[all-commits] [llvm/llvm-project] 0b9f1c: [SCEV] Disallow simplifying phi(undef, X) to X (#1...

[Yingwei Zheng via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 0b9f1cc024ca6c7e8d60524be07c0ddfcd08b23c
      https://github.com/llvm/llvm-project/commit/0b9f1cc024ca6c7e8d60524be07c0ddfcd08b23c
  Author: Yingwei Zheng <dtcxzyw2333 at gmail.com>
  Date:   2024-11-07 (Thu, 07 Nov 2024)

  Changed paths:
    M llvm/lib/Analysis/ScalarEvolution.cpp
    M llvm/test/Transforms/IndVarSimplify/invalidate-modified-lcssa-phi.ll
    M llvm/test/Transforms/IndVarSimplify/no-iv-rewrite.ll
    M llvm/test/Transforms/LoopStrengthReduce/X86/2012-01-13-phielim.ll
    A llvm/test/Transforms/LoopUnroll/pr114879.ll
    M llvm/test/Transforms/LoopVectorize/uniform-blend.ll
    M llvm/unittests/Analysis/ScalarEvolutionTest.cpp
    M polly/test/CodeGen/OpenMP/floord-as-argument-to-subfunction.ll
    M polly/test/CodeGen/inner_scev_sdiv_2.ll
    M polly/test/CodeGen/inner_scev_sdiv_3.ll
    M polly/test/CodeGen/non-affine-phi-node-expansion.ll
    M polly/test/CodeGen/phi-defined-before-scop.ll
    M polly/test/CodeGen/scop_expander_insert_point.ll
    M polly/test/CodeGen/stack-overflow-in-load-hoisting.ll
    M polly/test/ScopInfo/complex_domain_binary_condition.ll
    M polly/test/ScopInfo/scev-div-with-evaluatable-divisor.ll

  Log Message:
  -----------
  [SCEV] Disallow simplifying phi(undef, X) to X (#115109)

See the following case:
```
@GlobIntONE = global i32 0, align 4

define ptr @src() {
entry:
  br label %for.body.peel.begin

for.body.peel.begin:                              ; preds = %entry
  br label %for.body.peel

for.body.peel:                                    ; preds = %for.body.peel.begin
  br i1 true, label %cleanup.peel, label %cleanup.loopexit.peel

cleanup.loopexit.peel:                            ; preds = %for.body.peel
  br label %cleanup.peel

cleanup.peel:                                     ; preds = %cleanup.loopexit.peel, %for.body.peel
  %retval.2.peel = phi ptr [ undef, %for.body.peel ], [ @GlobIntONE, %cleanup.loopexit.peel ]
  br i1 true, label %for.body.peel.next, label %cleanup7

for.body.peel.next:                               ; preds = %cleanup.peel
  br label %for.body.peel.next1

for.body.peel.next1:                              ; preds = %for.body.peel.next
  br label %entry.peel.newph

entry.peel.newph:                                 ; preds = %for.body.peel.next1
  br label %for.body

for.body:                                         ; preds = %cleanup, %entry.peel.newph
  %retval.0 = phi ptr [ %retval.2.peel, %entry.peel.newph ], [ %retval.2, %cleanup ]
  br i1 false, label %cleanup, label %cleanup.loopexit

cleanup.loopexit:                                 ; preds = %for.body
  br label %cleanup

cleanup:                                          ; preds = %cleanup.loopexit, %for.body
  %retval.2 = phi ptr [ %retval.0, %for.body ], [ @GlobIntONE, %cleanup.loopexit ]
  br i1 false, label %for.body, label %cleanup7.loopexit

cleanup7.loopexit:                                ; preds = %cleanup
  %retval.2.lcssa.ph = phi ptr [ %retval.2, %cleanup ]
  br label %cleanup7

cleanup7:                                         ; preds = %cleanup7.loopexit, %cleanup.peel
  %retval.2.lcssa = phi ptr [ %retval.2.peel, %cleanup.peel ], [ %retval.2.lcssa.ph, %cleanup7.loopexit ]
  ret ptr %retval.2.lcssa
}

define ptr @tgt() {
entry:
  br label %for.body.peel.begin

for.body.peel.begin:                              ; preds = %entry
  br label %for.body.peel

for.body.peel:                                    ; preds = %for.body.peel.begin
  br i1 true, label %cleanup.peel, label %cleanup.loopexit.peel

cleanup.loopexit.peel:                            ; preds = %for.body.peel
  br label %cleanup.peel

cleanup.peel:                                     ; preds = %cleanup.loopexit.peel, %for.body.peel
  %retval.2.peel = phi ptr [ undef, %for.body.peel ], [ @GlobIntONE, %cleanup.loopexit.peel ]
  br i1 true, label %for.body.peel.next, label %cleanup7

for.body.peel.next:                               ; preds = %cleanup.peel
  br label %for.body.peel.next1

for.body.peel.next1:                              ; preds = %for.body.peel.next
  br label %entry.peel.newph

entry.peel.newph:                                 ; preds = %for.body.peel.next1
  br label %for.body

for.body:                                         ; preds = %cleanup, %entry.peel.newph
  br i1 false, label %cleanup, label %cleanup.loopexit

cleanup.loopexit:                                 ; preds = %for.body
  br label %cleanup

cleanup:                                          ; preds = %cleanup.loopexit, %for.body
  br i1 false, label %for.body, label %cleanup7.loopexit

cleanup7.loopexit:                                ; preds = %cleanup
  %retval.2.lcssa.ph = phi ptr [ %retval.2.peel, %cleanup ]
  br label %cleanup7

cleanup7:                                         ; preds = %cleanup7.loopexit, %cleanup.peel
  %retval.2.lcssa = phi ptr [ %retval.2.peel, %cleanup.peel ], [ %retval.2.lcssa.ph, %cleanup7.loopexit ]
  ret ptr %retval.2.lcssa
}
```
1. `simplifyInstruction(%retval.2.peel)` returns `@GlobIntONE`. Thus,
`ScalarEvolution::createNodeForPHI` returns SCEV expr `@GlobIntONE` for
`%retval.2.peel`.
2. `SimplifyIndvar::replaceIVUserWithLoopInvariant` tries to replace the
use of `%retval.2.peel` in `%retval.2.lcssa.ph` with `@GlobIntONE`.
3. `simplifyLoopAfterUnroll -> simplifyLoopIVs -> SCEVExpander::expand`
reuses `%retval.2.peel = phi ptr [ undef, %for.body.peel ], [
@GlobIntONE, %cleanup.loopexit.peel ]` to generate code for
`@GlobIntONE`. It is incorrect.

This patch disallows simplifying `phi(undef, X)` to `X` by setting
`CanUseUndef` to false.
Closes https://github.com/llvm/llvm-project/issues/114879.



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