cmd/compile: intrinsify cmp.Compare on common types such as strings

[mvdan]

#61725 optimized strings.Compare, which is great, but it did not optimize cmp.Compare[string], which is otherwise equivalent.

This is leading to users learning that they should avoid cmp.Compare[string] for the sake of performance, even writing linters for it like https://github.com/tklauser/lintcomparestrings, which in my opinion is really unfortunate. For the same reason, I find changes like https://go-review.googlesource.com/c/go/+/642038 unfortunate and unnecessary.

The compiler should be clever enough to optimize the generic cmp.Compare function just as well as specialized functions such as strings.Compare or any others that might exist for common comparable types such as integers. Then the developers don't have to remember facts about which one of them is faster.

Personally, I also find it pretty nice to consistently use cmp.Compare. Needing to mix different compare functions in an expression for the sake of performance is a bit odd.

[mvdan]

Even though this change may not seem hugely important or urgent, I would still argue for doing it sooner than later, before it's a "well known fact" that cmp.Compare is slow for common types and should be avoided.

[mknyszek]

CC @golang/compiler

[prattmic]

Is adding a type switch to the beginning of cmp.Compare sufficient to get good results, or does that not fully simplify in the compiler?

func Compare[T Ordered](x, y T) int {
	switch any(x).(type) {
	case string:
		return bytealg.CompareString(any(x).(string), any(y).(string))
	}
	...

[mateusz834]

@prattmic #59591 ?

[adonovan]

Is adding a type switch to the beginning of cmp.Compare sufficient to get good results...?

It actually makes it worse: the usual call boils down to runtime.cmpstring, but the switch defeats inlining.

[fengyoulin]

CL 578835 removed the redundant calls to runtime.cmpstring, but the redundant tests and branches cannot be removed. Maybe it's worth doing.

[gopherbot]

Change https://go.dev/cl/647235 mentions this issue: cmd/compile: intrinsify cmp.Compare for string type

[randall77]

I have a separate set of CLs for removing the test+branch stuff, not quite ready to mail yet.

[gopherbot]

Change https://go.dev/cl/648336 mentions this issue: cmd/compile: remove SignInt when applied to cmpstring