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

[Florian Hahn via llvm-commits]

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@@ -1120,6 +1122,133 @@ bool VPlanTransforms::handleMaxMinNumReductions(VPlan &Plan) {
   return true;
 }
 
+/// For argmin/argmax reductions with strict predicates, convert the existing
+/// FindLastIV reduction to a new UMin reduction of a wide canonical IV. If the
+/// original IV was not canonical, a new canonical wide IV is added, and the
+/// final result is scaled back to the original IV.
+static bool handleStrictArgMinArgMax(VPlan &Plan,
+                                     VPReductionPHIRecipe *MinMaxPhiR,
+                                     VPReductionPHIRecipe *FindIVPhiR,
+                                     VPWidenIntOrFpInductionRecipe *WideIV,
+                                     VPInstruction *MinMaxResult) {
+  Type *Ty = Plan.getVectorLoopRegion()->getCanonicalIVType();
+  if (Ty != VPTypeAnalysis(Plan).inferScalarType(FindIVPhiR))
+    return false;
+
+  // If the original wide IV is not canonical, create a new one. The wide IV is
+  // guaranteed to not wrap for all lanes that are active in the vector loop.
+  if (!WideIV->isCanonical()) {
+    VPValue *Zero = Plan.getOrAddLiveIn(ConstantInt::get(Ty, 0));
+    VPValue *One = Plan.getOrAddLiveIn(ConstantInt::get(Ty, 1));
+    auto *WidenCanIV = new VPWidenIntOrFpInductionRecipe(
+        nullptr, Zero, One, WideIV->getVFValue(),
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fhahn wrote:

Yep, I am looking into removing the reliance on the underlying values separately.

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