Only apply GCC 128-bit integer return behavior on Mingw64

The Windows x64 ABI passes values larger than 8 bytes in registers:

> Any argument that doesn't fit in 8 bytes, or isn't 1, 2, 4, or 8 bytes,
> must be passed by reference.

https://learn.microsoft.com/en-us/cpp/build/x64-calling-convention?view=msvc-170

The Mingw64 ABI chooses to treat 128-bit integers as vectors and thus
uses xmm0 but we should not do that for the standard Windows x64 ABI.

Author: wesleywiser

clang/lib/CodeGen/Targets/X86.cpp

@@ -3331,13 +3331,16 @@ ABIArgInfo WinX86_64ABIInfo::classify(QualType Ty, unsigned &FreeSSERegs,
       // If it's a parameter type, the normal ABI rule is that arguments larger
       // than 8 bytes are passed indirectly. GCC follows it. We follow it too,
       // even though it isn't particularly efficient.
-      if (!IsReturnType)
-        return ABIArgInfo::getIndirect(Align, /*ByVal=*/false);
-
-      // Mingw64 GCC returns i128 in XMM0. Coerce to v2i64 to handle that.
-      // Clang matches them for compatibility.
-      return ABIArgInfo::getDirect(llvm::FixedVectorType::get(
-          llvm::Type::getInt64Ty(getVMContext()), 2));
+      if (IsMingw64 && IsReturnType) {
+        // Mingw64 GCC returns i128 in XMM0. Coerce to v2i64 to handle that.
+        // Clang matches them for compatibility.
+        return ABIArgInfo::getDirect(llvm::FixedVectorType::get(
+            llvm::Type::getInt64Ty(getVMContext()), 2));
+      } else {
+          // Otherwise, for arguments in Mingw64 GCC or for MS x64 ABI,
+          // pass by reference.
+          return ABIArgInfo::getIndirect(Align, /*ByVal=*/false);
+      }
 
     default:
       break;

clang/test/CodeGen/win64-i128.c

@@ -8,12 +8,12 @@ typedef int int128_t __attribute__((mode(TI)));
 int128_t foo(void) { return 0; }
 
 // GNU64: define dso_local <2 x i64> @foo()
-// MSC64: define dso_local <2 x i64> @foo()
+// MSC64: define dso_local void @foo(ptr dead_on_unwind noalias writable sret(i128) align 16 %agg.result)
 
 int128_t bar(int128_t a, int128_t b) { return a * b; }
 
 // GNU64: define dso_local <2 x i64> @bar(ptr noundef %0, ptr noundef %1)
-// MSC64: define dso_local <2 x i64> @bar(ptr noundef %0, ptr noundef %1)
+// MSC64: define dso_local void @bar(ptr dead_on_unwind noalias writable sret(i128) align 16 %agg.result, ptr noundef %0, ptr noundef %1)
 
 void vararg(int a, ...) {
   // GNU64-LABEL: define{{.*}} void @vararg