[all-commits] [llvm/llvm-project] 06f066: [SeparateConstOffsetFromGEP] Fix: `b - a` matched ...

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  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 06f06644efb003e0748e94ac11cb230271a006f8
      https://github.com/llvm/llvm-project/commit/06f06644efb003e0748e94ac11cb230271a006f8
  Author: Liren Peng <liren.plr at gmail.com>
  Date:   2023-02-15 (Wed, 15 Feb 2023)

  Changed paths:
    M llvm/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp
    M llvm/test/Transforms/SeparateConstOffsetFromGEP/split-gep-sub.ll

  Log Message:
  -----------
  [SeparateConstOffsetFromGEP] Fix: `b - a` matched `a - b` during reuniteExts

During the SeparateConstOffsetFromGEP pass, a - b and b - a will be
considered equivalent in some instances.

An example- the IR contains:

  BB1:
      %add = add %a, 511
      br label %BB2
  BB2:
      %sub2 = sub %b,  %a
      br label %BB3
  BB3:
      %sub1 = sub %add, %b
      %gep = getelementptr float, ptr %p, %sub1

Step 1 in the SeparateConstOffsetFromGEP pass, after split constant index:

  BB1:
      %add = add %a, 511
      br label %BB2
  BB2:
      %sub2 = sub %b,  %a
      br label %BB3
  BB3:
      %sub.t = sub %a, %b
      %gep.base = getelementptr float, ptr %p, %sub.t
      %gep = getelementptr float, ptr %gep.base, 511

Step 2, after reuniteExts:

  BB1:
      br label %BB2
  BB2:
      %sub2 = sub %b,  %a
      br label %BB3
  BB3:
      %gep.base = getelementptr float, ptr %p, %sub2
      %gep = getelementptr float, ptr %gep.base, 511

Obviously, reuniteExts treated a - b and b - a as equivalent.
This patch fixes that.

Reviewed By: nikic, spatel

Differential Revision: https://reviews.llvm.org/D143542