[PATCH] D98232: [regalloc] Ensure Query::collectInterferringVregs is called before interval iteration

Thu Mar 25 05:18:56 PDT 2021

dmgreen added a comment.

I'm worried that this comes up in a lot of places. Perhaps rare still, but important cases. The aarch64 example we have is just a matrix multiply, and is 25% slower with all the cascading spills, https://bugs.llvm.org/show_bug.cgi?id=26810 quotes the same. Like I said before though, the option didn't fix some examples of the same thing that we were seeing in ARM, so I'm not sure how reliably better it is.

@aditya_nandakumar @qcolombet any idea when a better fix for that issue might be available?

Could we just make `consider-local-interval-cost` -O3 only in the meantime? That should alleviate some of the compile time worries, as we have genuine examples of where it is hurting performance.

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Comment at: llvm/test/CodeGen/X86/bug26810.ll:1
-; RUN: llc < %s -march=x86 -regalloc=greedy -stop-after=greedy | FileCheck %s
+; RUN: llc -consider-local-interval-cost < %s -march=x86 -regalloc=greedy -stop-after=greedy | FileCheck %s
 ; Make sure bad eviction sequence doesnt occur
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If we are enabling this flag by default, we should probably update the tests, not hide them behind a flag.

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https://reviews.llvm.org/D98232