ use rustc_middle::ty::query::Providers;
use rustc_middle::ty::{self, TyCtxt};
use rustc_session::config::OptLevel;
use rustc_session::Session;
use rustc_target::spec::abi::Abi;
use rustc_target::spec::{SanitizerSet, StackProbeType};
use crate::attributes;
@ -293,7 +294,7 @@ pub fn from_fn_attrs(cx: &CodegenCx<'ll, 'tcx>, llfn: &'ll Value, instance: ty::
// The target doesn't care; the subtarget reads our attribute.
apply_tune_cpu_attr(cx, llfn);
let function_features = codegen_fn_attrs
let mut function_features = codegen_fn_attrs
.target_features
.iter()
.map(|f| {
@ -305,23 +306,10 @@ pub fn from_fn_attrs(cx: &CodegenCx<'ll, 'tcx>, llfn: &'ll Value, instance: ty::
InstructionSetAttr::ArmT32 => "+thumb-mode".to_string(),
}))
.collect::<Vec<String>>();
if !function_features.is_empty() {
let mut global_features = llvm_util::llvm_global_features(cx.tcx.sess);
global_features.extend(function_features.into_iter());
let features = global_features.join(",");
let val = CString::new(features).unwrap();
llvm::AddFunctionAttrStringValue(
llfn,
llvm::AttributePlace::Function,
cstr!("target-features"),
&val,
);
}
// Note that currently the `wasm-import-module` doesn't do anything, but
// eventually LLVM 7 should read this and ferry the appropriate import
// module to the output file.
if cx.tcx.sess.target.is_like_wasm {
// If this function is an import from the environment but the wasm
// import has a specific module/name, apply them here.
if let Some(module) = wasm_import_module(cx.tcx, instance.def_id()) {
llvm::AddFunctionAttrStringValue(
llfn,
@ -340,6 +328,30 @@ pub fn from_fn_attrs(cx: &CodegenCx<'ll, 'tcx>, llfn: &'ll Value, instance: ty::
&name,
);
}
// The `"wasm"` abi on wasm targets automatically enables the
// `+multivalue` feature because the purpose of the wasm abi is to match
// the WebAssembly specification, which has this feature. This won't be
// needed when LLVM enables this `multivalue` feature by default.
if !cx.tcx.is_closure(instance.def_id()) {
let abi = cx.tcx.fn_sig(instance.def_id()).abi();
if abi == Abi::Wasm {
function_features.push("+multivalue".to_string());
}
}
}
if !function_features.is_empty() {
let mut global_features = llvm_util::llvm_global_features(cx.tcx.sess);
global_features.extend(function_features.into_iter());
let features = global_features.join(",");
let val = CString::new(features).unwrap();
llvm::AddFunctionAttrStringValue(
llfn,
llvm::AttributePlace::Function,
cstr!("target-features"),
&val,
);
}
}