cmd/compile: utf8.DecodeRune should be fast/slow-path inlineable

[josharian]

We manually partially inline utf8.DecodeRune in a few places in the tree. For example, in bytes.EqualFold, we have:

		if s[0] < utf8.RuneSelf {
			sr, s = rune(s[0]), s[1:]
		} else {
			r, size := utf8.DecodeRune(s)
			sr, s = r, s[size:]
		}

It'd be nice to rewrite utf8.DecodeRune like this:

func DecodeRune(p []byte) (r rune, size int) {
	if len(p) > 0 && p[0] < RuneSelf {
		return rune(p[0]), 1
	}
	return decodeRuneSlow(p)
}

and then let mid-stack inlining take it from there. (And delete all the manual partial inlining.)

Unfortunately, this has a cost of 89, over the budget of 80.

This is basically just another variant of #17566, but I'm filing it separately since that issue is super noisy.

cc @bradfitz @mvdan @dr2chase

[martisch]

While at it we should make it also work for utf8.DecodeRuneInString at the same time.

[mvdan]

Perhaps very clear cases like this can be used to start tweaking the inlining heuristic in the right direction, with the smallest steps possible.

[corona10]

@martisch @mvdan Can I take a look at this issue?

[mvdan]

@corona10 you're certainly welcome to take a look, but if you're looking for low-hanging fruit issues to work on, this isn't a good candidate. It likely requires someone with a good understanding of the compiler, and it's not a simple fix, as @josharian already gave that a try.

[corona10]

@mvdan
Oh.. @josharian already tried it. Looks like a not simple issue.
I thought it was just an issue of rewriting so that it can help the compiler to do a mid stack inlining, but I did not know that cost was an issue to consider. I will try to find some easier issues.