[llvm] [LV] Convert gather loads with invariant stride into strided loads (PR #147297)

[Mel Chen via llvm-commits]

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@@ -5106,3 +5133,191 @@ void VPlanTransforms::addExitUsersForFirstOrderRecurrences(VPlan &Plan,
     }
   }
 }
+
+static std::pair<VPValue *, VPValue *> matchStridedStart(VPValue *CurIndex) {
+  // TODO: Support VPWidenPointerInductionRecipe.
+  if (auto *WidenIV = dyn_cast<VPWidenIntOrFpInductionRecipe>(CurIndex))
+    return {WidenIV, WidenIV->getStepValue()};
+
+  auto *WidenR = dyn_cast<VPWidenRecipe>(CurIndex);
+  if (!WidenR || !WidenR->getUnderlyingInstr())
+    return {nullptr, nullptr};
+
+  unsigned Opcode = WidenR->getOpcode();
+  // TODO: Support Instruction::Add and Instruction::Or.
+  if (Opcode != Instruction::Shl && Opcode != Instruction::Mul)
+    return {nullptr, nullptr};
+
+  // Match the pattern binop(variant, uniform), or binop(uniform, variant) if
+  // the binary operator is commutative.
+  bool IsLHSUniform = vputils::isSingleScalar(WidenR->getOperand(0));
+  if (IsLHSUniform == vputils::isSingleScalar(WidenR->getOperand(1)) ||
+      (IsLHSUniform && !Instruction::isCommutative(Opcode)))
+    return {nullptr, nullptr};
+  unsigned VarIdx = IsLHSUniform ? 1 : 0;
+
+  auto [Start, Stride] = matchStridedStart(WidenR->getOperand(VarIdx));
+  if (!Start)
+    return {nullptr, nullptr};
+
+  SmallVector<VPValue *> StartOps(WidenR->operands());
+  StartOps[VarIdx] = Start;
+  auto *StartR = new VPReplicateRecipe(WidenR->getUnderlyingInstr(), StartOps,
+                                       /*IsUniform*/ true);
+  StartR->insertBefore(WidenR);
+
+  unsigned InvIdx = VarIdx == 0 ? 1 : 0;
+  auto *StrideR =
+      new VPInstruction(Opcode, {Stride, WidenR->getOperand(InvIdx)});
+  StrideR->insertBefore(WidenR);
+  return {StartR, StrideR};
+}
+
+/// Checks if the given VPWidenGEPRecipe \p WidenGEP represents a strided
+/// access. If so, it creates recipes representing the base pointer and stride
+/// in element units, and returns a tuple of {base pointer, stride, element
+/// type}. Otherwise, returns a tuple where all elements are nullptr.
+static std::tuple<VPValue *, VPValue *, Type *>
+determineBaseAndStride(VPWidenGEPRecipe *WidenGEP) {
+  // TODO: Check if the base pointer is strided.
+  if (!WidenGEP->isPointerLoopInvariant())
+    return {nullptr, nullptr, nullptr};
+
+  // Find the only one variant index.
+  std::optional<unsigned> VarIndex = WidenGEP->getUniqueVariantIndex();
+  if (!VarIndex)
+    return {nullptr, nullptr, nullptr};
+
+  Type *ElementTy = WidenGEP->getIndexedType(*VarIndex);
+  if (ElementTy->isScalableTy() || ElementTy->isStructTy() ||
+      ElementTy->isVectorTy())
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Mel-Chen wrote:

015eccf7f4e0f695bffd935e2429355eb606c9ca

https://github.com/llvm/llvm-project/pull/147297