[llvm] [VPlan] Move initial skeleton construction earlier (NFC). (PR #150848)

[via llvm-commits]

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@@ -488,12 +488,46 @@ void VPlanTransforms::prepareForVectorization(
 
   addCanonicalIVRecipes(Plan, HeaderVPBB, LatchVPBB, InductionTy, IVDL);
 
-  [[maybe_unused]] bool HandledUncountableEarlyExit = false;
+  // Create SCEV and VPValue for the trip count.
+  // We use the symbolic max backedge-taken-count, which works also when
+  // vectorizing loops with uncountable early exits.
+  const SCEV *BackedgeTakenCountSCEV = PSE.getSymbolicMaxBackedgeTakenCount();
+  assert(!isa<SCEVCouldNotCompute>(BackedgeTakenCountSCEV) &&
+         "Invalid backedge-taken count");
+  ScalarEvolution &SE = *PSE.getSE();
+  const SCEV *TripCount = SE.getTripCountFromExitCount(BackedgeTakenCountSCEV,
+                                                       InductionTy, TheLoop);
+  Plan.setTripCount(
+      vputils::getOrCreateVPValueForSCEVExpr(Plan, TripCount, SE));
+
+  VPBasicBlock *ScalarPH = Plan.createVPBasicBlock("scalar.ph");
+  VPBlockUtils::connectBlocks(ScalarPH, Plan.getScalarHeader());
+
+  // The connection order corresponds to the operands of the conditional branch,
+  // with the middle block already connected to the exit block.
+  VPBlockUtils::connectBlocks(MiddleVPBB, ScalarPH);
+  // Also connect the entry block to the scalar preheader.
+  // TODO: Also introduce a branch recipe together with the minimum trip count
+  // check.
+  VPBlockUtils::connectBlocks(Plan.getEntry(), ScalarPH);
+  Plan.getEntry()->swapSuccessors();
+}
+
+void VPlanTransforms::handleEarlyExitsAndAddMiddleCheck(
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ayalz wrote:

Better split into separate `handleEarlyExits()` and `addMiddleBranch()`?
(Former is unneeded for native, although perhaps better to keep aligned.)

https://github.com/llvm/llvm-project/pull/150848