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cmd/compile: loops prevent bounds checking elimination #40704

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elichai opened this issue Aug 11, 2020 · 3 comments
Open

cmd/compile: loops prevent bounds checking elimination #40704

elichai opened this issue Aug 11, 2020 · 3 comments

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@elichai
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@elichai elichai commented Aug 11, 2020

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

$ go version
go1.14

Does this issue reproduce with the latest release?

Yes

What did you do?

If I do some copies between arrays with a loop like:

func Convert(a *[32]byte, b *[4]uint64) {
	for i := 0; i < 4; i++ {
		binary.LittleEndian.PutUint64(a[i*8:], b[i])
	}
}

it bounds check on each array on every iteration.

but if I unroll it, it can figure out that it will never go out of bounds and eliminate the checks:

func Convert(a *[32]byte, b *[4]uint64) {
		binary.LittleEndian.PutUint64(a[:], b[0])
		binary.LittleEndian.PutUint64(a[8:], b[1])
		binary.LittleEndian.PutUint64(a[16:], b[2])
		binary.LittleEndian.PutUint64(a[24:], b[3])
}

godbolt: https://godbolt.org/z/E3c5r4

This means that when implementing performance critical code (ie cryptography) I find myself unrolling loops of 8-16 because they end up on pprof which really sucks for readability

@randall77 randall77 added this to the Unplanned milestone Aug 11, 2020
@randall77
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@randall77 randall77 commented Aug 11, 2020

We don't propagate known ranges through multiplies at the moment. Shouldn't be too hard to do in prove.go.

@mdempsky mdempsky changed the title loops prevent bounds checking elimination cmd/compile: loops prevent bounds checking elimination Aug 12, 2020
@gopherbot
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@gopherbot gopherbot commented Aug 17, 2020

Change https://golang.org/cl/248701 mentions this issue: cmd/compile: add support for Lsh64*64 and Mul64 to IsSliceInBounds

@gtank
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@gtank gtank commented Dec 23, 2020

Another cryptography implementer chiming in! It would be great to see this improved, and ideally for operations other than multiplication as well. It would specifically allow me to improve the readability of my code without suffering a notable (~30% on one task, see gtank/blake2#2 (comment)) performance penalty for not unrolling the loop.

I've tried hard to keep that library pure Go, and this makes it more likely that in a production deployment I'd have to pull in assembly to achieve the throughput expected of the algorithm itself (cc @FiloSottile).

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