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Ensure floats are returned losslessly by the Rust ABI on 32-bit x86

Solves #115567 for the (default) `"Rust"` ABI. When compiling for 32-bit x86, this PR changes the `"Rust"` ABI to return floats indirectly instead of in x87 registers (with the exception of single `f32`s, which this PR returns in general purpose registers as they are small enough to fit in one). No change is made to the `"C"` ABI as that ABI requires x87 register usage and therefore will need a different solution.
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bors committed Jul 12, 2024
2 parents 62c068f + cae9d48 commit c6727fc
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34 changes: 34 additions & 0 deletions compiler/rustc_ty_utils/src/abi.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -743,6 +743,40 @@ fn fn_abi_adjust_for_abi<'tcx>(
return;
}

// Avoid returning floats in x87 registers on x86 as loading and storing from x87
// registers will quiet signalling NaNs.
if cx.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;
}
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)) =>
{
// Larger than a pointer, return indirectly.
arg.make_indirect();
return;
}
_ => {}
};
}

match arg.layout.abi {
Abi::Aggregate { .. } => {}

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6 changes: 4 additions & 2 deletions src/doc/rustc/src/platform-support.md
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Expand Up @@ -41,10 +41,10 @@ target | notes
`x86_64-pc-windows-msvc` | 64-bit MSVC (Windows 10+, Windows Server 2016+)
`x86_64-unknown-linux-gnu` | 64-bit Linux (kernel 3.2+, glibc 2.17+)

[^x86_32-floats-return-ABI]: Due to limitations of the C ABI, floating-point support on `i686` targets is non-compliant: floating-point return values are passed via an x87 register, so NaN payload bits can be lost. See [issue #114479][x86-32-float-issue].
[^x86_32-floats-return-ABI]: Due to limitations of the C ABI, floating-point support on `i686` targets is non-compliant: floating-point return values are passed via an x87 register, so NaN payload bits can be lost. Functions with the default Rust ABI are not affected. See [issue #115567][x86-32-float-return-issue].

[77071]: https://github.com/rust-lang/rust/issues/77071
[x86-32-float-issue]: https://github.com/rust-lang/rust/issues/114479
[x86-32-float-return-issue]: https://github.com/rust-lang/rust/issues/115567

## Tier 1

Expand Down Expand Up @@ -209,6 +209,8 @@ target | std | notes

[^x86_32-floats-x87]: Floating-point support on `i586` targets is non-compliant: the `x87` registers and instructions used for these targets do not provide IEEE-754-compliant behavior, in particular when it comes to rounding and NaN payload bits. See [issue #114479][x86-32-float-issue].

[x86-32-float-issue]: https://github.com/rust-lang/rust/issues/114479

[wasi-rename]: https://github.com/rust-lang/compiler-team/issues/607

[Fortanix ABI]: https://edp.fortanix.com/
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328 changes: 328 additions & 0 deletions tests/assembly/x86-return-float.rs
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@@ -0,0 +1,328 @@
//@ assembly-output: emit-asm
//@ only-x86
// FIXME(#114479): LLVM miscompiles loading and storing `f32` and `f64` when SSE is disabled.
// There's no compiletest directive to ignore a test on i586 only, so just always explicitly enable
// SSE2.
// Use the same target CPU as `i686` so that LLVM orders the instructions in the same order.
//@ compile-flags: -Ctarget-feature=+sse2 -Ctarget-cpu=pentium4
// Force frame pointers to make ASM more consistent between targets
//@ compile-flags: -O -C force-frame-pointers
//@ filecheck-flags: --implicit-check-not fld --implicit-check-not fst
//@ revisions: unix windows
//@[unix] ignore-windows
//@[windows] only-windows

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

// Tests that returning `f32` and `f64` with the "Rust" ABI on 32-bit x86 doesn't use the x87
// floating point stack, as loading and storing `f32`s and `f64`s to and from the x87 stack quietens
// signalling NaNs.

// Returning individual floats

// CHECK-LABEL: return_f32:
#[no_mangle]
pub fn return_f32(x: f32) -> f32 {
// CHECK: movl {{.*}}(%ebp), %eax
// CHECK-NOT: ax
// CHECK: retl
x
}

// CHECK-LABEL: return_f64:
#[no_mangle]
pub fn return_f64(x: f64) -> f64 {
// CHECK: movl [[#%d,OFFSET:]](%ebp), %[[PTR:.*]]
// CHECK-NEXT: movsd [[#%d,OFFSET+4]](%ebp), %[[VAL:.*]]
// CHECK-NEXT: movsd %[[VAL]], (%[[PTR]])
// CHECK: retl
x
}

// Returning scalar pairs containing floats

// CHECK-LABEL: return_f32_f32:
#[no_mangle]
pub fn return_f32_f32(x: (f32, f32)) -> (f32, f32) {
// CHECK: movl [[#%d,OFFSET:]](%ebp), %[[PTR:.*]]
// CHECK-NEXT: movss [[#%d,OFFSET+4]](%ebp), %[[VAL1:.*]]
// CHECK-NEXT: movss [[#%d,OFFSET+8]](%ebp), %[[VAL2:.*]]
// CHECK-NEXT: movss %[[VAL1]], (%[[PTR]])
// CHECK-NEXT: movss %[[VAL2]], 4(%[[PTR]])
// CHECK: retl
x
}

// CHECK-LABEL: return_f64_f64:
#[no_mangle]
pub fn return_f64_f64(x: (f64, f64)) -> (f64, f64) {
// CHECK: movl [[#%d,OFFSET:]](%ebp), %[[PTR:.*]]
// CHECK-NEXT: movsd [[#%d,OFFSET+4]](%ebp), %[[VAL1:.*]]
// CHECK-NEXT: movsd [[#%d,OFFSET+12]](%ebp), %[[VAL2:.*]]
// CHECK-NEXT: movsd %[[VAL1]], (%[[PTR]])
// CHECK-NEXT: movsd %[[VAL2]], 8(%[[PTR]])
// CHECK: retl
x
}

// CHECK-LABEL: return_f32_f64:
#[no_mangle]
pub fn return_f32_f64(x: (f32, f64)) -> (f32, f64) {
// CHECK: movl [[#%d,OFFSET:]](%ebp), %[[PTR:.*]]
// CHECK-NEXT: movss [[#%d,OFFSET+4]](%ebp), %[[VAL1:.*]]
// CHECK-NEXT: movsd [[#%d,OFFSET+8]](%ebp), %[[VAL2:.*]]
// CHECK-NEXT: movss %[[VAL1]], (%[[PTR]])
// CHECK-NEXT: movsd %[[VAL2]], {{4|8}}(%[[PTR]])
// CHECK: retl
x
}

// CHECK-LABEL: return_f64_f32:
#[no_mangle]
pub fn return_f64_f32(x: (f64, f32)) -> (f64, f32) {
// CHECK: movl [[#%d,OFFSET:]](%ebp), %[[PTR:.*]]
// CHECK-NEXT: movsd [[#%d,OFFSET+4]](%ebp), %[[VAL1:.*]]
// CHECK-NEXT: movss [[#%d,OFFSET+12]](%ebp), %[[VAL2:.*]]
// CHECK-NEXT: movsd %[[VAL1]], (%[[PTR]])
// CHECK-NEXT: movss %[[VAL2]], 8(%[[PTR]])
// CHECK: retl
x
}

// CHECK-LABEL: return_f32_other:
#[no_mangle]
pub fn return_f32_other(x: (f32, usize)) -> (f32, usize) {
// CHECK: movl [[#%d,OFFSET:]](%ebp), %[[PTR:.*]]
// CHECK-NEXT: movss [[#%d,OFFSET+4]](%ebp), %[[VAL1:.*]]
// CHECK-NEXT: movl [[#%d,OFFSET+8]](%ebp), %[[VAL2:.*]]
// CHECK-NEXT: movss %[[VAL1]], (%[[PTR]])
// CHECK-NEXT: movl %[[VAL2]], 4(%[[PTR]])
// CHECK: retl
x
}

// CHECK-LABEL: return_f64_other:
#[no_mangle]
pub fn return_f64_other(x: (f64, usize)) -> (f64, usize) {
// CHECK: movl [[#%d,OFFSET:]](%ebp), %[[PTR:.*]]
// CHECK-NEXT: movsd [[#%d,OFFSET+4]](%ebp), %[[VAL1:.*]]
// CHECK-NEXT: movl [[#%d,OFFSET+12]](%ebp), %[[VAL2:.*]]
// CHECK-NEXT: movsd %[[VAL1]], (%[[PTR]])
// CHECK-NEXT: movl %[[VAL2]], 8(%[[PTR]])
// CHECK: retl
x
}

// CHECK-LABEL: return_other_f32:
#[no_mangle]
pub fn return_other_f32(x: (usize, f32)) -> (usize, f32) {
// CHECK: movl [[#%d,OFFSET:]](%ebp), %[[PTR:.*]]
// CHECK-NEXT: movl [[#%d,OFFSET+4]](%ebp), %[[VAL1:.*]]
// CHECK-NEXT: movss [[#%d,OFFSET+8]](%ebp), %[[VAL2:.*]]
// CHECK-NEXT: movl %[[VAL1]], (%[[PTR]])
// CHECK-NEXT: movss %[[VAL2]], 4(%[[PTR]])
// CHECK: retl
x
}

// CHECK-LABEL: return_other_f64:
#[no_mangle]
pub fn return_other_f64(x: (usize, f64)) -> (usize, f64) {
// CHECK: movl [[#%d,OFFSET:]](%ebp), %[[PTR:.*]]
// CHECK-NEXT: movl [[#%d,OFFSET+4]](%ebp), %[[VAL1:.*]]
// CHECK-NEXT: movsd [[#%d,OFFSET+8]](%ebp), %[[VAL2:.*]]
// CHECK-NEXT: movl %[[VAL1]], (%[[PTR]])
// CHECK-NEXT: movsd %[[VAL2]], {{4|8}}(%[[PTR]])
// CHECK: retl
x
}

// Calling functions returning floats

// CHECK-LABEL: call_f32:
#[no_mangle]
pub unsafe fn call_f32(x: &mut f32) {
extern "Rust" {
fn get_f32() -> f32;
}
// CHECK: movl {{.*}}(%ebp), %[[PTR:.*]]
// CHECK: calll {{()|_}}get_f32
// CHECK-NEXT: movl %eax, (%[[PTR]])
*x = get_f32();
}

// CHECK-LABEL: call_f64:
#[no_mangle]
pub unsafe fn call_f64(x: &mut f64) {
extern "Rust" {
fn get_f64() -> f64;
}
// CHECK: movl {{.*}}(%ebp), %[[PTR:.*]]
// CHECK: calll {{()|_}}get_f64
// CHECK: movsd {{.*}}(%{{ebp|esp}}), %[[VAL:.*]]
// CHECK-NEXT: movsd %[[VAL:.*]], (%[[PTR]])
*x = get_f64();
}

// Calling functions returning scalar pairs containing floats

// CHECK-LABEL: call_f32_f32:
#[no_mangle]
pub unsafe fn call_f32_f32(x: &mut (f32, f32)) {
extern "Rust" {
fn get_f32_f32() -> (f32, f32);
}
// CHECK: movl {{.*}}(%ebp), %[[PTR:.*]]
// CHECK: calll {{()|_}}get_f32_f32
// CHECK: movss [[#%d,OFFSET:]](%ebp), %[[VAL1:.*]]
// CHECK-NEXT: movss [[#%d,OFFSET+4]](%ebp), %[[VAL2:.*]]
// CHECK-NEXT: movss %[[VAL1]], (%[[PTR]])
// CHECK-NEXT: movss %[[VAL2]], 4(%[[PTR]])
*x = get_f32_f32();
}

// CHECK-LABEL: call_f64_f64:
#[no_mangle]
pub unsafe fn call_f64_f64(x: &mut (f64, f64)) {
extern "Rust" {
fn get_f64_f64() -> (f64, f64);
}
// CHECK: movl {{.*}}(%ebp), %[[PTR:.*]]
// CHECK: calll {{()|_}}get_f64_f64
// unix: movsd [[#%d,OFFSET:]](%ebp), %[[VAL1:.*]]
// unix-NEXT: movsd [[#%d,OFFSET+8]](%ebp), %[[VAL2:.*]]
// windows: movsd (%esp), %[[VAL1:.*]]
// windows-NEXT: movsd 8(%esp), %[[VAL2:.*]]
// CHECK-NEXT: movsd %[[VAL1]], (%[[PTR]])
// CHECK-NEXT: movsd %[[VAL2]], 8(%[[PTR]])
*x = get_f64_f64();
}

// CHECK-LABEL: call_f32_f64:
#[no_mangle]
pub unsafe fn call_f32_f64(x: &mut (f32, f64)) {
extern "Rust" {
fn get_f32_f64() -> (f32, f64);
}
// CHECK: movl {{.*}}(%ebp), %[[PTR:.*]]
// CHECK: calll {{()|_}}get_f32_f64
// unix: movss [[#%d,OFFSET:]](%ebp), %[[VAL1:.*]]
// unix-NEXT: movsd [[#%d,OFFSET+4]](%ebp), %[[VAL2:.*]]
// windows: movss (%esp), %[[VAL1:.*]]
// windows-NEXT: movsd 8(%esp), %[[VAL2:.*]]
// CHECK-NEXT: movss %[[VAL1]], (%[[PTR]])
// unix-NEXT: movsd %[[VAL2]], 4(%[[PTR]])
// windows-NEXT: movsd %[[VAL2]], 8(%[[PTR]])
*x = get_f32_f64();
}

// CHECK-LABEL: call_f64_f32:
#[no_mangle]
pub unsafe fn call_f64_f32(x: &mut (f64, f32)) {
extern "Rust" {
fn get_f64_f32() -> (f64, f32);
}
// CHECK: movl {{.*}}(%ebp), %[[PTR:.*]]
// CHECK: calll {{()|_}}get_f64_f32
// unix: movsd [[#%d,OFFSET:]](%ebp), %[[VAL1:.*]]
// unix-NEXT: movss [[#%d,OFFSET+8]](%ebp), %[[VAL2:.*]]
// windows: movsd (%esp), %[[VAL1:.*]]
// windows-NEXT: movss 8(%esp), %[[VAL2:.*]]
// CHECK-NEXT: movsd %[[VAL1]], (%[[PTR]])
// CHECK-NEXT: movss %[[VAL2]], 8(%[[PTR]])
*x = get_f64_f32();
}

// CHECK-LABEL: call_f32_other:
#[no_mangle]
pub unsafe fn call_f32_other(x: &mut (f32, usize)) {
extern "Rust" {
fn get_f32_other() -> (f32, usize);
}
// CHECK: movl {{.*}}(%ebp), %[[PTR:.*]]
// CHECK: calll {{()|_}}get_f32_other
// CHECK: movss [[#%d,OFFSET:]](%ebp), %[[VAL1:.*]]
// CHECK-NEXT: movl [[#%d,OFFSET+4]](%ebp), %[[VAL2:.*]]
// CHECK-NEXT: movss %[[VAL1]], (%[[PTR]])
// CHECK-NEXT: movl %[[VAL2]], 4(%[[PTR]])
*x = get_f32_other();
}

// CHECK-LABEL: call_f64_other:
#[no_mangle]
pub unsafe fn call_f64_other(x: &mut (f64, usize)) {
extern "Rust" {
fn get_f64_other() -> (f64, usize);
}
// CHECK: movl {{.*}}(%ebp), %[[PTR:.*]]
// CHECK: calll {{()|_}}get_f64_other
// unix: movsd [[#%d,OFFSET:]](%ebp), %[[VAL1:.*]]
// unix-NEXT: movl [[#%d,OFFSET+8]](%ebp), %[[VAL2:.*]]
// windows: movsd (%esp), %[[VAL1:.*]]
// windows-NEXT: movl 8(%esp), %[[VAL2:.*]]
// CHECK-NEXT: movsd %[[VAL1]], (%[[PTR]])
// CHECK-NEXT: movl %[[VAL2]], 8(%[[PTR]])
*x = get_f64_other();
}

// CHECK-LABEL: call_other_f32:
#[no_mangle]
pub unsafe fn call_other_f32(x: &mut (usize, f32)) {
extern "Rust" {
fn get_other_f32() -> (usize, f32);
}
// CHECK: movl {{.*}}(%ebp), %[[PTR:.*]]
// CHECK: calll {{()|_}}get_other_f32
// CHECK: movl [[#%d,OFFSET:]](%ebp), %[[VAL1:.*]]
// CHECK-NEXT: movss [[#%d,OFFSET+4]](%ebp), %[[VAL2:.*]]
// CHECK-NEXT: movl %[[VAL1]], (%[[PTR]])
// CHECK-NEXT: movss %[[VAL2]], 4(%[[PTR]])
*x = get_other_f32();
}

// CHECK-LABEL: call_other_f64:
#[no_mangle]
pub unsafe fn call_other_f64(x: &mut (usize, f64)) {
extern "Rust" {
fn get_other_f64() -> (usize, f64);
}
// CHECK: movl {{.*}}(%ebp), %[[PTR:.*]]
// CHECK: calll {{()|_}}get_other_f64
// unix: movl [[#%d,OFFSET:]](%ebp), %[[VAL1:.*]]
// unix-NEXT: movsd [[#%d,OFFSET+4]](%ebp), %[[VAL2:.*]]
// windows: movl (%esp), %[[VAL1:.*]]
// windows-NEXT: movsd 8(%esp), %[[VAL2:.*]]
// CHECK-NEXT: movl %[[VAL1]], (%[[PTR]])
// unix-NEXT: movsd %[[VAL2]], 4(%[[PTR]])
// windows-NEXT: movsd %[[VAL2]], 8(%[[PTR]])
*x = get_other_f64();
}

// The "C" ABI for `f16` and `f128` on x86 has never used the x87 floating point stack. Do some
// basic checks to ensure this remains the case for the "Rust" ABI.

// CHECK-LABEL: return_f16:
#[no_mangle]
pub fn return_f16(x: f16) -> f16 {
// CHECK: pinsrw $0, {{.*}}(%ebp), %xmm0
// CHECK-NOT: xmm0
// CHECK: retl
x
}

// CHECK-LABEL: return_f128:
#[no_mangle]
pub fn return_f128(x: f128) -> f128 {
// CHECK: movl [[#%d,OFFSET:]](%ebp), %[[PTR:.*]]
// CHECK-NEXT: movl [[#%d,OFFSET+16]](%ebp), %[[VAL4:.*]]
// CHECK-NEXT: movl [[#%d,OFFSET+4]](%ebp), %[[VAL1:.*]]
// CHECK-NEXT: movl [[#%d,OFFSET+8]](%ebp), %[[VAL2:.*]]
// CHECK-NEXT: movl [[#%d,OFFSET+12]](%ebp), %[[VAL3:.*]]
// CHECK-NEXT: movl %[[VAL4:.*]] 12(%[[PTR]])
// CHECK-NEXT: movl %[[VAL3:.*]] 8(%[[PTR]])
// CHECK-NEXT: movl %[[VAL2:.*]] 4(%[[PTR]])
// CHECK-NEXT: movl %[[VAL1:.*]] (%[[PTR]])
// CHECK: retl
x
}
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