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x86-32 float return for 'Rust' ABI: treat all float types consistently #131871

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x86-32 float return for 'Rust' ABI: treat all float types consistently #131871

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37 changes: 15 additions & 22 deletions compiler/rustc_ty_utils/src/abi.rs
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Original file line number Diff line number Diff line change
@@ -1,6 +1,5 @@
use std::iter;

use rustc_abi::Float::*;
use rustc_abi::Primitive::{Float, Pointer};
use rustc_abi::{Abi, AddressSpace, PointerKind, Scalar, Size};
use rustc_hir as hir;
Expand Down Expand Up @@ -695,37 +694,31 @@ fn fn_abi_adjust_for_abi<'tcx>(
}

// Avoid returning floats in x87 registers on x86 as loading and storing from x87
// registers will quiet signalling NaNs.
// registers will quiet signalling NaNs. Also avoid using SSE registers since they
// are not always available (depending on target features).
if tcx.sess.target.arch == "x86"
&& arg_idx.is_none()
// Intrinsics themselves are not actual "real" functions, so theres no need to
// change their ABIs.
&& abi != SpecAbi::RustIntrinsic
{
match arg.layout.abi {
// Handle similar to the way arguments with an `Abi::Aggregate` abi are handled
// below, by returning arguments up to the size of a pointer (32 bits on x86)
// cast to an appropriately sized integer.
Abi::Scalar(s) if s.primitive() == Float(F32) => {
// Same size as a pointer, return in a register.
arg.cast_to(Reg::i32());
return;
let has_float = match arg.layout.abi {
Abi::Scalar(s) => matches!(s.primitive(), Float(_)),
Abi::ScalarPair(s1, s2) => {
matches!(s1.primitive(), Float(_)) || matches!(s2.primitive(), Float(_))
Comment on lines +706 to +708
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Suggested change
Abi::Scalar(s) => matches!(s.primitive(), Float(_)),
Abi::ScalarPair(s1, s2) => {
matches!(s1.primitive(), Float(_)) || matches!(s2.primitive(), Float(_))
Abi::Scalar(s) => matches!(s.primitive(), Float(F16 | F32 | F64)),
Abi::ScalarPair(s1, s2) => {
matches!(s1.primitive(), Float(F16 | F32 | F64)) || matches!(s2.primitive(), Float(F16 | F32 | F64))

nit: There's no need to include f128 here as the 32-bit x86 ABI already guarantees it gets passed in memory.

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@RalfJung RalfJung Oct 18, 2024 •
edited
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IMO we should treat all float types uniformly here, that's more future-proof. The result is the same both ways but my code makes it more clear what happens, relying on fewer external assumptions.

We don't care about being consistent with the C ABI here, after all.

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Yeah, I kinda agree morally with beetrees but not actually: this seems fine as-is.

I don't think it's worth spending too much time on this. While I do need to be able to read this code to refactor it, I do intend to bulldoze it.

}
Abi::Scalar(s) if s.primitive() == Float(F64) => {
// Larger than a pointer, return indirectly.
arg.make_indirect();
return;
}
Abi::ScalarPair(s1, s2)
if matches!(s1.primitive(), Float(F32 | F64))
|| matches!(s2.primitive(), Float(F32 | F64)) =>
{
_ => false, // anyway not passed via registers on x86
};
if has_float {
if arg.layout.size <= Pointer(AddressSpace::DATA).size(cx) {
// Same size or smaller than pointer, return in a register.
arg.cast_to(Reg { kind: RegKind::Integer, size: arg.layout.size });
} else {
// Larger than a pointer, return indirectly.
arg.make_indirect();
return;
}
_ => {}
};
return;
}
}

match arg.layout.abi {
Expand Down
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