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cmd/compile: Compiler doesn't know bits.Len and friends return non-negative integers #51963

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greatroar opened this issue Mar 26, 2022 · 1 comment

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@greatroar
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@greatroar greatroar commented Mar 26, 2022

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

$ go version
go version go1.18 linux/amd64

Does this issue reproduce with the latest release?

Reproduces with

$ gotip version
go version devel go1.19-ab0f7611d73 Mon Mar 14 09:19:01 2022 +0000 linux/amd64

What did you do?

func nextpow2(x uint) uint {
	return 1 << bits.Len(x)
}

What did you expect to see?

The same code as for

func nextpow2(x uint) uint {
	return 1 << uint(bits.Len(x))
}

What did you see instead?

The compiler generates a check that boils down to

if bits.Len(x) < 0 {
	runtime.panicshift()
}

This happens because bits.Len (and LeadingZeros, TrailingZeros, OnesCount) have return type int instead of uint. In the example given, this doesn't matter much because I can add the cast. But that's clutter and also adds to the inlining cost, despite the generated code being smaller.

It looks like the compiler knows that cap and len never return negative integers, so perhaps the math/bits functions could be dealt with in the same way?

@gopherbot
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@gopherbot gopherbot commented Mar 26, 2022

Change https://go.dev/cl/396036 mentions this issue: cmd/compile: upgrade prove pass to know results of math/bits ops are nonnegative

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