[PATCH] D112117: [LV] Drop FP flags from instructions that need predication

[Diego Caballero via Phabricator via llvm-commits]

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See D111846 <https://reviews.llvm.org/D111846> for more context. LV propagates FP flags in instructions that are
originally guarded by a condition and its respective control flow. It may happen that
the FP flags are only valid when the condition and control flow is in place. When the code
is vectorized and the control flow within the loop is linearized, the FP flags may no
longer hold and lead to a poison value if they are propagated.

TODO: Determine which FP flags should be dropped. The obvious ones are `HasNoNaNs`,
`HasNoInfs` and `HasNoSignedZeros`. I'm not sure if not dropping any of remaining would
lead to correctness issues or dropping them would actually be problematic.

TODO: Find proper tests that show that propagating the FP flags would be incorrect.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D112117

Files:
  llvm/lib/Transforms/Vectorize/LoopVectorize.cpp


Index: llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
===================================================================
--- llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -3049,6 +3049,11 @@
   }
   if (isa<PossiblyExactOperator>(NewInst))
     NewInst->setIsExact(false);
+  if (isa<FPMathOperator>(NewInst)) {
+    NewInst->setHasNoNaNs(false);
+    NewInst->setHasNoInfs(false);
+    NewInst->setHasNoSignedZeros(false);
+  }
 }
 
 void InnerLoopVectorizer::scalarizeInstruction(Instruction *Instr,


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