[llvm] [VPlan] Introduce explicit ExtractFromEnd recipes for live-outs. (PR #100658)

[via llvm-commits]

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@@ -8517,20 +8534,121 @@ static void addLiveOutsForFirstOrderRecurrences(VPlan &Plan) {
   }
   VPValue *OneVPV = Plan.getOrAddLiveIn(
       ConstantInt::get(Plan.getCanonicalIV()->getScalarType(), 1));
+  VPValue *TwoVPV = Plan.getOrAddLiveIn(
+      ConstantInt::get(Plan.getCanonicalIV()->getScalarType(), 2));
 
   for (auto &HeaderPhi : VectorRegion->getEntryBasicBlock()->phis()) {
     auto *FOR = dyn_cast<VPFirstOrderRecurrencePHIRecipe>(&HeaderPhi);
     if (!FOR)
       continue;
 
+    // This is the second phase of vectorizing first-order recurrences. An
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ayalz wrote:

Would be good to clarify how many phases there are and what this second one does, see below.

https://github.com/llvm/llvm-project/pull/100658