[llvm] [VPlan] Implement VPWidenCastRecipe::computeCost(). (NFCI) (PR #111339)

[Florian Hahn via llvm-commits]

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@@ -7258,12 +7258,29 @@ LoopVectorizationPlanner::precomputeCosts(VPlan &Plan, ElementCount VF,
     const auto &ChainOps = RdxDesc.getReductionOpChain(RedPhi, OrigLoop);
     SetVector<Instruction *> ChainOpsAndOperands(ChainOps.begin(),
                                                  ChainOps.end());
+    auto isZExtOrSExt = [](const unsigned Opcode) -> bool {
+      return Opcode == Instruction::ZExt || Opcode == Instruction::SExt;
+    };
     // Also include the operands of instructions in the chain, as the cost-model
     // may mark extends as free.
+    //
+    // For ARM, some of the instruction can folded into the reducion
+    // instruction. So we need to mark all folded instructions free.
+    // For example: We can fold reduce(mul(ext(A), ext(B))) into one
+    // instruction.
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fhahn wrote:

Would be good if there would be a generic way to check this and avoid hard-coding here, but I cannot think of a better alternative :( 

https://github.com/llvm/llvm-project/pull/111339