[CodeGen]: fix error message for "=r" asm constraint

[glider]

Summary

Nico Weber reported that the following code:

char buf[9];
asm("" : "=r" (buf));

yields the "impossible constraint in asm: can't store struct into a register"
error message, although |buf| is not a struct (see
http://crbug.com/999160).

Make the error message more generic and add a test for it.
Also make sure other tests in x86_64-PR42672.c check for the full error
message.

[glider]

clang-format

[glider]

Minor comment fix.

clang/lib/CodeGen/CGStmt.cpp

if ((i < ResultTypeRequiresCast.size()) && ResultTypeRequiresCast[i]) {
       unsigned Size = getContext().getTypeSize(ResultRegQualTys[i]);
       Address A = Builder.CreateBitCast(Dest.getAddress(),
                                         ResultRegTypes[i]->getPointerTo());
       QualType Ty = getContext().getIntTypeForBitwidth(Size, /*Signed*/ false);
       if (Ty.isNull()) {
         const Expr *OutExpr = S.getOutputExpr(i);
         CGM.Error(
             OutExpr->getExprLoc(),
-            "impossible constraint in asm: can't store struct into a register");
+            "impossible constraint in asm: can't store value into a register");
         return;
       }
       Dest = MakeAddrLValue(A, Ty);
     }
     EmitStoreThroughLValue(RValue::get(Tmp), Dest);
   }
 }
 

clang/test/CodeGen/x86_64-PR42672.c

 // RUN: %clang_cc1 -triple=x86_64-unknown-linux-gnu -DSTRUCT -emit-llvm %s -o - 2>&1 | FileCheck %s --check-prefix=CHECK-STRUCT
 // RUN: %clang_cc1 -triple=x86_64-unknown-linux-gnu -USTRUCT -emit-llvm %s -o - 2>&1 | FileCheck %s --check-prefix=CHECK-NOSTRUCT
 // RUN: not %clang_cc1 -triple=x86_64-unknown-linux-gnu -DIMPOSSIBLE_ODD -emit-llvm %s -o - 2>&1 | FileCheck %s --check-prefix=CHECK-IMPOSSIBLE_ODD
 // RUN: not %clang_cc1 -triple=x86_64-unknown-linux-gnu -DIMPOSSIBLE_BIG -emit-llvm %s -o - 2>&1 | FileCheck %s --check-prefix=CHECK-IMPOSSIBLE_BIG
 // RUN: %clang_cc1 -triple=x86_64-unknown-linux-gnu -DPOSSIBLE_X -emit-llvm %s -o - 2>&1 | FileCheck %s --check-prefix=CHECK-X
 // RUN: not %clang_cc1 -triple=x86_64-unknown-linux-gnu -DIMPOSSIBLE_X -emit-llvm %s -o - 2>&1 | FileCheck %s --check-prefix=CHECK-IMPOSSIBLE_X
+// RUN: not %clang_cc1 -triple=x86_64-unknown-linux-gnu -DIMPOSSIBLE_9BYTES -emit-llvm %s -o - 2>&1 | FileCheck %s --check-prefix=CHECK-IMPOSSIBLE_9BYTES
 
 // Make sure Clang doesn't treat |lockval| as asm input.
 void _raw_spin_lock(void) {
 #ifdef STRUCT
   struct {
     unsigned short owner, next;
   } lockval;
   lockval.owner = 1;
#ifdef IMPOSSIBLE_ODD
   struct __attribute__((__packed__)) {
     unsigned short first;
     unsigned char second;
   } str;
   asm("nop"
       : "=r"(str));
 #endif
 }
-// CHECK-IMPOSSIBLE_ODD: impossible constraint in asm
+// CHECK-IMPOSSIBLE_ODD: impossible constraint in asm: can't store value into a register
 
 // Check Clang reports an error if attempting to return a big structure via a register.
 void big_struct(void) {
 #ifdef IMPOSSIBLE_BIG
   struct {
     long long int v1, v2, v3, v4;
   } str;
   asm("nop"
       : "=r"(str));
 #endif
 }
-// CHECK-IMPOSSIBLE_BIG: impossible constraint in asm
+// CHECK-IMPOSSIBLE_BIG: impossible constraint in asm: can't store value into a register
 
 // Clang is able to emit LLVM IR for an 16-byte structure.
 void x_constraint_fit() {
 #ifdef POSSIBLE_X
   struct S {
     unsigned x[4];
   } z;
   asm volatile("nop"
struct S {
     unsigned x[8];
   } z;
   asm volatile("nop"
                : "=x"(z));
 #endif
 }
 
 // CHECK-IMPOSSIBLE_X: invalid output size for constraint
+
+// http://crbug.com/999160
+// Clang used to report the following message
+//   impossible constraint in asm: can't store struct into a register
+// for the assembly directive below, although there's no struct.
+void crbug_999160_regtest() {
+#ifdef IMPOSSIBLE_9BYTES
+  char buf[9];
+  asm(""
+      : "=r"(buf));
+#endif
+}
+
+// CHECK-IMPOSSIBLE_9BYTES: impossible constraint in asm: can't store value into a register