[llvm] [LV] Optimize FindLast recurrences to FindIV (NFCI). (PR #177870)

[Florian Hahn via llvm-commits]

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@@ -5349,3 +5349,96 @@ void VPlanTransforms::addExitUsersForFirstOrderRecurrences(VPlan &Plan,
     }
   }
 }
+
+void VPlanTransforms::optimizeFindIVReductions(VPlan &Plan,
+                                               PredicatedScalarEvolution &PSE,
+                                               Loop &L) {
+  ScalarEvolution &SE = *PSE.getSE();
+  VPRegionBlock *VectorLoopRegion = Plan.getVectorLoopRegion();
+
+  // Helper lambda to check if the IV range excludes the sentinel value.
+  auto CheckSentinel = [&SE](const SCEV *IVSCEV, bool IsFindLast,
+                             bool Signed) -> std::optional<APInt> {
+    unsigned BW = IVSCEV->getType()->getScalarSizeInBits();
+    APInt Sentinel =
+        IsFindLast
+            ? (Signed ? APInt::getSignedMinValue(BW) : APInt::getMinValue(BW))
+            : (Signed ? APInt::getSignedMaxValue(BW) : APInt::getMaxValue(BW));
+
+    ConstantRange IVRange =
+        Signed ? SE.getSignedRange(IVSCEV) : SE.getUnsignedRange(IVSCEV);
+    if (!IVRange.contains(Sentinel))
+      return Sentinel;
+    return std::nullopt;
+  };
+
+  for (VPRecipeBase &Phi :
+       make_early_inc_range(VectorLoopRegion->getEntryBasicBlock()->phis())) {
+    auto *PhiR = dyn_cast<VPReductionPHIRecipe>(&Phi);
+    if (!PhiR || !RecurrenceDescriptor::isFindLastRecurrenceKind(
+                     PhiR->getRecurrenceKind()))
+      continue;
+
+    // Get the IV from the backedge value of the reduction phi.
+    // The backedge value should be a select between the phi and the IV.
+    VPValue *BackedgeVal = PhiR->getBackedgeValue();
+    VPValue *TrueVal, *FalseVal;
+    if (!match(BackedgeVal,
+               m_Select(m_VPValue(), m_VPValue(TrueVal), m_VPValue(FalseVal))))
+      continue;
+
+    // The non-phi operand of the select is the IV.
+    VPValue *IV = (TrueVal == PhiR) ? FalseVal : TrueVal;
+
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fhahn wrote:

Added an assert that PhiR is in the operands, thanks!

https://github.com/llvm/llvm-project/pull/177870