[PATCH] D73727: [SLP] drop poison-generating flags for shuffle reduction ops (PR44536)

We may calculate reassociable math ops in arbitrary order when creating a shuffle reduction,
so there's no guarantee that things like 'nsw' hold on those intermediate values. Drop all
poison-generating flags for safety.

This change is limited to shuffle reductions because I don't think we have a problem in the
general case (where we intersect flags of each scalar op that goes into a vector op), but if
there's evidence of other cases being wrong, we can extend this fix to cover those cases.

https://bugs.llvm.org/show_bug.cgi?id=44536

Differential Revision: https://reviews.llvm.org/D73727

1 files changed, 5 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopUtils.cpp b/llvm/lib/Transforms/Utils/LoopUtils.cpp
index b1d0c83..472be10 100644
--- a/llvm/lib/Transforms/Utils/LoopUtils.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUtils.cpp
@@ -937,6 +937,11 @@ llvm::getShuffleReduction(IRBuilder<> &Builder, Value *Src, unsigned Op,
     }
     if (!RedOps.empty())
       propagateIRFlags(TmpVec, RedOps);
+
+    // We may compute the reassociated scalar ops in a way that does not
+    // preserve nsw/nuw etc. Conservatively, drop those flags.
+    if (auto *ReductionInst = dyn_cast<Instruction>(TmpVec))
+      ReductionInst->dropPoisonGeneratingFlags();
   }
   // The result is in the first element of the vector.
   return Builder.CreateExtractElement(TmpVec, Builder.getInt32(0));