runtime: speed up growslice by avoiding divisions

Only compute the number of maximum allowed elements per slice once.
Special case newcap computation for slices with byte sized elements.

name              old time/op  new time/op  delta
GrowSliceBytes-2  61.1ns ± 1%  43.4ns ± 1%  -29.00%  (p=0.000 n=20+20)
GrowSliceInts-2   85.9ns ± 1%  75.7ns ± 1%  -11.80%  (p=0.000 n=20+20)

Change-Id: I5d9c0d5987cdd108ac29dc32e31912dcefa2324d
Reviewed-on: https://go-review.googlesource.com/20653Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/append_test.go

@@ -7,6 +7,24 @@ import "testing"
 
 const N = 20
 
+func BenchmarkGrowSliceBytes(b *testing.B) {
+	b.StopTimer()
+	var x = make([]byte, 8)
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		_ = append([]byte(nil), x...)
+	}
+}
+
+func BenchmarkGrowSliceInts(b *testing.B) {
+	b.StopTimer()
+	var x = make([]int, 8)
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		_ = append([]int(nil), x...)
+	}
+}
+
 func BenchmarkAppend(b *testing.B) {
 	b.StopTimer()
 	x := make([]int, 0, N)

src/runtime/slice.go

@@ -38,10 +38,6 @@ func makeslice(t *slicetype, len64, cap64 int64) slice {
 // and it returns a new slice with at least that capacity, with the old data
 // copied into it.
 func growslice(t *slicetype, old slice, cap int) slice {
-	if cap < old.cap || t.elem.size > 0 && uintptr(cap) > _MaxMem/t.elem.size {
-		panic(errorString("growslice: cap out of range"))
-	}
-
 	if raceenabled {
 		callerpc := getcallerpc(unsafe.Pointer(&t))
 		racereadrangepc(old.array, uintptr(old.len*int(t.elem.size)), callerpc, funcPC(growslice))
@@ -52,11 +48,19 @@ func growslice(t *slicetype, old slice, cap int) slice {
 
 	et := t.elem
 	if et.size == 0 {
+		if cap < old.cap {
+			panic(errorString("growslice: cap out of range"))
+		}
 		// append should not create a slice with nil pointer but non-zero len.
 		// We assume that append doesn't need to preserve old.array in this case.
 		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
 	if newcap+newcap < cap {
 		newcap = cap
@@ -73,12 +77,18 @@ func growslice(t *slicetype, old slice, cap int) slice {
 		}
 	}
 
-	if uintptr(newcap) >= _MaxMem/et.size {
+	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 {
+		newcap = int(capmem)
+	} else {
 		newcap = int(capmem / et.size)
+	}
+
 	var p unsafe.Pointer
 	if et.kind&kindNoPointers != 0 {
 		p = rawmem(capmem)