[llvm] [LV] Vectorize selecting last IV of min/max element. (PR #141431)

[Mel Chen via llvm-commits]

================
@@ -214,6 +214,40 @@ static bool checkOrderedReduction(RecurKind Kind, Instruction *ExactFPMathInst,
   return true;
 }
 
+static std::optional<RecurrenceDescriptor>
+getMultiUseMinMax(PHINode *Phi, RecurKind Kind, Loop *TheLoop) {
+  BasicBlock *Latch = TheLoop->getLoopLatch();
+  if (!Latch)
+    return std::nullopt;
+  Value *Inc = Phi->getIncomingValueForBlock(Latch);
+  RecurKind RK;
+  if (Phi->hasOneUse() ||
+      !RecurrenceDescriptor::isIntMinMaxRecurrenceKind(Kind))
+    return std::nullopt;
+
+  Value *A, *B;
+  if (match(Inc, m_OneUse(m_UMin(m_Value(A), m_Value(B)))))
+    RK = RecurKind::UMin;
+  else if (match(Inc, m_OneUse(m_UMax(m_Value(A), m_Value(B)))))
+    RK = RecurKind::UMax;
+  else if (match(Inc, m_OneUse(m_SMax(m_Value(A), m_Value(B)))))
+    RK = RecurKind::SMax;
+  else if (match(Inc, m_OneUse(m_SMin(m_Value(A), m_Value(B)))))
+    RK = RecurKind::SMin;
+  else
+    return std::nullopt;
+
+  if (A == B || (A != Phi && B != Phi))
+    return std::nullopt;
+
+  SmallPtrSet<Instruction *, 4> CastInsts;
+  Value *RdxStart = Phi->getIncomingValueForBlock(TheLoop->getLoopPreheader());
+  RecurrenceDescriptor RD(RdxStart, nullptr, nullptr, RK, FastMathFlags(),
+                          nullptr, Phi->getType(), false, false, CastInsts, -1U,
+                          true);
----------------
Mel-Chen wrote:

Could add /* arg name*/ for each nullptr and false?

https://github.com/llvm/llvm-project/pull/141431