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

[Shih-Po Hung via llvm-commits]

================
@@ -2878,11 +2878,20 @@ 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));
+  case Instruction::SDiv:
+  case Instruction::SRem: {
+    auto *LHS = I->getOperand(0);
+    auto *RHS = I->getOperand(1);
+    // If RHS is loop-invariant, signed-division overflow is possible
+    // when RHS == −1 or when it isn’t a known constant.
+    return !Legal->isInvariant(RHS) ||
+           (!Legal->isInvariant(LHS) &&
+            (!isa<ConstantInt>(RHS) ||
+             (isa<ConstantInt>(RHS) && cast<ConstantInt>(RHS)->isMinusOne())));
----------------
arcbbb wrote:

Thanks! I forgot to look into what is checked in isSafeToSpeculativelyExecute.

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