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

[via llvm-commits]

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@@ -214,6 +214,51 @@ static bool checkOrderedReduction(RecurKind Kind, Instruction *ExactFPMathInst,
   return true;
 }
 
+/// Returns true if \p Phi is a min/max reduction matching \p Kind where \p Phi
+/// is used in the loop outside the reduction chain. This is common for loops
+/// selecting the index of a minimum/maximum value (argmin/argmax).
+static bool isMinMaxReductionWithLoopUsersOutsideReductionChain(
+    PHINode *Phi, RecurKind Kind, Loop *TheLoop, RecurrenceDescriptor &RedDes) {
+  BasicBlock *Latch = TheLoop->getLoopLatch();
+  if (!Latch)
+    return false;
+  assert(Phi->getNumIncomingValues() == 2 && "phi must have 2 incoming values");
+  Value *Inc = Phi->getIncomingValueForBlock(Latch);
+  if (Phi->hasOneUse() || !Inc->hasOneUse() ||
+      !RecurrenceDescriptor::isIntMinMaxRecurrenceKind(Kind))
+    return false;
+
+  Value *A, *B;
+  bool IsMinMax = [&]() {
+    switch (Kind) {
+    case RecurKind::UMax:
+      return match(Inc, m_UMax(m_Value(A), m_Value(B)));
+    case RecurKind::UMin:
+      return match(Inc, m_UMin(m_Value(A), m_Value(B)));
+    case RecurKind::SMax:
+      return match(Inc, m_SMax(m_Value(A), m_Value(B)));
+    case RecurKind::SMin:
+      return match(Inc, m_SMin(m_Value(A), m_Value(B)));
+    default:
+      llvm_unreachable("all min/max kinds must be handled");
+    }
+  }();
+  if (!IsMinMax)
+    return false;
+
+  if (A == B || (A != Phi && B != Phi))
----------------
ayalz wrote:

Nit: can be written as `!((A == Phi) ^ (B == Phi))` or `((A == Phi) == (B == Phi))`, but probably clearer as written.

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