[llvm-dev] canonical form loops

[Sjoerd Meijer via llvm-dev]

Hello,

Quick question to see if I haven't missed anything: I would like convert counting down loops, i.e. loops with a constant -1 step value, to counting up loops, because the vectoriser is able to better deal with these loops (see e.g. D76838 that I was discussing today with Ayal). It looks like LoopSimplifyCFG and IndVarSimplify don't do this. So was just curious if I haven't missed anything here or in another pass I haven't yet considered. I was perhaps also expecting this to be the canonical form of loops, but couldn't find any evidence of that in [1] or in source-code.
The obvious follow-up question is if there would be any objections to adding this to e.g. LoopSimplifyCFG, and adding LoopSimplifyCFG to the optimisation pipeline just before the vectoriser.

Cheers,
Sjoerd.

[1] https://llvm.org/docs/LoopTerminology.html


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