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cmd/compile: allow ; on the RHS of rewrite rules for side-effects #30818

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josharian opened this issue Mar 13, 2019 · 3 comments
Open

cmd/compile: allow ; on the RHS of rewrite rules for side-effects #30818

josharian opened this issue Mar 13, 2019 · 3 comments
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NeedsFix The path to resolution is known, but the work has not been done.
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Unplanned

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@josharian
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Consider this rewrite rule:

(ADDQ x l:(MOVQload [off] {sym} ptr mem)) && canMergeLoadClobber(v, l, x) && clobber(l) -> (ADDQload x [off] {sym} ptr mem)

The canMergeLoadClobber condition is pure (free from side-effects). The clobber condition is not really a condition at all, but pretends to be one and is there for side-effects instead.

It'd be nice, for both clarity and reusability, to be able to put side-effects on the RHS of the rule. Something like:

(ADDQ x l:(MOVQload [off] {sym} ptr mem)) && canMergeLoadClobber(v, l, x) -> clobber(l); (ADDQload x [off] {sym} ptr mem)

noteRule, warnRule, and clobberIfDead all belong on the RHS.

@mundaym's hypothetical mergeSuccessor function could also be naturally split into pure conditional LHS and impure RHS.

Thoughts?

cc @randall77 @cherrymui @mvdan @mundaym
@randall77
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Sure, I like it.
It's a little bit strange that in A; B A is a Go statement and B is an s-expression. But I'm ok with that.

@ALTree ALTree added the NeedsDecision Feedback is required from experts, contributors, and/or the community before a change can be made. label Mar 14, 2019
@josharian josharian added NeedsFix The path to resolution is known, but the work has not been done. and removed NeedsDecision Feedback is required from experts, contributors, and/or the community before a change can be made. labels Mar 14, 2019
@josharian
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This is waiting on a decision on #30810. (I don't want to implement twice.)

@mvdan
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mvdan commented Mar 22, 2019

I think both should happen, and I think this would be easier to implement with #30810 done :)

@josharian josharian mentioned this issue Feb 24, 2020
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@seankhliao seankhliao added this to the Unplanned milestone Aug 20, 2022
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