Index: lib/CodeGen/CGBuiltin.cpp
===================================================================
--- lib/CodeGen/CGBuiltin.cpp
+++ lib/CodeGen/CGBuiltin.cpp
@@ -945,12 +945,26 @@
                                                Result.Val.getFloat()));
   }
 
+  bool EmitMathIntrinsic = true;
+  const Triple& Triple = getTarget().getTriple();
+  if (Triple.isWindowsGNUEnvironment() &&
+      Triple.getArch() == llvm::Triple::x86_64) {
+    // On x86_64 mingw, the calling convention handles long doubles as
+    // structs. This detail is normally handled within clang, but
+    // intrinsics lowered into libcalls in LLVM don't handle this.
+    // For these cases, skip emitting an intrinsic and emit a normal
+    // function call with the right calling convention instead.
+    const char *TypeStr = getContext().BuiltinInfo.getTypeString(BuiltinID);
+    if (strstr(TypeStr, "Ld")) // long double
+      EmitMathIntrinsic = false;
+  }
+
   // There are LLVM math intrinsics/instructions corresponding to math library
   // functions except the LLVM op will never set errno while the math library
   // might. Also, math builtins have the same semantics as their math library
   // twins. Thus, we can transform math library and builtin calls to their
   // LLVM counterparts if the call is marked 'const' (known to never set errno).
-  if (FD->hasAttr<ConstAttr>()) {
+  if (FD->hasAttr<ConstAttr>() && EmitMathIntrinsic) {
     switch (BuiltinID) {
     case Builtin::BIceil:
     case Builtin::BIceilf:
Index: test/CodeGen/mingw-x86_64-long-double-builtins.c
===================================================================
--- /dev/null
+++ test/CodeGen/mingw-x86_64-long-double-builtins.c
@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 -triple x86_64-mingw32 -emit-llvm -o - %s | FileCheck %s
+
+// Check that long double math functions are emitted as direct function
+// calls instead of as intrinsics.
+
+long double floorl(long double);
+
+long double foo(long double a) {
+  return floorl(a);
+// CHECK: call void @floorl(x86_fp80* sret {{.*}}, x86_fp80* {{.*}})
+}