[llvm] [LV] Ignore some costs when loop gets fully unrolled (PR #106699)

[David Sherwood via llvm-commits]

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@@ -5544,14 +5544,28 @@ InstructionCost LoopVectorizationCostModel::computePredInstDiscount(
 InstructionCost LoopVectorizationCostModel::expectedCost(ElementCount VF) {
   InstructionCost Cost;
 
+  // If with the given VF loop gets fully unrolled, ignore the costs of
+  // comparison and induction instructions, as they'll get simplified away
+  SmallPtrSet<const Value *, 16> ValuesToIgnoreForVF;
+  auto TC = PSE.getSE()->getSmallConstantTripCount(TheLoop);
+  auto *Cmp = TheLoop->getLatchCmpInst();
+  if (Cmp && TC == VF.getKnownMinValue()) {
+    ValuesToIgnoreForVF.insert(Cmp);
+    for (const auto &[IV, IndDesc] : Legal->getInductionVars()) {
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david-arm wrote:

I don't think this looks right, since there could be outside users of the induction variables and the increment will remain even after the loop is removed. I think you need to make sure that the only use of the increment is the phi in the loop header.

https://github.com/llvm/llvm-project/pull/106699