diff --git a/clang/lib/CodeGen/Targets/X86.cpp b/clang/lib/CodeGen/Targets/X86.cpp
index b7a1374d5b399..b36a6e1396653 100644
--- a/clang/lib/CodeGen/Targets/X86.cpp
+++ b/clang/lib/CodeGen/Targets/X86.cpp
@@ -3390,6 +3390,9 @@ ABIArgInfo WinX86_64ABIInfo::classify(QualType Ty, unsigned &FreeSSERegs,
 
     case BuiltinType::Int128:
     case BuiltinType::UInt128:
+    case BuiltinType::Float128:
+      // 128-bit float and integer types share the same ABI.
+
       // 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.
@@ -3400,6 +3403,8 @@ ABIArgInfo WinX86_64ABIInfo::classify(QualType Ty, unsigned &FreeSSERegs,
 
       // Mingw64 GCC returns i128 in XMM0. Coerce to v2i64 to handle that.
       // Clang matches them for compatibility.
+      // NOTE: GCC actually returns f128 indirectly but will hopefully change.
+      // See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115054#c8.
       return ABIArgInfo::getDirect(llvm::FixedVectorType::get(
           llvm::Type::getInt64Ty(getVMContext()), 2));
 
diff --git a/clang/test/CodeGen/win-fp128.c b/clang/test/CodeGen/win-fp128.c
new file mode 100644
index 0000000000000..328a7aaa7df57
--- /dev/null
+++ b/clang/test/CodeGen/win-fp128.c
@@ -0,0 +1,19 @@
+// RUN: %clang_cc1 -triple x86_64-windows-gnu -emit-llvm -o - %s \
+// RUN:    | FileCheck %s --check-prefix=CHECK-GNU64
+// __float128 is unsupported on MSVC
+
+__float128 fp128_ret(void) { return 0; }
+// CHECK-GNU64: define dso_local <2 x i64>  @fp128_ret()
+
+__float128 fp128_args(__float128 a, __float128 b) { return a * b; }
+// CHECK-GNU64: define dso_local <2 x i64> @fp128_args(ptr noundef %0, ptr noundef %1)
+
+void fp128_vararg(int a, ...) {
+  // CHECK-GNU64-LABEL: define dso_local void @fp128_vararg
+  __builtin_va_list ap;
+  __builtin_va_start(ap, a);
+  __float128 i = __builtin_va_arg(ap, __float128);
+  // CHECK-GNU64: load ptr, ptr
+  // CHECK-GNU64: load fp128, ptr
+  __builtin_va_end(ap);
+}