cmd/gc: add //go:nowritebarrier to diagnose unintended write barriers

//go:nowritebarrier can only be used in package runtime.
It does not disable write barriers; it is an assertion, checked
by the compiler, that the following function needs no write
barriers.

Change-Id: Id7978b779b66dc1feea39ee6bda9fd4d80280b7c
Reviewed-on: https://go-review.googlesource.com/1224Reviewed-by: Rick Hudson <rlh@golang.org>

src/cmd/gc/go.h

@@ -271,6 +271,7 @@ struct	Node
 	uchar	noescape;	// func arguments do not escape
 	uchar	nosplit;	// func should not execute on separate stack
 	uchar	builtin;	// built-in name, like len or close
+	uchar	nowritebarrier;	// emit compiler error instead of write barrier
 	uchar	walkdef;
 	uchar	typecheck;
 	uchar	local;
@@ -987,6 +988,7 @@ EXTERN	int	flag_race;
 EXTERN	int	flag_largemodel;
 EXTERN	int	noescape;
 EXTERN	int	nosplit;
+EXTERN	int	nowritebarrier;
 EXTERN	int	debuglive;
 EXTERN	Link*	ctxt;
 

src/cmd/gc/go.y

@@ -1319,6 +1319,7 @@ xfndcl:
 		$$->endlineno = lineno;
 		$$->noescape = noescape;
 		$$->nosplit = nosplit;
+		$$->nowritebarrier = nowritebarrier;
 		funcbody($$);
 	}
 
@@ -1502,6 +1503,7 @@ xdcl_list:
 		nointerface = 0;
 		noescape = 0;
 		nosplit = 0;
+		nowritebarrier = 0;
 	}
 
 vardcl_list:

src/cmd/gc/lex.c

@@ -1655,6 +1655,12 @@ go:
 		nosplit = 1;
 		goto out;
 	}
+	if(strcmp(lexbuf, "go:nowritebarrier") == 0) {
+		if(!compiling_runtime)
+			yyerror("//go:nowritebarrier only allowed in runtime");
+		nowritebarrier = 1;
+		goto out;
+	}
 	
 out:
 	return c;

src/cmd/gc/walk.c

@@ -2010,6 +2010,8 @@ applywritebarrier(Node *n, NodeList **init)
 	char name[32];
 
 	if(n->left && n->right && needwritebarrier(n->left, n->right)) {
+		if(curfn && curfn->nowritebarrier)
+			yyerror("write barrier prohibited");
 		t = n->left->type;
 		l = nod(OADDR, n->left, N);
 		l->etype = 1; // addr does not escape

src/runtime/proc1.go

@@ -1577,6 +1577,7 @@ func goexit0(gp *g) {
 }
 
 //go:nosplit
+//go:nowritebarrier
 func save(pc, sp uintptr) {
 	_g_ := getg()
 
@@ -1585,7 +1586,7 @@ func save(pc, sp uintptr) {
 	_g_.sched.lr = 0
 	_g_.sched.ret = 0
 	_g_.sched.ctxt = nil
-	// write as uintptr to avoid write barrier, which will smash _g_.sched.
+	// _g_.sched.g = _g_, but avoid write barrier, which smashes _g_.sched
 	*(*uintptr)(unsafe.Pointer(&_g_.sched.g)) = uintptr(unsafe.Pointer(_g_))
 }