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

[via llvm-commits]

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@@ -8452,7 +8452,104 @@ static void addUsersInExitBlock(
            return P && Inductions.contains(P);
          })))
       continue;
-    Plan.addLiveOut(&ExitPhi, V);
+
+    auto MiddleVPBB =
+        cast<VPBasicBlock>(Plan.getVectorLoopRegion()->getSingleSuccessor());
+    VPBuilder B(MiddleVPBB);
+    if (auto *Terminator = MiddleVPBB->getTerminator()) {
+      auto *Condition = dyn_cast<VPInstruction>(Terminator->getOperand(0));
+      assert((!Condition || Condition->getParent() == MiddleVPBB) &&
+             "Condition expected in MiddleVPBB");
+      B.setInsertPoint(Condition ? Condition : Terminator);
+    }
+
+    VPValue *Ext;
+    if (auto *FOR = dyn_cast_or_null<VPFirstOrderRecurrencePHIRecipe>(
+            V->getDefiningRecipe())) {
+      // This is the second phase of vectorizing first-order recurrences. An
+      // overview of the transformation is described below. Suppose we have the
+      // following loop with some use after the loop of the last a[i-1],
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ayalz wrote:

This explanation is moved from adjustFixedOrderRecurrences() to here (addUsersInExitBlock()), yet deserves to still be in a method dedicated to (the 2nd phase of) FORs. How about moving it, along with setting the users of FOR in exit block, to addLiveOutsForFixedOrderRecurrences()?

The documentation of these methods may need updating, once settled.

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