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>

1 files changed, 8 insertions, 0 deletions

diff --git a/src/runtime/append_test.go b/src/runtime/append_test.go
index 3170870b0e..6d7836a351 100644
--- a/src/runtime/append_test.go
+++ b/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)