runtime: speed up growslice by avoiding divisions 2

This is a follow-up of https://go-review.googlesource.com/#/c/20653/

Special case computation for slices with elements of byte size or
pointer size.

name                      old time/op  new time/op  delta
GrowSliceBytes-4          86.2ns ± 3%  75.4ns ± 2%  -12.50%  (p=0.000 n=20+20)
GrowSliceInts-4            161ns ± 3%   136ns ± 3%  -15.59%  (p=0.000 n=19+19)
GrowSlicePtr-4             239ns ± 2%   233ns ± 2%   -2.52%  (p=0.000 n=20+20)
GrowSliceStruct24Bytes-4   258ns ± 3%   256ns ± 3%     ~     (p=0.134 n=20+20)

Change-Id: Ice5fa648058fe9d7fa89dee97ca359966f671128
Reviewed-on: https://go-review.googlesource.com/21101Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/append_test.go

@@ -9,7 +9,7 @@ const N = 20
 
 func BenchmarkGrowSliceBytes(b *testing.B) {
 	b.StopTimer()
-	var x = make([]byte, 8)
+	var x = make([]byte, 9)
 	b.StartTimer()
 	for i := 0; i < b.N; i++ {
 		_ = append([]byte(nil), x...)
@@ -18,13 +18,33 @@ func BenchmarkGrowSliceBytes(b *testing.B) {
 
 func BenchmarkGrowSliceInts(b *testing.B) {
 	b.StopTimer()
-	var x = make([]int, 8)
+	var x = make([]int, 9)
 	b.StartTimer()
 	for i := 0; i < b.N; i++ {
 		_ = append([]int(nil), x...)
 	}
 }
 
+func BenchmarkGrowSlicePtr(b *testing.B) {
+	b.StopTimer()
+	var x = make([]*byte, 9)
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		_ = append([]*byte(nil), x...)
+	}
+}
+
+type struct24 struct{ a, b, c int64 }
+
+func BenchmarkGrowSliceStruct24Bytes(b *testing.B) {
+	b.StopTimer()
+	var x = make([]struct24, 9)
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		_ = append([]struct24(nil), x...)
+	}
+}
+
 func BenchmarkAppend(b *testing.B) {
 	b.StopTimer()
 	x := make([]int, 0, N)

src/runtime/slice.go

@@ -56,11 +56,6 @@ func growslice(t *slicetype, old slice, cap int) slice {
 		return slice{unsafe.Pointer(&zerobase), old.len, cap}
 	}
 
-	maxcap := _MaxMem / et.size
-	if cap < old.cap || uintptr(cap) > maxcap {
-		panic(errorString("growslice: cap out of range"))
-	}
-
 	newcap := old.cap
 	doublecap := newcap + newcap
 	if cap > doublecap {
@@ -73,17 +68,30 @@ func growslice(t *slicetype, old slice, cap int) slice {
 				newcap += newcap / 4
 			}
 		}
-		if uintptr(newcap) > maxcap {
-			panic(errorString("growslice: cap out of range"))
-		}
 	}
 
-	lenmem := uintptr(old.len) * et.size
-	capmem := roundupsize(uintptr(newcap) * et.size)
-	if et.size == 1 {
+	var lenmem, capmem, maxcap uintptr
+	const ptrSize = unsafe.Sizeof((*byte)(nil))
+	switch et.size {
+	case 1:
+		lenmem = uintptr(old.len)
+		capmem = roundupsize(uintptr(newcap))
 		newcap = int(capmem)
-	} else {
+		maxcap = _MaxMem
+	case ptrSize:
+		lenmem = uintptr(old.len) * ptrSize
+		capmem = roundupsize(uintptr(newcap) * ptrSize)
+		newcap = int(capmem / ptrSize)
+		maxcap = _MaxMem / ptrSize
+	default:
+		lenmem = uintptr(old.len) * et.size
+		capmem = roundupsize(uintptr(newcap) * et.size)
 		newcap = int(capmem / et.size)
+		maxcap = _MaxMem / et.size
+	}
+
+	if cap < old.cap || uintptr(newcap) > maxcap {
+		panic(errorString("growslice: cap out of range"))
 	}
 
 	var p unsafe.Pointer