runtime: change Note from union to struct

Unions can break precise GC.
Update #5193.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/8362046

src/pkg/runtime/lock_futex.c

@@ -111,9 +111,9 @@ runtime·noteclear(Note *n)
 void
 runtime·notewakeup(Note *n)
 {
-	if(runtime·xchg(&n->key, 1))
+	if(runtime·xchg((uint32*)&n->key, 1))
 		runtime·throw("notewakeup - double wakeup");
-	runtime·futexwakeup(&n->key, 1);
+	runtime·futexwakeup((uint32*)&n->key, 1);
 }
 
 void
@@ -121,8 +121,8 @@ runtime·notesleep(Note *n)
 {
 	if(m->profilehz > 0)
 		runtime·setprof(false);
-	while(runtime·atomicload(&n->key) == 0)
-		runtime·futexsleep(&n->key, 0, -1);
+	while(runtime·atomicload((uint32*)&n->key) == 0)
+		runtime·futexsleep((uint32*)&n->key, 0, -1);
 	if(m->profilehz > 0)
 		runtime·setprof(true);
 }
@@ -137,15 +137,15 @@ runtime·notetsleep(Note *n, int64 ns)
 		return;
 	}
 
-	if(runtime·atomicload(&n->key) != 0)
+	if(runtime·atomicload((uint32*)&n->key) != 0)
 		return;
 
 	if(m->profilehz > 0)
 		runtime·setprof(false);
 	deadline = runtime·nanotime() + ns;
 	for(;;) {
-		runtime·futexsleep(&n->key, 0, ns);
-		if(runtime·atomicload(&n->key) != 0)
+		runtime·futexsleep((uint32*)&n->key, 0, ns);
+		if(runtime·atomicload((uint32*)&n->key) != 0)
 			break;
 		now = runtime·nanotime();
 		if(now >= deadline)

src/pkg/runtime/lock_sema.c

@@ -118,7 +118,7 @@ runtime·unlock(Lock *l)
 void
 runtime·noteclear(Note *n)
 {
-	n->waitm = nil;
+	n->key = 0;
 }
 
 void
@@ -127,8 +127,8 @@ runtime·notewakeup(Note *n)
 	M *mp;
 
 	do
-		mp = runtime·atomicloadp(&n->waitm);
-	while(!runtime·casp(&n->waitm, mp, (void*)LOCKED));
+		mp = runtime·atomicloadp((void**)&n->key);
+	while(!runtime·casp((void**)&n->key, mp, (void*)LOCKED));
 
 	// Successfully set waitm to LOCKED.
 	// What was it before?
@@ -148,8 +148,8 @@ runtime·notesleep(Note *n)
 {
 	if(m->waitsema == 0)
 		m->waitsema = runtime·semacreate();
-	if(!runtime·casp(&n->waitm, nil, m)) {  // must be LOCKED (got wakeup)
-		if(n->waitm != (void*)LOCKED)
+	if(!runtime·casp((void**)&n->key, nil, m)) {  // must be LOCKED (got wakeup)
+		if(n->key != LOCKED)
 			runtime·throw("notesleep - waitm out of sync");
 		return;
 	}
@@ -176,8 +176,8 @@ runtime·notetsleep(Note *n, int64 ns)
 		m->waitsema = runtime·semacreate();
 
 	// Register for wakeup on n->waitm.
-	if(!runtime·casp(&n->waitm, nil, m)) {  // must be LOCKED (got wakeup already)
-		if(n->waitm != (void*)LOCKED)
+	if(!runtime·casp((void**)&n->key, nil, m)) {  // must be LOCKED (got wakeup already)
+		if(n->key != LOCKED)
 			runtime·throw("notetsleep - waitm out of sync");
 		return;
 	}
@@ -212,10 +212,10 @@ runtime·notetsleep(Note *n, int64 ns)
 	// so that any notewakeup racing with the return does not
 	// try to grant us the semaphore when we don't expect it.
 	for(;;) {
-		mp = runtime·atomicloadp(&n->waitm);
+		mp = runtime·atomicloadp((void**)&n->key);
 		if(mp == m) {
 			// No wakeup yet; unregister if possible.
-			if(runtime·casp(&n->waitm, mp, nil))
+			if(runtime·casp((void**)&n->key, mp, nil))
 				return;
 		} else if(mp == (M*)LOCKED) {
 			// Wakeup happened so semaphore is available.

src/pkg/runtime/runtime.h

@@ -54,7 +54,7 @@ typedef	struct	Lock		Lock;
 typedef	struct	M		M;
 typedef	struct	P		P;
 typedef	struct	Mem		Mem;
-typedef	union	Note		Note;
+typedef	struct	Note		Note;
 typedef	struct	Slice		Slice;
 typedef	struct	Stktop		Stktop;
 typedef	struct	String		String;
@@ -163,10 +163,12 @@ struct	Lock
 	// Used to be a union, but unions break precise GC.
 	uintptr	key;
 };
-union	Note
+struct	Note
 {
-	uint32	key;	// futex-based impl
-	M*	waitm;	// waiting M (sema-based impl)
+	// Futex-based impl treats it as uint32 key,
+	// while sema-based impl as M* waitm.
+	// Used to be a union, but unions break precise GC.
+	uintptr	key;
 };
 struct String
 {