[all-commits] [llvm/llvm-project] 2cb4d1: [LSR] Do not create duplicated PHI nodes while pre...

[Sergey Kachkov via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 2cb4d1b1bd7bde2724b79976e859684bd3f5c771
      https://github.com/llvm/llvm-project/commit/2cb4d1b1bd7bde2724b79976e859684bd3f5c771
  Author: Sergey Kachkov <109674256+skachkov-sc at users.noreply.github.com>
  Date:   2024-09-06 (Fri, 06 Sep 2024)

  Changed paths:
    M llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp
    M llvm/test/Transforms/LoopStrengthReduce/2011-10-03-CritEdgeMerge.ll
    M llvm/test/Transforms/LoopStrengthReduce/AMDGPU/lsr-invalid-ptr-extend.ll
    M llvm/test/Transforms/LoopStrengthReduce/X86/2011-11-29-postincphi.ll
    M llvm/test/Transforms/LoopStrengthReduce/X86/expander-crashes.ll
    M llvm/test/Transforms/LoopStrengthReduce/X86/missing-phi-operand-update.ll
    M llvm/test/Transforms/LoopStrengthReduce/preserve-lcssa.ll

  Log Message:
  -----------
  [LSR] Do not create duplicated PHI nodes while preserving LCSSA form (#107380)

Motivating example: https://godbolt.org/z/eb97zrxhx
Here we have 2 induction variables in the loop: one is corresponding to
i variable (add rdx, 4), the other - to res (add rax, 2). The second
induction variable can be removed by rewriteLoopExitValues() method
(final value of res at loop exit is unroll_iter * -2); however, this
doesn't happen because we have duplicated LCSSA phi nodes at loop exit:
```
; Preheader:
for.body.preheader.new:                           ; preds = %for.body.preheader
  %unroll_iter = and i64 %N, -4
  br label %for.body

; Loop:
for.body:                                         ; preds = %for.body, %for.body.preheader.new
  %lsr.iv = phi i64 [ %lsr.iv.next, %for.body ], [ 0, %for.body.preheader.new ]
  %i.07 = phi i64 [ 0, %for.body.preheader.new ], [ %inc.3, %for.body ]
  %inc.3 = add nuw i64 %i.07, 4
  %lsr.iv.next = add nsw i64 %lsr.iv, -2
  %niter.ncmp.3.not = icmp eq i64 %unroll_iter, %inc.3
  br i1 %niter.ncmp.3.not, label %for.end.loopexit.unr-lcssa.loopexit, label %for.body, !llvm.loop !7

; Exit blocks
for.end.loopexit.unr-lcssa.loopexit:              ; preds = %for.body
  %inc.3.lcssa = phi i64 [ %inc.3, %for.body ]
  %lsr.iv.next.lcssa11 = phi i64 [ %lsr.iv.next, %for.body ]
  %lsr.iv.next.lcssa = phi i64 [ %lsr.iv.next, %for.body ]
  br label %for.end.loopexit.unr-lcssa
```
rewriteLoopExitValues requires %lsr.iv.next value to have only 2 uses:
one in LCSSA phi node, the other - in induction phi node. Here we have 3
uses of this value because of duplicated lcssa nodes, so the transform
doesn't apply and leads to an extra add operation inside the loop. The
proposed solution is to accumulate inserted instructions that will
require LCSSA form update into SetVector and then call
formLCSSAForInstructions for this SetVector once, so the same
instructions don't process twice.



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