x86_win64 ABI: do not use xmm0 with softfloat ABI

Author: RalfJung

compiler/rustc_target/src/callconv/x86_win64.rs

@@ -1,11 +1,11 @@
 use rustc_abi::{BackendRepr, Float, Integer, Primitive, RegKind, Size};
 
 use crate::callconv::{ArgAbi, FnAbi, Reg};
-use crate::spec::HasTargetSpec;
+use crate::spec::{HasTargetSpec, RustcAbi};
 
 // Win64 ABI: https://docs.microsoft.com/en-us/cpp/build/parameter-passing
 
-pub(crate) fn compute_abi_info<Ty>(_cx: &impl HasTargetSpec, fn_abi: &mut FnAbi<'_, Ty>) {
+pub(crate) fn compute_abi_info<Ty>(cx: &impl HasTargetSpec, fn_abi: &mut FnAbi<'_, Ty>) {
     let fixup = |a: &mut ArgAbi<'_, Ty>, is_ret: bool| {
         match a.layout.backend_repr {
             BackendRepr::Uninhabited | BackendRepr::Memory { sized: false } => {}
@@ -23,7 +23,10 @@ pub(crate) fn compute_abi_info<Ty>(_cx: &impl HasTargetSpec, fn_abi: &mut FnAbi<
                 // (probably what clang calls "illegal vectors").
             }
             BackendRepr::Scalar(scalar) => {
-                if is_ret && matches!(scalar.primitive(), Primitive::Int(Integer::I128, _)) {
+                if is_ret
+                    && cx.target_spec().rustc_abi != Some(RustcAbi::X86Softfloat)
+                    && matches!(scalar.primitive(), Primitive::Int(Integer::I128, _))
+                {
                     // `i128` is returned in xmm0 by Clang and GCC
                     // FIXME(#134288): This may change for the `-msvc` targets in the future.
                     let reg = Reg { kind: RegKind::Vector, size: Size::from_bits(128) };