[llvm-dev] loop invariant code hoisting

Sjoerd Meijer via llvm-dev llvm-dev at lists.llvm.org
Mon Dec 7 05:25:36 PST 2020


I would like to restrict LICM in the number of memory operations it hoists out of a loop. Blindly hoisting out all instructions out could result in a lot of spilling/reloading, which is what I'd like to avoid. We see this for example on (inner)loops with small constant bounds that are unrolled. I am drafting something in https://reviews.llvm.org/D92488, which contains a reduced version of my motivating example.
It was brought to my attention that LICM might not have any restrictions by design because hoisting out all instructions could be some sort of canonical form that other passes depend on, so sinking back defs closer to their user(s) might be a back-end problem. I was wondering if there are any opinions on this.

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