runtime: random offset for map iteration

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5285042

doc/go_spec.html

@@ -4085,7 +4085,8 @@ a single byte in the string.
 </li>
 
 <li>
-The iteration order over maps is not specified.
+The iteration order over maps is not specified
+and is not guaranteed to be the same from one iteration to the next.
 If map entries that have not yet been reached are deleted during iteration,
 the corresponding iteration values will not be produced. If map entries are
 inserted during iteration, the behavior is implementation-dependent, but the

src/cmd/gc/go.h

@@ -80,16 +80,18 @@ typedef	struct	Hiter	Hiter;
 struct	Hiter
 {
 	uchar	data[8];		// return val from next
-	int32	elemsize;		// size of elements in table */
-	int32	changes;		// number of changes observed last time */
-	int32	i;			// stack pointer in subtable_state */
-	uchar	last[8];		// last hash value returned */
-	uchar	h[8];			// the hash table */
+	int32	elemsize;		// size of elements in table
+	int32	changes;		// number of changes observed last time
+	int32	i;			// stack pointer in subtable_state
+	int32	cycled;		// actually a bool but pad for next field, a pointer
+	uchar	last[8];		// last hash value returned
+	uchar	cycle[8];		// the value where we started and will stop
+	uchar	h[8];			// the hash table
 	struct
 	{
-		uchar	sub[8];		// pointer into subtable */
-		uchar	start[8];	// pointer into start of subtable */
-		uchar	end[8];		// pointer into end of subtable */
+		uchar	sub[8];		// pointer into subtable
+		uchar	start[8];	// pointer into start of subtable
+		uchar	end[8];		// pointer into end of subtable
 		uchar	pad[8];
 	} sub[4];
 };

src/pkg/runtime/hashmap.c

@@ -506,20 +506,27 @@ iter_restart (struct hash_iter *it, struct hash_subtable *st, int32 used)
 static void *
 hash_next (struct hash_iter *it)
 {
-	int32 elemsize = it->elemsize;
-	struct hash_iter_sub *sub = &it->subtable_state[it->i];
-	struct hash_entry *e = sub->e;
-	struct hash_entry *last = sub->last;
-	hash_hash_t e_hash = 0;
+	int32 elemsize;
+	struct hash_iter_sub *sub;
+	struct hash_entry *e;
+	struct hash_entry *last;
+	hash_hash_t e_hash;
 
 	if (it->changes != it->h->changes) {    /* hash table's structure changed; recompute */
+		if (~it->last_hash == 0)
+			return (0);
 		it->changes = it->h->changes;
 		it->i = 0;
 		iter_restart (it, it->h->st, 0);
+	}
+	elemsize = it->elemsize;
+
+Again:
+	e_hash = 0;
 	sub = &it->subtable_state[it->i];
 	e = sub->e;
 	last = sub->last;
-	}
+
 	if (e != sub->start && it->last_hash != HASH_OFFSET (e, -elemsize)->hash) {
 		struct hash_entry *start = HASH_OFFSET (e, -(elemsize * it->h->max_probes));
 		struct hash_entry *pe = HASH_OFFSET (e, -elemsize);
@@ -542,8 +549,20 @@ hash_next (struct hash_iter *it)
 		}
 		if (e > last) {
 			if (it->i == 0) {
-				it->last_hash = HASH_OFFSET (e, -elemsize)->hash;
-				sub->e = e;
+				if(!it->cycled) {
+					// Wrap to zero and iterate up until it->cycle.
+					it->cycled = true;
+					it->last_hash = 0;
+					it->subtable_state[0].e = it->h->st->entry;
+					it->subtable_state[0].start = it->h->st->entry;
+					it->subtable_state[0].last = it->h->st->last;
+					goto Again;
+				}
+				// Set last_hash to impossible value and
+				// break it->changes, so that check at top of
+				// hash_next will be used if we get called again.
+				it->last_hash = ~(uintptr_t)0;
+				it->changes--;
 				return (0);
 			} else {
 				it->i--;
@@ -552,6 +571,15 @@ hash_next (struct hash_iter *it)
 				last = sub->last;
 			}
 		} else if ((e_hash & HASH_MASK) != HASH_SUBHASH) {
+			if(it->cycled && e->hash > it->cycle) {
+				// Already returned this.
+				// Set last_hash to impossible value and
+				// break it->changes, so that check at top of
+				// hash_next will be used if we get called again.
+				it->last_hash = ~(uintptr_t)0;
+				it->changes--;
+				return (0);
+			}
 			it->last_hash = e->hash;
 			sub->e = HASH_OFFSET (e, elemsize);
 			return (e->data);
@@ -581,6 +609,17 @@ hash_iter_init (Hmap *h, struct hash_iter *it)
 	it->subtable_state[0].e = h->st->entry;
 	it->subtable_state[0].start = h->st->entry;
 	it->subtable_state[0].last = h->st->last;
+	
+	// fastrand1 returns 31 useful bits.
+	// We don't care about not having a bottom bit but we
+	// do want top bits.
+	if(sizeof(void*) == 8)
+		it->cycle = (uint64)runtime·fastrand1()<<33 | (uint64)runtime·fastrand1()<<2;
+	else
+		it->cycle = runtime·fastrand1()<<1;
+	it->cycled = false;
+	it->last_hash = it->cycle;
+	iter_restart(it, it->h->st, 0);
 }
 
 static void

src/pkg/runtime/hashmap.h

@@ -82,7 +82,9 @@ struct hash_iter {
 	int32	elemsize;	/* size of elements in table */
 	int32	changes;	/* number of changes observed last time */
 	int32	i;		/* stack pointer in subtable_state */
+	bool cycled;		/* have reached the end and wrapped to 0 */
 	hash_hash_t last_hash;	/* last hash value returned */
+	hash_hash_t cycle;	/* hash value where we started */
 	struct Hmap *h;		/* the hash table */
 	struct hash_iter_sub {
 		struct hash_entry *e;		/* pointer into subtable */