[llvm] [LoopVectorize] Widen selected-base loads with masked loads (PR #213227)

[via llvm-commits]

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@@ -5439,6 +5440,118 @@ void VPlanTransforms::makeMemOpWideningDecisions(VPlan &Plan, VFRange &Range,
         return false;
       });
 
+  // Recognize loads through a pointer selected per iteration between two
+  // loop-invariant bases. Although the combined address is not consecutive,
+  // each selected address chain is. When profitable, use two complementary
+  // masked consecutive loads and blend their results instead of gathering a
+  // vector of selected pointers.
+  VPlanTransforms::runPass(
+      "widenSelectedBaseLoads", ProcessSubset, Plan, [&](VPInstruction *VPI) {
+        SelectedBaseLoadPattern Pattern;
+        if (!match(VPI, m_SelectedBaseLoad(Pattern)))
+          return false;
+
+        if (!vputils::isUniformAcrossVFsAndUFs(Pattern.TrueBase) ||
+            !vputils::isUniformAcrossVFsAndUFs(Pattern.FalseBase) ||
+            vputils::isUniformAcrossVFsAndUFs(Pattern.Cond))
+          return false;
+
+        auto *LI = cast<LoadInst>(VPI->getUnderlyingInstr());
+        Type *ScalarTy = LI->getType();
+        // The ingredient's alignment only applies to the address selected by
+        // each scalar iteration. It does not imply that either candidate base
+        // is aligned when that candidate is masked off. Use a conservative
+        // alignment for the newly-speculated base pointers.
+        const Align SplitAlignment(1);
+        if (!CostCtx.Config.isLegalMaskedLoadOrStore(
+                /*IsLoad=*/true, ScalarTy, SplitAlignment,
+                LI->getPointerAddressSpace()))
+          return false;
+
+        VPBuilder Builder(VPI);
+        SmallVector<VPValue *> TrueOps = Pattern.GEPOperands;
+        SmallVector<VPValue *> FalseOps = Pattern.GEPOperands;
+        // replace the baseOp
+        TrueOps[0] = Pattern.TrueBase;
+        FalseOps[0] = Pattern.FalseBase;
+        auto *TrueAddr =
+            Builder.insert(Pattern.Addr->cloneWithOperands(TrueOps));
+        auto *FalseAddr =
+            Builder.insert(Pattern.Addr->cloneWithOperands(FalseOps));
+
+        // Splitting the selected base must expose two unit-stride accesses.
+        // Remove the speculative address recipes again if either proof fails.
+        if (getConstantStride(TrueAddr, ScalarTy, CostCtx.PSE, CostCtx.L) !=
+                1 ||
+            getConstantStride(FalseAddr, ScalarTy, CostCtx.PSE, CostCtx.L) !=
+                1) {
+          TrueAddr->eraseFromParent();
+          FalseAddr->eraseFromParent();
+          return false;
+        }
+
+        bool HasActiveMask = RecipeBuilder.isPredicatedInst(LI);
+        auto IsProfitable = [&](ElementCount VF) {
+          // Compare against the legacy decision for each VF so a scalarization
+          // choice is not replaced just because the gather is expensive.
+          if (VF.isScalable() || CostCtx.willBeScalarized(LI, VF) ||
+              !CostCtx.Config.isLegalGatherOrScatter(LI, VF))
+            return false;
+          Type *VecTy = VectorType::get(ScalarTy, VF);
+          InstructionCost BlendedCost = slpvectorizer::getBlendedLoadCost(
+              CostCtx.TTI, VecTy, SplitAlignment,
+              LI->getPointerAddressSpace(), CostCtx.CostKind);
+          if (HasActiveMask) {
+            Type *MaskTy = CmpInst::makeCmpResultType(VecTy);
+            BlendedCost +=
+                2 * CostCtx.TTI.getArithmeticInstrCost(Instruction::And,
+                                                       MaskTy,
+                                                       CostCtx.CostKind);
+          }
+          // Reuse the legacy decision's cost. This is the gather cost for the
+          // usual case, but also keeps the comparison correct if the legacy
+          // model selected scalarization.
+          InstructionCost LegacyCost = CostCtx.getLegacyCost(LI, VF);
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OHNope wrote:

Oh ,the legacy cost came in when I addressed the earlier review comment about reusing the existing cost model — I reached for the legacy one rather than the recipe costs,sry ;; 

https://github.com/llvm/llvm-project/pull/213227