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

[David Sherwood via llvm-commits]

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@@ -5557,14 +5576,23 @@ InstructionCost LoopVectorizationCostModel::computePredInstDiscount(
 InstructionCost LoopVectorizationCostModel::expectedCost(ElementCount VF) {
   InstructionCost Cost;
 
+  // If the vector loop gets executed exactly once with the given VF, ignore the
+  // costs of comparison and induction instructions, as they'll get simplified
+  // away.
+  SmallPtrSet<Instruction *, 2> ValuesToIgnoreForVF;
+  auto TC = PSE.getSE()->getSmallConstantTripCount(TheLoop);
+  if (VF.isFixed() && TC == VF.getFixedValue() && !foldTailByMasking())
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david-arm wrote:

For example, suppose TC=8 and the max VF=16. In this case we *could* choose a VF of 16 and there will be a remainder, but the cost model may prefer VF=8 where there isn't a remainder.

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