Index: clang/lib/Basic/Targets/X86.cpp
===================================================================
--- clang/lib/Basic/Targets/X86.cpp
+++ clang/lib/Basic/Targets/X86.cpp
@@ -1779,14 +1779,17 @@
     LLVM_FALLTHROUGH;
   case 'v':
   case 'x':
-    if (SSELevel >= AVX512F)
-      // 512-bit zmm registers can be used if target supports AVX512F.
-      return Size <= 512U;
-    else if (SSELevel >= AVX)
-      // 256-bit ymm registers can be used if target supports AVX.
-      return Size <= 256U;
-    return Size <= 128U;
-
+    return Size <= 512U;
+// FIXME: We should check the size based on the SSE level, but that requires
+// supporting the target attribute. The X86 backend seems to tolerate large
+// sizes and will produce an error message.
+//    if (SSELevel >= AVX512F)
+//      // 512-bit zmm registers can be used if target supports AVX512F.
+//      return Size <= 512U;
+//    else if (SSELevel >= AVX)
+//      // 256-bit ymm registers can be used if target supports AVX.
+//      return Size <= 256U;
+//    return Size <= 128U;
   }
 
   return true;
Index: clang/test/CodeGen/x86_32-inline-asm.c
===================================================================
--- clang/test/CodeGen/x86_32-inline-asm.c
+++ clang/test/CodeGen/x86_32-inline-asm.c
@@ -46,7 +46,7 @@
   __asm__ volatile("foo1 %0" : : "f" (val256)); // expected-error {{invalid input size for constraint 'f'}}
   __asm__ volatile("foo1 %0" : : "t" (val256)); // expected-error {{invalid input size for constraint 't'}}
   __asm__ volatile("foo1 %0" : : "u" (val256)); // expected-error {{invalid input size for constraint 'u'}}
-  __asm__ volatile("foo1 %0" : : "x" (val512)); // expected-error {{invalid input size for constraint 'x'}}
+  __asm__ volatile("foo1 %0" : : "x" (val512)); // No error.
 
   __asm__ volatile("foo1 %0" : "=R" (val)); // expected-error {{invalid output size for constraint '=R'}}
   __asm__ volatile("foo1 %0" : "=q" (val)); // expected-error {{invalid output size for constraint '=q'}}
@@ -60,13 +60,8 @@
   __asm__ volatile("foo1 %0" : "=A" (val128)); // expected-error {{invalid output size for constraint '=A'}}
   __asm__ volatile("foo1 %0" : "=t" (val256)); // expected-error {{invalid output size for constraint '=t'}}
   __asm__ volatile("foo1 %0" : "=u" (val256)); // expected-error {{invalid output size for constraint '=u'}}
-  __asm__ volatile("foo1 %0" : "=x" (val512)); // expected-error {{invalid output size for constraint '=x'}}
+  __asm__ volatile("foo1 %0" : "=x" (val512)); // No error.
 
-#ifdef __AVX__
   __asm__ volatile("foo1 %0" : : "x" (val256)); // No error.
   __asm__ volatile("foo1 %0" : "=x" (val256));  // No error.
-#else
-  __asm__ volatile("foo1 %0" : : "x" (val256)); // expected-error {{invalid input size for constraint 'x'}}
-  __asm__ volatile("foo1 %0" : "=x" (val256)); // expected-error {{invalid output size for constraint '=x'}}
-#endif
 }
Index: clang/test/CodeGen/x86_64-PR42672.c
===================================================================
--- clang/test/CodeGen/x86_64-PR42672.c
+++ clang/test/CodeGen/x86_64-PR42672.c
@@ -100,7 +100,7 @@
 #endif
 }
 
-// CHECK-IMPOSSIBLE_X: invalid output size for constraint
+// CHECK-IMPOSSIBLE_X: impossible constraint in asm: can't store value into a register
 
 // http://crbug.com/999160
 // Clang used to report the following message: