[llvm] [LV] Don't predicate uniform divides with loop-invariant divisor. (PR #98904)

[Florian Hahn via llvm-commits]

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@@ -3710,6 +3713,15 @@ bool LoopVectorizationCostModel::isPredicatedInst(Instruction *I) const {
   case Instruction::SDiv:
   case Instruction::SRem:
   case Instruction::URem:
+    // When folding the tail, at least one of the lanes must execute
+    // unconditionally. If the divisor is loop-invariant no predication is
+    // needed, as predication would not prevent the divide-by-0 on the executed
+    // lane.
+    if (foldTailByMasking() && !Legal->blockNeedsPredication(I->getParent()) &&
+        TheLoop->isLoopInvariant(I->getOperand(1)) &&
+        (IsKnownUniform || isUniformAfterVectorization(I, VF)))
+      return false;
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fhahn wrote:

Thanks I restructured the code and generalized it so the unconditionally-executed but tail-folded cases are all handled at the ned.

Removed the IsKnownUniform requirement.

https://github.com/llvm/llvm-project/pull/98904