[llvm] [LV] Vectorize conditional scalar assignments (PR #158088)

[Florian Hahn via llvm-commits]

================
@@ -997,6 +998,73 @@ bool VPlanTransforms::handleMaxMinNumReductions(VPlan &Plan) {
   return true;
 }
 
+bool VPlanTransforms::handleFindLastReductions(VPlan &Plan,
+                                               VPRecipeBuilder &RecipeBuilder) {
+  if (Plan.hasScalarVFOnly())
+    return false;
+
+  // We want to create the following nodes:
+  // vec.body:
+  //   mask.phi = phi <VF x i1> [ all.false, vec.ph ], [ new.mask, vec.body ]
+  //   ...data.phi already exists, but needs updating...
+  //   data.phi = phi <VF x Ty> [ default.val, vec.ph ], [ new.data, vec.body ]
+  //
+  //   ...'data' and 'compare' created by existing nodes...
+  //
+  //   any_active = i1 any_of_reduction(compare)
+  //   new.mask = select any_active, compare, mask.phi
+  //   new.data = select any_active, data, data.phi
+  //
+  // middle.block:
+  //   result = extract-last-active new.data, new.mask, default.val
+
+  for (auto &Phi : Plan.getVectorLoopRegion()->getEntryBasicBlock()->phis()) {
+    auto *PhiR = dyn_cast<VPReductionPHIRecipe>(&Phi);
+    if (!PhiR || !RecurrenceDescriptor::isFindLastRecurrenceKind(
+                     PhiR->getRecurrenceKind()))
+      continue;
+
+    // Find the condition for the select
+    auto *SelectR = cast<VPSingleDefRecipe>(&PhiR->getBackedgeRecipe());
+    VPValue *Cond = nullptr, *Op1 = nullptr, *Op2 = nullptr;
+    if (!match(SelectR,
+               m_Select(m_VPValue(Cond), m_VPValue(Op1), m_VPValue(Op2))))
+      return false;
+
+    // Add mask phi
----------------
fhahn wrote:

nit:
```suggestion
    // Add mask phi.
```

for consitency with other comments

https://github.com/llvm/llvm-project/pull/158088