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

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.

Reviewers: eli.friedman, thakis

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66948

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@370444 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/CGStmt.cpp

@@ -2326,7 +2326,7 @@ void CodeGenFunction::EmitAsmStmt(const AsmStmt &S) {
         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);

test/CodeGen/x86_64-PR42672.c

@@ -4,6 +4,7 @@
 // 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) {
@@ -57,7 +58,7 @@ void odd_struct(void) {
       : "=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) {
@@ -69,7 +70,7 @@ void big_struct(void) {
       : "=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() {
@@ -100,3 +101,17 @@ void x_constraint_nofit() {
 }
 
 // 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