[clang][bytecode] Implement __noop (llvm#106714)

This does nothing and returns 0.

Author: tbaederr

clang/lib/AST/ByteCode/Function.cpp

@@ -61,7 +61,8 @@ bool Function::isVirtual() const {
 static bool isUnevaluatedBuiltin(unsigned BuiltinID) {
   return BuiltinID == Builtin::BI__builtin_classify_type ||
          BuiltinID == Builtin::BI__builtin_os_log_format_buffer_size ||
-         BuiltinID == Builtin::BI__builtin_constant_p;
+         BuiltinID == Builtin::BI__builtin_constant_p ||
+         BuiltinID == Builtin::BI__noop;
 }
 
 bool Function::isUnevaluatedBuiltin() const {

clang/lib/AST/ByteCode/InterpBuiltin.cpp

@@ -1593,6 +1593,10 @@ bool InterpretBuiltin(InterpState &S, CodePtr OpPC, const Function *F,
       return false;
     break;
 
+  case Builtin::BI__noop:
+    pushInteger(S, 0, Call->getType());
+    break;
+
   default:
     S.FFDiag(S.Current->getLocation(OpPC),
              diag::note_invalid_subexpr_in_const_expr)

clang/test/AST/ByteCode/ms.cpp

@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -verify=ref,both %s -fms-extensions
-// RUN: %clang_cc1 -verify=expected,both %s -fexperimental-new-constant-interpreter -fms-extensions
+// RUN: %clang_cc1 -verify=ref,both %s -fms-extensions -fcxx-exceptions
+// RUN: %clang_cc1 -verify=expected,both %s -fexperimental-new-constant-interpreter -fms-extensions -fcxx-exceptions
 
 // ref-no-diagnostics
 // expected-no-diagnostics
@@ -8,3 +8,14 @@
 static_assert(_rotl(0x01, 5) == 32);
 
 static_assert(alignof(__unaligned int) == 1, "");
+
+static_assert(__noop() == 0, "");
+
+constexpr int noopIsActuallyNoop() {
+    int a = 0;
+    __noop(throw);
+    __noop(++a);
+    __noop(a = 100);
+    return a;
+}
+static_assert(noopIsActuallyNoop() == 0);