What version of Go are you using (go version)?

go version go1.12 darwin/amd64

Does this issue reproduce with the latest release?

Need to try.

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/hl/8pj040l94nlfzdfz8yzxjmr82d2t40/T/go-build481657979=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

package main

import "testing"

func BenchmarkCap(b *testing.B) {
	for i := 0; i < b.N; i++ {
		x := make([]string, 0, 500)
		for s := 0; s < 500; s++ {
			x = append(x, "taco")
		}
	}
}

func BenchmarkLen(b *testing.B) {
	for i := 0; i < b.N; i++ {
		x := make([]string, 500)
		for s := 0; s < 500; s++ {
			x[s] = "taco"
		}
	}
}

func BenchmarkLenReverse(b *testing.B) {
	for i := 0; i < b.N; i++ {
		x := make([]string, 500)
		for s := 499; s >= 0; s-- {
			x[s] = "taco"
		}
	}
}

What did you expect to see?

I figured the second and third benchmarks would be about the same

What did you see instead?

Going through the array in reverse is consistently a tiny bit faster. Is this just a quirk of my CPU architecture? I tried switching it around a little to use 1 instead of 0 for the comparison and that narrowed the gap slightly but in reverse was still consistently faster.