runtime: speed up makeslice by avoiding divisions

Only compute the number of maximum allowed elements per slice once.

name         old time/op  new time/op  delta
MakeSlice-2  55.5ns ± 1%  45.6ns ± 2%  -17.88%  (p=0.000 n=99+100)

Change-Id: I951feffda5d11910a75e55d7e978d306d14da2c5
Reviewed-on: https://go-review.googlesource.com/21801
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/append_test.go

@@ -7,6 +7,14 @@ import "testing"
 
 const N = 20
 
+func BenchmarkMakeSlice(b *testing.B) {
+	var x []byte
+	for i := 0; i < b.N; i++ {
+		x = make([]byte, 32)
+		_ = x
+	}
+}
+
 func BenchmarkGrowSliceBytes(b *testing.B) {
 	b.StopTimer()
 	var x = make([]byte, 9)

src/runtime/slice.go

@@ -16,19 +16,27 @@ type slice struct {
 
 // TODO: take uintptrs instead of int64s?
 func makeslice(t *slicetype, len64, cap64 int64) slice {
-	// NOTE: The len > MaxMem/elemsize check here is not strictly necessary,
+	// NOTE: The len > maxElements check here is not strictly necessary,
 	// but it produces a 'len out of range' error instead of a 'cap out of range' error
 	// when someone does make([]T, bignumber). 'cap out of range' is true too,
 	// but since the cap is only being supplied implicitly, saying len is clearer.
 	// See issue 4085.
+
+	maxElements := ^uintptr(0)
+	if t.elem.size > 0 {
+		maxElements = _MaxMem / t.elem.size
+	}
+
 	len := int(len64)
-	if len64 < 0 || int64(len) != len64 || t.elem.size > 0 && uintptr(len) > _MaxMem/t.elem.size {
+	if len64 < 0 || int64(len) != len64 || uintptr(len) > maxElements {
 		panic(errorString("makeslice: len out of range"))
 	}
+
 	cap := int(cap64)
-	if cap < len || int64(cap) != cap64 || t.elem.size > 0 && uintptr(cap) > _MaxMem/t.elem.size {
+	if cap < len || int64(cap) != cap64 || uintptr(cap) > maxElements {
 		panic(errorString("makeslice: cap out of range"))
 	}
+
 	p := newarray(t.elem, uintptr(cap))
 	return slice{p, len, cap}
 }