[llvm] [VPlan] Implement VPlan-based unit-strideness speculation (PR #182595)

[Luke Lau via llvm-commits]

================
@@ -5521,6 +5531,184 @@ void VPlanTransforms::makeMemOpWideningDecisions(VPlan &Plan, VFRange &Range,
                            });
 }
 
+void VPlanTransforms::multiversionForUnitStridedMemOps(
+    VPlan &Plan, VPCostContext &CostCtx, VPRecipeBuilder &RecipeBuilder,
+    VFRange &Range, SmallVectorImpl<VPInstruction *> &MemOps) {
+  ScalarEvolution *SE = CostCtx.PSE.getSE();
+  PredicatedScalarEvolution StrideMVPSE(*SE, const_cast<Loop &>(*CostCtx.L));
+  SCEVUnionPredicate StridePredicates({}, *SE);
+
+  for (VPInstruction *VPI : MemOps) {
+    auto *PtrOp = VPI->getOpcode() == Instruction::Load ? VPI->getOperand(0)
+                                                        : VPI->getOperand(1);
+
+    const SCEV *PtrSCEV =
+        vputils::getSCEVExprForVPValue(PtrOp, CostCtx.PSE, CostCtx.L);
+    const SCEV *Start, *Stride;
+
+    if (!match(PtrSCEV, m_scev_AffineAddRec(m_SCEV(Start), m_SCEV(Stride),
+                                            m_SpecificLoop(CostCtx.L))))
+      continue;
+
+    Type *ScalarTy = VPI->getOpcode() == Instruction::Load
+                         ? VPI->getScalarType()
+                         : VPI->getOperand(0)->getScalarType();
+
+    if (VPI->getMask()) {
+      Instruction *I = VPI->getUnderlyingInstr();
+      bool IsLoad = VPI->getOpcode() == Instruction::Load;
+      if (!LoopVectorizationPlanner::getDecisionAndClampRange(
+              [&](ElementCount VF) -> bool {
+                return CostCtx.Config.isLegalMaskedLoadOrStore(
+                    IsLoad, ScalarTy, getLoadStoreAlignment(I),
+                    getLoadStoreAddressSpace(I));
+              },
+              Range))
+        continue;
+    }
+
----------------
lukel97 wrote:

That makes sense. Can we add a comment to explain that. Also since this will clamp the VF, should we move this down further after the SCEV checks? Maybe beside the backedge check which also clamps VF

https://github.com/llvm/llvm-project/pull/182595