cmd/compile: strange performance difference between two implementations

[go101]

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

$ go version
go version go1.17.3 linux/amd64

Does this issue reproduce with the latest release?

Yes

What did you do?

package pointers

import "testing"

const N = 10000

type T struct {
	x int
}

//go:noinline
func f(t *T) {
	t.x = 0
	for i := 0; i < N; i++ {
		t.x += i
	}
}

//go:noinline
func g(t *T) {
	var x = 0
	for i := 0; i < N; i++ {
		x += i
	}
	t.x = x
}

func Benchmark_f(b *testing.B) {
	var t = &T{}
	for i := 0; i < b.N; i++ { f(t) }
}

func Benchmark_g(b *testing.B) {
	var t = &T{}
	for i := 0; i < b.N; i++ { g(t) }
}

What did you expect to see?

Similar performances.

What did you see instead?

goos: linux
goarch: amd64
pkg: example.com
cpu: Intel(R) Core(TM) i5-4210U CPU @ 1.70GHz
Benchmark_f-4   	   48352	     24403 ns/op
Benchmark_g-4   	  292581	      3956 ns/op

I checked the generated assembly instructions. Yes, they are different, but the complexities are similar. So it is some strange that the performance difference is so large.

[randall77]

The inner loop in f still has writes in it, which is probably why it is slower than g (whose inner loop is completely in registers).

To fix this I think we'd have to promote t.x from memory to register somehow. That seems pretty challenging.
(If the loop were unrolled it might get much of that effect automatically.)

[go101]

The problem also exists for reads:

package pointers

import "testing"

const N = 1000
var a [N]int
var r int

//go:noinline
func g1(a *[N]int) int {
	var r int
	_ = *a
	for i := range a {
		r += a[i]
	}
	return r
}

//go:noinline
func g0(a *[N]int) int {
	var r int
	for i := range a {
		r += a[i]
	}
	return r
}

func Benchmark_g1(b *testing.B) {
	for i := 0; i < b.N; i++ { r = g1(&a) }
}

func Benchmark_g0(b *testing.B) {
	for i := 0; i < b.N; i++ { r = g0(&a) }
}

Benchmark_g1-4   	 2178316	       556.8 ns/op
Benchmark_g0-4   	 1949654	       611.8 ns/op

[go101]

It looks the read case is different from the write case. The compiler generates one more instruction TESTB AL, (AX) for the g0 function in the read case.

[randall77]

@go101 In your example it's just that the nil pointer check is outside the loop in g1 but inside the loop in g0. We'd need to lift the nil check out of the loop to make them the same speed. Which I believe is #41666.