[all-commits] [llvm/llvm-project] 79af68: [SCEV] Handle more adds in computeConstantDifferen...

[Nikita Popov via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 79af6892f8eee1d0fb96c243716c8b03936751c9
      https://github.com/llvm/llvm-project/commit/79af6892f8eee1d0fb96c243716c8b03936751c9
  Author: Nikita Popov <npopov at redhat.com>
  Date:   2024-08-02 (Fri, 02 Aug 2024)

  Changed paths:
    M llvm/lib/Analysis/ScalarEvolution.cpp
    M llvm/unittests/Analysis/ScalarEvolutionTest.cpp

  Log Message:
  -----------
  [SCEV] Handle more adds in computeConstantDifference() (#101339)

Currently it only deals with the case where we're subtracting adds with
at most one non-constant operand. This patch extends it to cancel out
common operands for the subtraction of arbitrary add expressions.

The background here is that I want to replace a getMinusSCEV() call in
LAA with computeConstantDifference():

https://github.com/llvm/llvm-project/blob/93fecc2577ece0329f3bbe2719bbc5b4b9b30010/llvm/lib/Analysis/LoopAccessAnalysis.cpp#L1602-L1603

This particular call is very expensive in some cases (e.g. lencod with
LTO) and computeConstantDifference() could achieve this much more
cheaply, because it does not need to construct new SCEV expressions.

However, the current computeConstantDifference() implementation is too
weak for this and misses many basic cases. This is a step towards making
it more powerful while still keeping it pretty fast.



To unsubscribe from these emails, change your notification settings at https://github.com/llvm/llvm-project/settings/notifications