[llvm] [VPlan] Model FOR extract of exit value in VPlan. (PR #93395)

[Florian Hahn via llvm-commits]

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@@ -558,6 +560,27 @@ Value *VPInstruction::generatePerPart(VPTransformState &State, unsigned Part) {
 
     return ReducedPartRdx;
   }
+  case VPInstruction::ExtractRecurrenceResult: {
+    if (Part != 0)
+      return State.get(this, 0, /*IsScalar*/ true);
+
+    // Extract the second last element in the middle block for users outside the
+    // loop.
+    Value *Res;
+    if (State.VF.isVector()) {
+      Res = State.get(
+          getOperand(0),
+          VPIteration(State.UF - 1, VPLane::getLastLaneForVF(State.VF, 2)));
+    } else {
+      assert(State.UF > 1 && "VF and UF cannot both be 1");
+      // When loop is unrolled without vectorizing, retrieve the value just
+      // prior to the final unrolled value. This is analogous to the vectorized
+      // case above: extracting the second last element when VF > 1.
+      Res = State.get(getOperand(0), State.UF - 2);
+    }
+    Res->setName(Name);
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fhahn wrote:

This is used to retain the original name for the IR values, which still seems useful to keep the IR readable and reduce the test changes (in particular it makes it easier to trace back to where the IR instruction is coming from)

https://github.com/llvm/llvm-project/pull/93395