[llvm] [LV] Cost EVL width-adjusting zext/trunc as free (PR #225016)

[Pengcheng Wang via llvm-commits]

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@@ -1360,9 +1360,17 @@ InstructionCost VPInstruction::computeCost(ElementCount VF,
   // NOTE: At the moment it seems only possible to expose this path for
   // the trunc, zext and sext opcodes.
   // TODO: Update VF arg to use onlyFirstLaneUsed once WidenCast is unified.
-  if (Instruction::isCast(getOpcode()))
+  if (Instruction::isCast(getOpcode())) {
+    // A scalar zext/trunc that only adjusts the width of an
+    // ExplicitVectorLength to the canonical IV type is free: it feeds only
+    // the IV increment and AVL decrement, which are modeled as free below.
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wangpc-pp wrote:

Normally all these casts in LV can be free, I cannot imagine one that is not.

https://github.com/llvm/llvm-project/pull/225016