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

[via llvm-commits]

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@@ -1432,7 +1433,143 @@ bool VPlanTransforms::handleFindLastReductions(VPlan &Plan) {
   return true;
 }
 
-bool VPlanTransforms::handleMultiUseReductions(VPlan &Plan) {
+/// Given a first argmin/argmax pattern with strict predicate consisting of
+/// 1) a MinOrMax reduction \p MinOrMaxPhiR producing \p MinOrMaxResult,
+/// 2) a wide induction \p WideIV,
+/// 3) a FindLastIV reduction \p FindLastIVPhiR,
+/// return the smallest index of the FindLastIV reduction result using UMin,
+/// unless \p MinOrMaxResult equals the start value of its MinOrMax reduction.
+/// In that case, return the start value of the FindLastIV reduction instead.
+/// If \p WideIV is not canonical, a new canonical wide IV is added, and the
+/// final result is scaled back to the non-canonical \p WideIV.
+/// The final value of the FindLastIV reduction was originally computed using
+/// \p FindIVSelect, \p FindIVCmp, and \p FindIVRdxResult, which are replaced
+/// and removed.
+/// Returns true if the pattern was handled successfully, false otherwise.
+static bool handleFirstArgMinOrMax(
+    VPlan &Plan, VPReductionPHIRecipe *MinOrMaxPhiR,
+    VPReductionPHIRecipe *FindLastIVPhiR, VPWidenIntOrFpInductionRecipe *WideIV,
+    VPInstruction *MinOrMaxResult, VPInstruction *FindIVSelect,
+    VPRecipeBase *FindIVCmp, VPInstruction *FindIVRdxResult) {
+  Type *Ty = Plan.getVectorLoopRegion()->getCanonicalIVType();
+  // TODO: Support non (i.e., narrower than) canonical IV types.
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ayalz wrote:

Nit/todo: provide ORE message whenever false is returned.

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