[llvm] [LV] Trim VFs that cause vector splitting when tail-folding with EVL (PR #222836)

[Elvis Wang via llvm-commits]

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@@ -653,3 +676,35 @@ void VPlanTransforms::convertEVLExitCond(VPlan &Plan) {
   LatchBr->setOperand(
       0, Builder.createICmp(CmpInst::ICMP_EQ, AVLNext, Plan.getZero(AVLTy)));
 }
+
+void VPlanTransforms::trimVFsCausingSplits(VPlan &Plan,
+                                           VPCostContext &CostCtx) {
+  if (!Plan.hasScalableVF())
+    return;
+
+  // Get the widest type in the vector region.
+  unsigned WidestType = 8;
+  VPRegionBlock *VectorLoop = Plan.getVectorLoopRegion();
+  for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(
+           vp_depth_first_deep(VectorLoop->getEntry())))
+    for (VPRecipeBase &BaseR : make_range(VPBB->getFirstNonPhi(), VPBB->end()))
+      if (auto *R = dyn_cast<VPSingleDefRecipe>(&BaseR);
+          R && !vputils::onlyScalarValuesUsed(R)) {
+        // Skip the partial reduction since its VF is scaled down and can
+        // be calculated by its operands.
+        if (isa<VPExpressionRecipe>(R))
+          continue;
+        WidestType =
+            std::max(WidestType, R->getScalarType()->getScalarSizeInBits());
+      }
+
+  // Trim the VF that will generate vectors need to split.
+  SmallVector<ElementCount, 4> VFs = to_vector(Plan.vectorFactors());
+  for (auto VF : reverse(VFs)) {
+    if (CostCtx.TTI.getRegUsageForType(VectorType::get(
+            Type::getIntNTy(Plan.getContext(), WidestType), VF)) <= 8)
+      break;
+    Plan.removeVF(VF);
----------------
ElvisWang123 wrote:

Yes, but using `getDecisionAndClampRange` here (late optimization pipeline) requires lots of changes (need to pass the range here, delay VF attached on plan ...). So I'd prefer to trim the VF from the Plan.

https://github.com/llvm/llvm-project/pull/222836