[clang] [llvm] [X86][Windows] Return `fp128` on the stack (PR #204887)

[Trevor Gross via cfe-commits]

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@@ -670,6 +670,20 @@ bool X86TargetLowering::CanLowerReturn(
     CallingConv::ID CallConv, MachineFunction &MF, bool isVarArg,
     const SmallVectorImpl<ISD::OutputArg> &Outs, LLVMContext &Context,
     const Type *RetTy) const {
+  // Mingw64 GCC returns f128 via sret, which matches the documentation of the
+  // Windows x64 calling convention:
+  //
+  // https://learn.microsoft.com/en-us/cpp/build/x64-calling-convention?view=msvc-170#return-values
+  //
+  // > Otherwise, the caller must allocate memory for the return value and pass
+  // a pointer to it as the first argument.
+  //
+  // Return false, which will perform sret demotion.
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tgross35 wrote:

This could probably be rephrased to be clear that it is one reasonable interpretation of the CC, and that what we're picking also matches GCC. Since the spec also says:

> Nonscalar types including floats, doubles, and vector types such as __m128, __m128i, __m128d are returned in XMM0

which could easily be taken to mean f128 too.

https://github.com/llvm/llvm-project/pull/204887