runtime: refactor evacuate x/y handling

This change unifies the x and y cases.

It shrinks evacuate's machine code by ~25% and its stack size by ~15%.

It also eliminates a critical branch.
Whether an entry should go to x or y is designed to be unpredictable.
As a result, half of the branch predictions for useX were wrong.
Mispredicting that branch can easily incur an expensive cache miss.
Switching to an xy array allows elimination of that branch,
which in turn reduces cache misses.

Change-Id: Ie9cef53744b96c724c377ac0985b487fc50b49b1
Reviewed-on: https://go-review.googlesource.com/54653
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/hashmap.go

@@ -298,6 +298,10 @@ func makemap(t *maptype, hint int64, h *hmap, bucket unsafe.Pointer) *hmap {
 	if dataOffset%uintptr(t.elem.align) != 0 {
 		throw("need padding in bucket (value)")
 	}
+	if evacuatedX+1 != evacuatedY {
+		// evacuate relies on this relationship
+		throw("bad evacuatedN")
+	}
 
 	// find size parameter which will hold the requested # of elements
 	B := uint8(0)
@@ -1044,24 +1048,30 @@ func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
 		// TODO: reuse overflow buckets instead of using new ones, if there
 		// is no iterator using the old buckets.  (If !oldIterator.)
 
-		var (
-			x, y   *bmap          // current low/high buckets in new map
-			xi, yi int            // key/val indices into x and y
-			xk, yk unsafe.Pointer // pointers to current x and y key storage
-			xv, yv unsafe.Pointer // pointers to current x and y value storage
-		)
-		x = (*bmap)(add(h.buckets, oldbucket*uintptr(t.bucketsize)))
-		xi = 0
-		xk = add(unsafe.Pointer(x), dataOffset)
-		xv = add(xk, bucketCnt*uintptr(t.keysize))
+		// evacDst is an evacuation destination.
+		type evacDst struct {
+			b *bmap          // current destination bucket
+			i int            // key/val index into b
+			k unsafe.Pointer // pointer to current key storage
+			v unsafe.Pointer // pointer to current value storage
+		}
+
+		// xy contains the x and y (low and high) evacuation destinations.
+		var xy [2]evacDst
+		x := &xy[0]
+		x.b = (*bmap)(add(h.buckets, oldbucket*uintptr(t.bucketsize)))
+		x.k = add(unsafe.Pointer(x.b), dataOffset)
+		x.v = add(x.k, bucketCnt*uintptr(t.keysize))
+
 		if !h.sameSizeGrow() {
 			// Only calculate y pointers if we're growing bigger.
 			// Otherwise GC can see bad pointers.
-			y = (*bmap)(add(h.buckets, (oldbucket+newbit)*uintptr(t.bucketsize)))
-			yi = 0
-			yk = add(unsafe.Pointer(y), dataOffset)
-			yv = add(yk, bucketCnt*uintptr(t.keysize))
+			y := &xy[1]
+			y.b = (*bmap)(add(h.buckets, (oldbucket+newbit)*uintptr(t.bucketsize)))
+			y.k = add(unsafe.Pointer(y.b), dataOffset)
+			y.v = add(y.k, bucketCnt*uintptr(t.keysize))
 		}
+
 		for ; b != nil; b = b.overflow(t) {
 			k := add(unsafe.Pointer(b), dataOffset)
 			v := add(k, bucketCnt*uintptr(t.keysize))
@@ -1078,7 +1088,7 @@ func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
 				if t.indirectkey {
 					k2 = *((*unsafe.Pointer)(k2))
 				}
-				useX := true
+				var useY uint8
 				if !h.sameSizeGrow() {
 					// Compute hash to make our evacuation decision (whether we need
 					// to send this key/value to bucket x or bucket y).
@@ -1105,54 +1115,37 @@ func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
 							top += minTopHash
 						}
 					}
-					useX = hash&newbit == 0
-				}
-				if useX {
-					b.tophash[i] = evacuatedX
-					if xi == bucketCnt {
-						newx := h.newoverflow(t, x)
-						x = newx
-						xi = 0
-						xk = add(unsafe.Pointer(x), dataOffset)
-						xv = add(xk, bucketCnt*uintptr(t.keysize))
+					if hash&newbit != 0 {
+						useY = 1
 					}
-					x.tophash[xi] = top
-					if t.indirectkey {
-						*(*unsafe.Pointer)(xk) = k2 // copy pointer
-					} else {
-						typedmemmove(t.key, xk, k) // copy value
 				}
-					if t.indirectvalue {
-						*(*unsafe.Pointer)(xv) = *(*unsafe.Pointer)(v)
-					} else {
-						typedmemmove(t.elem, xv, v)
+
+				b.tophash[i] = evacuatedX + useY // evacuatedX + 1 == evacuatedY, enforced in makemap
+				dst := &xy[useY]                 // evacuation destination
+
+				if dst.i == bucketCnt {
+					dst.b = h.newoverflow(t, dst.b)
+					dst.i = 0
+					dst.k = add(unsafe.Pointer(dst.b), dataOffset)
+					dst.v = add(dst.k, bucketCnt*uintptr(t.keysize))
 				}
-					xi++
-					xk = add(xk, uintptr(t.keysize))
-					xv = add(xv, uintptr(t.valuesize))
-				} else {
-					b.tophash[i] = evacuatedY
-					if yi == bucketCnt {
-						newy := h.newoverflow(t, y)
-						y = newy
-						yi = 0
-						yk = add(unsafe.Pointer(y), dataOffset)
-						yv = add(yk, bucketCnt*uintptr(t.keysize))
-					}
-					y.tophash[yi] = top
+				dst.b.tophash[dst.i] = top
 				if t.indirectkey {
-						*(*unsafe.Pointer)(yk) = k2
+					*(*unsafe.Pointer)(dst.k) = k2 // copy pointer
 				} else {
-						typedmemmove(t.key, yk, k)
+					typedmemmove(t.key, dst.k, k) // copy value
 				}
 				if t.indirectvalue {
-						*(*unsafe.Pointer)(yv) = *(*unsafe.Pointer)(v)
+					*(*unsafe.Pointer)(dst.v) = *(*unsafe.Pointer)(v)
 				} else {
-						typedmemmove(t.elem, yv, v)
-					}
-					yi++
-					yk = add(yk, uintptr(t.keysize))
-					yv = add(yv, uintptr(t.valuesize))
+					typedmemmove(t.elem, dst.v, v)
+				}
+				dst.i++
+				// If we're at the end of the bucket, don't update k/v,
+				// to avoid pointers pointing past the end of the bucket.
+				if dst.i < bucketCnt {
+					dst.k = add(dst.k, uintptr(t.keysize))
+					dst.v = add(dst.v, uintptr(t.valuesize))
 				}
 			}
 		}