[llvm] [VPlan] Introduce ExitPhi VPInstruction, use to create phi for FOR. (PR #94760)

[via llvm-commits]

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@@ -8635,6 +8604,49 @@ LoopVectorizationPlanner::tryToBuildVPlanWithVPRecipes(VFRange &Range) {
          "VPBasicBlock");
   RecipeBuilder.fixHeaderPhis();
 
+  auto *MiddleVPBB =
+      cast<VPBasicBlock>(Plan->getVectorLoopRegion()->getSingleSuccessor());
+
+  VPBasicBlock *ScalarPH = nullptr;
+  for (VPBlockBase *Succ : MiddleVPBB->getSuccessors()) {
+    auto *VPBB = dyn_cast<VPBasicBlock>(Succ);
+    if (VPBB && !isa<VPIRBasicBlock>(VPBB)) {
+      ScalarPH = VPBB;
+      break;
+    }
+  }
+
+  if (ScalarPH) {
+    for (auto &H : HeaderVPBB->phis()) {
+      auto *FOR = dyn_cast<VPFirstOrderRecurrencePHIRecipe>(&H);
+      if (!FOR)
+        continue;
+      VPBuilder B(ScalarPH);
+      VPBuilder MiddleBuilder;
+      // Set insert point so new recipes are inserted before terminator and
+      // condition, if there is either the former or both.
+      if (MiddleVPBB->getNumSuccessors() != 2)
+        MiddleBuilder.setInsertPoint(MiddleVPBB);
+      else if (isa<VPInstruction>(MiddleVPBB->getTerminator()->getOperand(0)))
+        MiddleBuilder.setInsertPoint(
+            &*std::prev(MiddleVPBB->getTerminator()->getIterator()));
+      else
+        MiddleBuilder.setInsertPoint(MiddleVPBB->getTerminator());
+
+      // Extract the resume value and create a new VPLiveOut for it.
+      auto *Resume = MiddleBuilder.createNaryOp(
+          VPInstruction::ExtractFromEnd,
+          {FOR->getBackedgeValue(),
+           Plan->getOrAddLiveIn(
+               ConstantInt::get(Plan->getCanonicalIV()->getScalarType(), 1))},
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ayalz wrote:

Can the VPValue or Value of 1 be created once at the outset, and reused here for all FORs?

https://github.com/llvm/llvm-project/pull/94760