[llvm] [LV] Support argmin/argmax with strict predicates. (PR #170223)

[via llvm-commits]

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@@ -1473,33 +1603,50 @@ bool VPlanTransforms::handleMultiUseReductions(VPlan &Plan) {
                            FindIVPhiR->getRecurrenceKind()))
       return false;
 
+    assert(!FindIVPhiR->isInLoop() && !FindIVPhiR->isOrdered() &&
+           "cannot handle inloop/ordered reductions yet");
+
     // TODO: Support cases where IVOp is the IV increment.
     if (!match(IVOp, m_TruncOrSelf(m_VPValue(IVOp))) ||
         !isa<VPWidenIntOrFpInductionRecipe>(IVOp))
       return false;
 
-    CmpInst::Predicate RdxPredicate = [RdxKind]() {
+    // Check if the predicate is compatible with the reduction kind.
+    bool IsValidKindPred = [RdxKind, Pred]() {
       switch (RdxKind) {
       case RecurKind::UMin:
-        return CmpInst::ICMP_UGE;
+        return Pred == CmpInst::ICMP_UGE || Pred == CmpInst::ICMP_UGT;
       case RecurKind::UMax:
-        return CmpInst::ICMP_ULE;
+        return Pred == CmpInst::ICMP_ULE || Pred == CmpInst::ICMP_ULT;
       case RecurKind::SMax:
-        return CmpInst::ICMP_SLE;
+        return Pred == CmpInst::ICMP_SLE || Pred == CmpInst::ICMP_SLT;
       case RecurKind::SMin:
-        return CmpInst::ICMP_SGE;
+        return Pred == CmpInst::ICMP_SGE || Pred == CmpInst::ICMP_SGT;
       default:
         llvm_unreachable("unhandled recurrence kind");
       }
     }();
-
-    // TODO: Strict predicates need to find the first IV value for which the
-    // predicate holds, not the last.
-    if (Pred != RdxPredicate)
+    if (!IsValidKindPred) {
+      ORE->emit([&]() {
+        return OptimizationRemarkMissed(
+                   DEBUG_TYPE, "VectorizationMultiUseReductionPredicate",
+                   TheLoop->getStartLoc(), TheLoop->getHeader())
+               << "Multi-use reduction with predicate "
+               << CmpInst::getPredicateName(Pred)
+               << " incompatible with reduction kind";
+      });
       return false;
+    }
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ayalz wrote:

Just noting this is tested in select-index-remarks.ll

https://github.com/llvm/llvm-project/pull/170223