1. What is a short input program that triggers the error?
package p
import (
"testing"
)
var slice = make([]byte, 256)
func loop() (t byte) {
for i := 0; i < len(slice); i++ {
t ^= slice[i]
}
return
}
func rangeloop() (t byte) {
for _, b := range slice {
t ^= b
}
return
}
func BenchmarkLoop(b *testing.B) {
for i := 0; i < b.N; i++ {
loop()
}
}
func BenchmarkRange(b *testing.B) {
for i := 0; i < b.N; i++ {
rangeloop()
}
}
2. What is the full compiler output?
GOARCH=386:
BenchmarkLoop 1000000 1239 ns/op
BenchmarkRange 5000000 695 ns/op
The generated range code is nearly optimal:
--- prog list "rangeloop" ---
[...]
0032 (loop_test.go:17) LEAL autotmp_0005+-12(SP),BX
0033 (loop_test.go:17) MOVL (BX),CX
0034 (loop_test.go:17) JMP ,36
0035 (loop_test.go:17) INCL ,AX
0036 (loop_test.go:17) CMPL AX,SI
0037 (loop_test.go:17) JGE $0,42
0038 (loop_test.go:17) MOVBLZX (CX),BP
0039 (loop_test.go:17) INCL ,CX
0040 (loop_test.go:18) XORB BP,t+0(FP)
0041 (loop_test.go:17) JMP ,35
0042 (loop_test.go:20) RET ,
GOARCH=amd64:
BenchmarkLoop 5000000 341 ns/op
BenchmarkRange 5000000 533 ns/op
The generate code for the loop body is quite shocking:
--- prog list "rangeloop" ---
[...]
0031 (loop_test.go:17) LEAQ autotmp_0000+-16(SP),BX
0032 (loop_test.go:17) MOVQ (BX),CX
0033 (loop_test.go:17) JMP ,35
0034 (loop_test.go:17) INCL ,AX
0035 (loop_test.go:17) MOVL DI,BP
0036 (loop_test.go:17) CMPL AX,DI
0037 (loop_test.go:17) JGE ,49
0038 (loop_test.go:17) MOVQ CX,BX
0039 (loop_test.go:17) MOVB (CX),BP
0040 (loop_test.go:17) MOVB BP,SI
0041 (loop_test.go:17) MOVB SI,BX
0042 (loop_test.go:17) MOVB BX,DX // 4 mov to do MOVB (CX), DX ?
0043 (loop_test.go:17) INCQ ,CX
0044 (loop_test.go:17) MOVQ CX,BX // BX is clobbered below
0045 (loop_test.go:18) MOVB DX,BX
0046 (loop_test.go:18) XORB BX,R8
0047 (loop_test.go:18) MOVB R8,t+0(FP)
0048 (loop_test.go:17) JMP ,34
0049 (loop_test.go:20) RET ,
3. What version of the compiler are you using? (Run it with the -V flag.)
$ /usr/bin/go tool 6g -V
6g version go1.0.2
