diff --git a/lib/Basic/Targets.cpp b/lib/Basic/Targets.cpp
index c67e4cd..f97a531 100644
--- a/lib/Basic/Targets.cpp
+++ b/lib/Basic/Targets.cpp
@@ -2927,6 +2927,13 @@ X86TargetInfo::validateAsmConstraint(const char *&Name,
     case 'm':  // any MMX register, when inter-unit moves enabled.
       break;   // falls through to setAllowsRegister.
   }
+  case 'f': // any x87 floating point stack register.
+    // Constraint 'f' cannot be used for output operands.
+    if (Info.ConstraintStr[0] == '=')
+      return false;
+
+    Info.setAllowsRegister();
+    return true;
   case 'a': // eax.
   case 'b': // ebx.
   case 'c': // ecx.
@@ -2934,7 +2941,6 @@ X86TargetInfo::validateAsmConstraint(const char *&Name,
   case 'S': // esi.
   case 'D': // edi.
   case 'A': // edx:eax.
-  case 'f': // any x87 floating point stack register.
   case 't': // top of floating point stack.
   case 'u': // second from top of floating point stack.
   case 'q': // Any register accessible as [r]l: a, b, c, and d.
diff --git a/test/Sema/asm.c b/test/Sema/asm.c
index 22b7497..89c8c57 100644
--- a/test/Sema/asm.c
+++ b/test/Sema/asm.c
@@ -157,3 +157,9 @@ struct foo {
 };
 register struct foo bar asm("sp"); // expected-error {{bad type for named register variable}}
 register float baz asm("sp"); // expected-error {{bad type for named register variable}}
+
+double f_output_constraint(void) {
+  double result;
+  __asm("foo1": "=f" (result)); // expected-error {{invalid output constraint '=f' in asm}}
+  return result;
+}