[llvm] [LV] Add extra check for signed oveflow for SDiv/SRem (PR #170818)

[Philip Reames via llvm-commits]

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@@ -2878,11 +2878,40 @@ bool LoopVectorizationCostModel::isPredicatedInst(Instruction *I) const {
              TheLoop->isLoopInvariant(cast<StoreInst>(I)->getValueOperand()));
   }
   case Instruction::UDiv:
-  case Instruction::SDiv:
-  case Instruction::SRem:
   case Instruction::URem:
     // If the divisor is loop-invariant no predication is needed.
     return !Legal->isInvariant(I->getOperand(1));
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preames wrote:

Meta comment: I think your change might be trying to do too much.  It looks like you're trying to handle some of the loop varying cases where the udiv case doesn't.  My suggestion would be something like the following:
```
if (!Legal->isInvariant(I->getOperand(1)))
   return true;
return RHSCouldBeMinusOne && LHSCouldBeSignedMin;
```

In particular, I think the poison comments may be a red herring as the udiv case doesn't ensure the invariant RHS is non-poison either.  

https://github.com/llvm/llvm-project/pull/170818