[llvm] [VPlan] Convert EVL loops to variable-length stepping after dissolution (PR #147222)

[Florian Hahn via llvm-commits]

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@@ -2357,6 +2357,71 @@ bool VPlanTransforms::tryAddExplicitVectorLength(
   return true;
 }
 
+void VPlanTransforms::simplifyEVLIVs(VPlan &Plan) {
+  using namespace llvm::VPlanPatternMatch;
+  // Find EVL loop entries by locating VPEVLBasedIVPHIRecipe
+  // There should be only one EVL PHI in the entire plan
+  VPEVLBasedIVPHIRecipe *EVLPhi = nullptr;
+
+  for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(
+           vp_depth_first_shallow(Plan.getEntry())))
+    for (VPRecipeBase &R : VPBB->phis())
+      if (auto *PhiR = dyn_cast<VPEVLBasedIVPHIRecipe>(&R)) {
+        assert(!EVLPhi && "Found multiple EVL PHIs. Only one expected");
+        EVLPhi = PhiR;
+      }
+
+  // Early return if no EVL PHI is found
+  if (!EVLPhi)
+    return;
+
+  VPBasicBlock *Entry = EVLPhi->getParent();
+  VPValue *EVLIncrement = EVLPhi->getBackedgeValue();
+
+  // Convert EVLPhi to concrete recipe.
+  auto *ScalarR =
+      VPBuilder(EVLPhi).createScalarPhi({EVLPhi->getStartValue(), EVLIncrement},
+                                        EVLPhi->getDebugLoc(), "evl.based.iv");
+  EVLPhi->replaceAllUsesWith(ScalarR);
+  EVLPhi->eraseFromParent();
+
+  // Replace CanonicalIVInc with EVL-PHI increment
+  VPRecipeBase *CanonicalIV = &*Entry->begin();
+  assert(dyn_cast<VPPhi>(CanonicalIV) && "Unexpected canoincal iv");
+  VPValue *Backedge = CanonicalIV->getOperand(1);
+  Backedge->replaceAllUsesWith(EVLIncrement);
+
+  // Remove unused phi
+  VPRecipeBase *CanonicalIVIncrement = Backedge->getDefiningRecipe();
+  CanonicalIVIncrement->eraseFromParent();
+  CanonicalIV->eraseFromParent();
+
+  // Find the latch-exiting block and replace use of VectorTripCount
+  // Before: (branch-on-count EVLIVInc, VectorTripCount)
+  // After: (branch-on-count EVLIVInc, TripCount)
+  auto Range =
+      VPBlockUtils::blocksOnly<VPBasicBlock>(vp_depth_first_shallow(Entry));
+  auto It = find_if(Range, [&Entry](VPBasicBlock *VPBB) {
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fhahn wrote:

looking for the latch could be done by just getting the second predecessor of the header?

https://github.com/llvm/llvm-project/pull/147222