From ad7448fe982d83de15deec9c55c56d0cd9261c6c Mon Sep 17 00:00:00 2001 From: =?utf8?q?Martin=20Mo=CC=88hrmann?= Date: Sun, 10 Apr 2016 17:32:35 +0200 Subject: [PATCH] runtime: speed up makeslice by avoiding divisions MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit 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 TryBot-Result: Gobot Gobot Reviewed-by: Ian Lance Taylor --- src/runtime/append_test.go | 8 ++++++++ src/runtime/slice.go | 14 +++++++++++--- 2 files changed, 19 insertions(+), 3 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) diff --git a/src/runtime/slice.go b/src/runtime/slice.go index 4ab221056c..f36ec0b466 100644 --- a/src/runtime/slice.go +++ b/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} } -- 2.48.1