Auto merge of #137779 - tgross35:windows-f128-abi-experiment, r=<try>

[do not merge] Windows f128 abi experiment

Running tests with llvm/llvm-project#115052 and llvm/llvm-project#128848.

r? `@ghost`

try-job: dist-x86_64-msvc
try-job: dist-x86_64-mingw
try-job: x86_64-msvc-1
try-job: x86_64-msvc-2
try-job: x86_64-mingw-1
try-job: x86_64-mingw-2

Author: bors

.gitmodules

@@ -28,8 +28,8 @@
 	shallow = true
 [submodule "src/llvm-project"]
 	path = src/llvm-project
-	url = https://github.com/rust-lang/llvm-project.git
-	branch = rustc/20.1-2025-02-13
+	url = https://github.com/tgross35/llvm-project.git
+	branch = windows-f128-abi-combined
 	shallow = true
 [submodule "src/doc/embedded-book"]
 	path = src/doc/embedded-book

compiler/rustc_target/src/callconv/x86_win64.rs

@@ -23,20 +23,24 @@ 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
+                    && matches!(
+                        scalar.primitive(),
+                        Primitive::Int(Integer::I128, _) | Primitive::Float(Float::F128)
+                    )
+                {
+                    // i128 and f128 have the same ABI on Windows.
                     if cx.target_spec().rustc_abi == Some(RustcAbi::X86Softfloat) {
-                        // Use the native `i128` LLVM type for the softfloat ABI -- in other words, adjust nothing.
+                        // Use the native `i128`/`f128` LLVM type for the softfloat ABI -- in other
+                        // words, adjust nothing.
                     } else {
-                        // `i128` is returned in xmm0 by Clang and GCC
+                        // `i128` and `f128` are returned in xmm0 by Clang. `i128` is returned in
+                        // xmm0 by GCC but `f128` is indirect (Clang and GCC have a mismatch).
                         // FIXME(#134288): This may change for the `-msvc` targets in the future.
                         let reg = Reg { kind: RegKind::Vector, size: Size::from_bits(128) };
                         a.cast_to(reg);
                     }
-                } else if a.layout.size.bytes() > 8
-                    && !matches!(scalar.primitive(), Primitive::Float(Float::F128))
-                {
-                    // Match what LLVM does for `f128` so that `compiler-builtins` builtins match up
-                    // with what LLVM expects.
+                } else if a.layout.size.bytes() > 8 {
                     a.make_indirect();
                 } else {
                     a.extend_integer_width_to(32);

tests/assembly/x86_64-windows-float-abi.rs

@@ -37,7 +37,7 @@ pub extern "C" fn second_f64(_: f64, x: f64) -> f64 {
 }
 
 // CHECK-LABEL: second_f128
-// CHECK: movaps %xmm1, %xmm0
+// CHECK: movaps (%rdi), %xmm0
 // CHECK-NEXT: retq
 #[no_mangle]
 pub extern "C" fn second_f128(_: f128, x: f128) -> f128 {