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Rollup merge of rust-lang#128388 - beetrees:f16-f128-slightly-improve…
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…-windows-abi, r=tgross35

Match LLVM ABI in `extern "C"` functions for `f128` on Windows

As MSVC doesn't support `_Float128`, x86-64 Windows doesn't have a defined ABI for `f128`. Currently, Rust will pass and return `f128` indirectly for `extern "C"` functions. This is inconsistent with LLVM, which passes and returns `f128` in XMM registers, meaning that e.g. the ABI of `extern "C"` compiler builtins won't match. This PR fixes this discrepancy by making the x86-64 Windows `extern "C"` ABI pass `f128` directly through to LLVM, so that Rust will follow whatever LLVM does. This still leaves the difference between LLVM and GCC (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115054) but this PR is still an improvement as at least Rust is now consistent with it's primary codegen backend and compiler builtins from `compiler-builtins` will now work.

I've also fixed the x86-64 Windows `has_reliable_f16` match arm in `std` `build.rs` to refer to the correct target, and added an equivalent match arm to `has_reliable_f128` as the LLVM-GCC ABI difference affects both `f16` and `f128`.

Tracking issue: rust-lang#116909

try-job: x86_64-msvc
try-job: x86_64-mingw
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tgross35 authored Jul 31, 2024
2 parents b91b069 + fe6478c commit 84cc3ab
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Showing 3 changed files with 49 additions and 4 deletions.
10 changes: 7 additions & 3 deletions compiler/rustc_target/src/abi/call/x86_win64.rs
Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
use crate::abi::call::{ArgAbi, FnAbi, Reg};
use crate::abi::Abi;
use crate::abi::{Abi, Float, Primitive};

// Win64 ABI: https://docs.microsoft.com/en-us/cpp/build/parameter-passing

Expand All @@ -18,8 +18,12 @@ pub fn compute_abi_info<Ty>(fn_abi: &mut FnAbi<'_, Ty>) {
// FIXME(eddyb) there should be a size cap here
// (probably what clang calls "illegal vectors").
}
Abi::Scalar(_) => {
if a.layout.size.bytes() > 8 {
Abi::Scalar(scalar) => {
// Match what LLVM does for `f128` so that `compiler-builtins` builtins match up
// with what LLVM expects.
if a.layout.size.bytes() > 8
&& !matches!(scalar.primitive(), Primitive::Float(Float::F128))
{
a.make_indirect();
} else {
a.extend_integer_width_to(32);
Expand Down
4 changes: 3 additions & 1 deletion library/std/build.rs
Original file line number Diff line number Diff line change
Expand Up @@ -94,7 +94,7 @@ fn main() {
// Unsupported <https://github.com/llvm/llvm-project/issues/94434>
("arm64ec", _) => false,
// MinGW ABI bugs <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115054>
("x86", "windows") => false,
("x86_64", "windows") => false,
// x86 has ABI bugs that show up with optimizations. This should be partially fixed with
// the compiler-builtins update. <https://github.com/rust-lang/rust/issues/123885>
("x86" | "x86_64", _) => false,
Expand Down Expand Up @@ -122,6 +122,8 @@ fn main() {
("nvptx64", _) => false,
// ABI unsupported <https://github.com/llvm/llvm-project/issues/41838>
("sparc", _) => false,
// MinGW ABI bugs <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115054>
("x86_64", "windows") => false,
// 64-bit Linux is about the only platform to have f128 symbols by default
(_, "linux") if target_pointer_width == 64 => true,
// Same as for f16, except MacOS is also missing f128 symbols.
Expand Down
39 changes: 39 additions & 0 deletions tests/assembly/x86_64-windows-float-abi.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,39 @@
//@ assembly-output: emit-asm
//@ compile-flags: -O
//@ only-windows
//@ only-x86_64

#![feature(f16, f128)]
#![crate_type = "lib"]

// CHECK-LABEL: second_f16
// CHECK: movaps %xmm1, %xmm0
// CHECK-NEXT: retq
#[no_mangle]
pub extern "C" fn second_f16(_: f16, x: f16) -> f16 {
x
}

// CHECK-LABEL: second_f32
// CHECK: movaps %xmm1, %xmm0
// CHECK-NEXT: retq
#[no_mangle]
pub extern "C" fn second_f32(_: f32, x: f32) -> f32 {
x
}

// CHECK-LABEL: second_f64
// CHECK: movaps %xmm1, %xmm0
// CHECK-NEXT: retq
#[no_mangle]
pub extern "C" fn second_f64(_: f64, x: f64) -> f64 {
x
}

// CHECK-LABEL: second_f128
// CHECK: movaps %xmm1, %xmm0
// CHECK-NEXT: retq
#[no_mangle]
pub extern "C" fn second_f128(_: f128, x: f128) -> f128 {
x
}

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