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cmd/compile: ARM's MOVWnop causes regalloc to insert unnecessary copies #37096

josharian opened this issue Feb 6, 2020 · 1 comment
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@josharian josharian commented Feb 6, 2020

MOVWnop is a nop/copy designed to ensure type safety. It generates no code. However, it is not free; regalloc sometimes makes a copy of a value to provide to MOVWnop, since it doesn't know that MOVWnop doesn't modify its argument.

I noticed this while tracking down regressions due to changes in rewrite rule order application.

I don't know what the best approach to fixing it is. Suggestions?

cc @cherrymui

@josharian josharian added this to the Backlog milestone Feb 6, 2020
@cagedmantis cagedmantis added the NeedsInvestigation label Feb 7, 2020
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@cagedmantis cagedmantis commented Feb 7, 2020

/cc @randall77 @griesemer

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