================
@@ -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.
+  if (Subtarget.isCallingConvWin64(CallConv) &&
----------------
tgross35 wrote:

Does `isCallingConvWin64` match swift? Either way, I don't think it's worth an 
exception here unless Swift (or any other CC) actually document an ABI 
specifically for f128.

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