runtime: refactor hmap.extra.overflow array into two separate fields

This makes it easier to deduce from the field names which overflow
field corresponds to h.buckets and which to h.oldbuckets by aligning
the naming with the buckets fields in hmap.

Change-Id: I8d6a729229a190db0212bac012ead1a3c13cf5d0
Reviewed-on: https://go-review.googlesource.com/62411
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/gc/reflect.go

@@ -295,7 +295,8 @@ func hiter(t *types.Type) *types.Type {
 	//    h           *hmap
 	//    buckets     *bmap
 	//    bptr        *bmap
-	//    overflow    [2]unsafe.Pointer // [2]*[]*bmap
+	//    overflow    unsafe.Pointer // *[]*bmap
+	//    oldoverflow unsafe.Pointer // *[]*bmap
 	//    startBucket uintptr
 	//    offset      uint8
 	//    wrapped     bool
@@ -312,7 +313,8 @@ func hiter(t *types.Type) *types.Type {
 		makefield("h", types.NewPtr(hmap)),
 		makefield("buckets", types.NewPtr(bmap)),
 		makefield("bptr", types.NewPtr(bmap)),
-		makefield("overflow", types.NewArray(types.Types[TUNSAFEPTR], 2)),
+		makefield("overflow", types.Types[TUNSAFEPTR]),
+		makefield("oldoverflow", types.Types[TUNSAFEPTR]),
 		makefield("startBucket", types.Types[TUINTPTR]),
 		makefield("offset", types.Types[TUINT8]),
 		makefield("wrapped", types.Types[TBOOL]),

src/runtime/hashmap.go

@@ -126,12 +126,13 @@ type mapextra struct {
 	// If both key and value do not contain pointers and are inline, then we mark bucket
 	// type as containing no pointers. This avoids scanning such maps.
 	// However, bmap.overflow is a pointer. In order to keep overflow buckets
-	// alive, we store pointers to all overflow buckets in hmap.overflow.
-	// Overflow is used only if key and value do not contain pointers.
-	// overflow[0] contains overflow buckets for hmap.buckets.
-	// overflow[1] contains overflow buckets for hmap.oldbuckets.
+	// alive, we store pointers to all overflow buckets in hmap.overflow and h.map.oldoverflow.
+	// overflow and oldoverflow are only used if key and value do not contain pointers.
+	// overflow contains overflow buckets for hmap.buckets.
+	// oldoverflow contains overflow buckets for hmap.oldbuckets.
 	// The indirection allows to store a pointer to the slice in hiter.
-	overflow [2]*[]*bmap
+	overflow    *[]*bmap
+	oldoverflow *[]*bmap
 
 	// nextOverflow holds a pointer to a free overflow bucket.
 	nextOverflow *bmap
@@ -160,7 +161,8 @@ type hiter struct {
 	h           *hmap
 	buckets     unsafe.Pointer // bucket ptr at hash_iter initialization time
 	bptr        *bmap          // current bucket
-	overflow    [2]*[]*bmap    // keeps overflow buckets alive
+	overflow    *[]*bmap       // keeps overflow buckets of hmap.buckets alive
+	oldoverflow *[]*bmap       // keeps overflow buckets of hmap.oldbuckets alive
 	startBucket uintptr        // bucket iteration started at
 	offset      uint8          // intra-bucket offset to start from during iteration (should be big enough to hold bucketCnt-1)
 	wrapped     bool           // already wrapped around from end of bucket array to beginning
@@ -257,7 +259,7 @@ func (h *hmap) newoverflow(t *maptype, b *bmap) *bmap {
 	h.incrnoverflow()
 	if t.bucket.kind&kindNoPointers != 0 {
 		h.createOverflow()
-		*h.extra.overflow[0] = append(*h.extra.overflow[0], ovf)
+		*h.extra.overflow = append(*h.extra.overflow, ovf)
 	}
 	b.setoverflow(t, ovf)
 	return ovf
@@ -267,8 +269,8 @@ func (h *hmap) createOverflow() {
 	if h.extra == nil {
 		h.extra = new(mapextra)
 	}
-	if h.extra.overflow[0] == nil {
-		h.extra.overflow[0] = new([]*bmap)
+	if h.extra.overflow == nil {
+		h.extra.overflow = new([]*bmap)
 	}
 }
 
@@ -703,6 +705,7 @@ func mapiterinit(t *maptype, h *hmap, it *hiter) {
 		// while we are iterating.
 		h.createOverflow()
 		it.overflow = h.extra.overflow
+		it.oldoverflow = h.extra.oldoverflow
 	}
 
 	// decide where to start
@@ -904,13 +907,13 @@ func hashGrow(t *maptype, h *hmap) {
 	h.nevacuate = 0
 	h.noverflow = 0
 
-	if h.extra != nil && h.extra.overflow[0] != nil {
+	if h.extra != nil && h.extra.overflow != nil {
 		// Promote current overflow buckets to the old generation.
-		if h.extra.overflow[1] != nil {
-			throw("overflow is not nil")
+		if h.extra.oldoverflow != nil {
+			throw("oldoverflow is not nil")
 		}
-		h.extra.overflow[1] = h.extra.overflow[0]
-		h.extra.overflow[0] = nil
+		h.extra.oldoverflow = h.extra.overflow
+		h.extra.overflow = nil
 	}
 	if nextOverflow != nil {
 		if h.extra == nil {
@@ -1123,7 +1126,7 @@ func advanceEvacuationMark(h *hmap, t *maptype, newbit uintptr) {
 		// If they are still referenced by an iterator,
 		// then the iterator holds a pointers to the slice.
 		if h.extra != nil {
-			h.extra.overflow[1] = nil
+			h.extra.oldoverflow = nil
 		}
 		h.flags &^= sameSizeGrow
 	}