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

[via llvm-commits]

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@@ -1120,6 +1122,129 @@ 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 handleFirstArgMinArgMax(VPlan &Plan,
+                                    VPReductionPHIRecipe *MinMaxPhiR,
+                                    VPReductionPHIRecipe *FindIVPhiR,
+                                    VPWidenIntOrFpInductionRecipe *WideIV,
+                                    VPInstruction *MinMaxResult) {
+  Type *Ty = Plan.getVectorLoopRegion()->getCanonicalIVType();
+  // TODO: Support different IV types.
+  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.getConstantInt(Ty, 0);
+    VPValue *One = Plan.getConstantInt(Ty, 1);
+    auto *WidenCanIV = new VPWidenIntOrFpInductionRecipe(
+        nullptr, Zero, One, WideIV->getVFValue(),
+        WideIV->getInductionDescriptor(),
+        VPIRFlags::WrapFlagsTy(/*HasNUW=*/true, /*HasNSW=*/false),
+        WideIV->getDebugLoc());
+    WidenCanIV->insertBefore(WideIV);
+
+    // Update the select to use the wide canonical IV.
+    auto *SelectR = cast<VPSingleDefRecipe>(
+        FindIVPhiR->getBackedgeValue()->getDefiningRecipe());
+    assert(match(SelectR, m_Select(m_VPValue(), m_VPValue(), m_VPValue())) &&
+           "backedge value must be a select");
+    WideIV->replaceUsesWithIf(WidenCanIV, [SelectR](const VPUser &U, unsigned) {
+      return SelectR == &U;
+    });
+  }
+
+  // Create the new UMin reduction recipe to track the minimum index.
----------------
ayalz wrote:

Creating a UMin reduction may be confusing - as it conceptually suggests recording the first iteration that updates the running min/max. The header phi itself remains the same, vectorizing Find[Last]IV, albeit one with a simpler sentinel-less final reduction (more below).

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