MS extension: Make __noop be the integer zero, not void

We still don't accept '__noop;', and we don't consider __noop to be the
integer literal zero.  More work is needed.

llvm-svn: 212839

clang/include/clang/Basic/Builtins.def

@@ -679,7 +679,7 @@ BUILTIN(__builtin_rindex, "c*cC*i", "Fn")
 // Microsoft builtins.  These are only active with -fms-extensions.
 LANGBUILTIN(_alloca,      "v*z", "n", ALL_MS_LANGUAGES)
 LANGBUILTIN(__assume,     "vb",  "n", ALL_MS_LANGUAGES)
-LANGBUILTIN(__noop,       "v.",  "n", ALL_MS_LANGUAGES)
+LANGBUILTIN(__noop,       "i.",  "n", ALL_MS_LANGUAGES)
 LANGBUILTIN(__debugbreak, "v",   "n", ALL_MS_LANGUAGES)
 LANGBUILTIN(__va_start,   "vc**.", "nt", ALL_MS_LANGUAGES)
 LANGBUILTIN(_InterlockedCompareExchange, "LiLiD*LiLi", "n", ALL_MS_LANGUAGES)

clang/lib/CodeGen/CGBuiltin.cpp

@@ -1515,7 +1515,8 @@ RValue CodeGenFunction::EmitBuiltinExpr(const FunctionDecl *FD,
     return EmitBuiltinNewDeleteCall(FD->getType()->castAs<FunctionProtoType>(),
                                     E->getArg(0), true);
   case Builtin::BI__noop:
-    return RValue::get(nullptr);
+    // __noop always evaluates to an integer literal zero.
+    return RValue::get(ConstantInt::get(IntTy, 0));
   case Builtin::BI_InterlockedExchange:
   case Builtin::BI_InterlockedExchangePointer:
     return EmitBinaryAtomic(*this, llvm::AtomicRMWInst::Xchg, E);

clang/test/CodeGen/builtin-ms-noop.cpp

@@ -5,9 +5,9 @@ class A {
   ~A() {}
 };
 
-void f() {
+int f() {
 // CHECK: @_Z1fv
 // CHECK-NOT: call void @_ZN1AD1Ev
-// CHECK: ret void
-  __noop(A());
+// CHECK: ret i32 0
+  return __noop(A());
 };