[llvm] [LV] Transform to handle exits in the scalar loop (PR #148626)

[Gaëtan Bossu via llvm-commits]

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@@ -3558,42 +3561,45 @@ void VPlanTransforms::handleUncountableEarlyExit(VPBasicBlock *EarlyExitingVPBB,
   // block if CondToEarlyExit.
   VPValue *IsEarlyExitTaken =
       Builder.createNaryOp(VPInstruction::AnyOf, {CondToEarlyExit});
-  VPBasicBlock *NewMiddle = Plan.createVPBasicBlock("middle.split");
-  VPBasicBlock *VectorEarlyExitVPBB =
-      Plan.createVPBasicBlock("vector.early.exit");
-  VPBlockUtils::insertOnEdge(LatchVPBB, MiddleVPBB, NewMiddle);
-  VPBlockUtils::connectBlocks(NewMiddle, VectorEarlyExitVPBB);
-  NewMiddle->swapSuccessors();
-
-  VPBlockUtils::connectBlocks(VectorEarlyExitVPBB, EarlyExitVPBB);
-
-  // Update the exit phis in the early exit block.
-  VPBuilder MiddleBuilder(NewMiddle);
-  VPBuilder EarlyExitB(VectorEarlyExitVPBB);
-  for (VPRecipeBase &R : EarlyExitVPBB->phis()) {
-    auto *ExitIRI = cast<VPIRPhi>(&R);
-    // Early exit operand should always be last, i.e., 0 if EarlyExitVPBB has
-    // a single predecessor and 1 if it has two.
-    unsigned EarlyExitIdx = ExitIRI->getNumOperands() - 1;
-    if (ExitIRI->getNumOperands() != 1) {
-      // The first of two operands corresponds to the latch exit, via MiddleVPBB
-      // predecessor. Extract its last lane.
-      ExitIRI->extractLastLaneOfFirstOperand(MiddleBuilder);
-    }
+  if (!Plan.shouldEarlyExitContinueInScalarLoop()) {
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gbossu wrote:

I think it would be worth updating `llvm/docs/Vectorizers.rst` now that there is a new strategy to handle EE loops.

Given that this new strategy is currently toggled using `cl::opt<bool> HandleEarlyExitsInScalarTail`, do you plan to make it the default eventually or will there be a new TTI hook?

https://github.com/llvm/llvm-project/pull/148626