Add LoadCombine pass.

This pass is disabled by default. Use -combine-loads to enable in -O[1-3]

Differential revision: http://reviews.llvm.org/D3580

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209791 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/LoadCombine.cpp

@@ -0,0 +1,268 @@
+//===- LoadCombine.cpp - Combine Adjacent Loads ---------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+/// \file
+/// This transformation combines adjacent loads.
+///
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Scalar.h"
+
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/TargetFolder.h"
+#include "llvm/Pass.h"
+#include "llvm/IR/DataLayout.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/IRBuilder.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/MathExtras.h"
+#include "llvm/Support/raw_ostream.h"
+
+using namespace llvm;
+
+#define DEBUG_TYPE "load-combine"
+
+STATISTIC(NumLoadsAnalyzed, "Number of loads analyzed for combining");
+STATISTIC(NumLoadsCombined, "Number of loads combined");
+
+namespace {
+struct PointerOffsetPair {
+  Value *Pointer;
+  uint64_t Offset;
+};
+
+struct LoadPOPPair {
+  LoadPOPPair(LoadInst *L, PointerOffsetPair P, unsigned O)
+      : Load(L), POP(P), InsertOrder(O) {}
+  LoadPOPPair() {}
+  LoadInst *Load;
+  PointerOffsetPair POP;
+  /// \brief The new load needs to be created before the first load in IR order.
+  unsigned InsertOrder;
+};
+
+class LoadCombine : public BasicBlockPass {
+  LLVMContext *C;
+  const DataLayout *DL;
+
+public:
+  LoadCombine()
+      : BasicBlockPass(ID),
+        C(nullptr), DL(nullptr) {
+    initializeSROAPass(*PassRegistry::getPassRegistry());
+  }
+  bool doInitialization(Function &) override;
+  bool runOnBasicBlock(BasicBlock &BB) override;
+  void getAnalysisUsage(AnalysisUsage &AU) const override;
+
+  const char *getPassName() const override { return "LoadCombine"; }
+  static char ID;
+
+  typedef IRBuilder<true, TargetFolder> BuilderTy;
+
+private:
+  BuilderTy *Builder;
+
+  PointerOffsetPair getPointerOffsetPair(LoadInst &);
+  bool combineLoads(DenseMap<const Value *, SmallVector<LoadPOPPair, 8>> &);
+  bool aggregateLoads(SmallVectorImpl<LoadPOPPair> &);
+  bool combineLoads(SmallVectorImpl<LoadPOPPair> &);
+};
+}
+
+bool LoadCombine::doInitialization(Function &F) {
+  DEBUG(dbgs() << "LoadCombine function: " << F.getName() << "\n");
+  C = &F.getContext();
+  DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
+  if (!DLP) {
+    DEBUG(dbgs() << "  Skipping LoadCombine -- no target data!\n");
+    return false;
+  }
+  DL = &DLP->getDataLayout();
+  return true;
+}
+
+PointerOffsetPair LoadCombine::getPointerOffsetPair(LoadInst &LI) {
+  PointerOffsetPair POP;
+  POP.Pointer = LI.getPointerOperand();
+  POP.Offset = 0;
+  while (isa<BitCastInst>(POP.Pointer) || isa<GetElementPtrInst>(POP.Pointer)) {
+    if (auto *GEP = dyn_cast<GetElementPtrInst>(POP.Pointer)) {
+      unsigned BitWidth = DL->getPointerTypeSizeInBits(GEP->getType());
+      APInt Offset(BitWidth, 0);
+      if (GEP->accumulateConstantOffset(*DL, Offset))
+        POP.Offset += Offset.getZExtValue();
+      else
+        // Can't handle GEPs with variable indices.
+        return POP;
+      POP.Pointer = GEP->getPointerOperand();
+    } else if (auto *BC = dyn_cast<BitCastInst>(POP.Pointer))
+      POP.Pointer = BC->getOperand(0);
+  }
+  return POP;
+}
+
+bool LoadCombine::combineLoads(
+    DenseMap<const Value *, SmallVector<LoadPOPPair, 8>> &LoadMap) {
+  bool Combined = false;
+  for (auto &Loads : LoadMap) {
+    if (Loads.second.size() < 2)
+      continue;
+    std::sort(Loads.second.begin(), Loads.second.end(),
+              [](const LoadPOPPair &A, const LoadPOPPair &B) {
+      return A.POP.Offset < B.POP.Offset;
+    });
+    if (aggregateLoads(Loads.second))
+      Combined = true;
+  }
+  return Combined;
+}
+
+/// \brief Try to aggregate loads from a sorted list of loads to be combined.
+///
+/// It is guaranteed that no writes occur between any of the loads. All loads
+/// have the same base pointer. There are at least two loads.
+bool LoadCombine::aggregateLoads(SmallVectorImpl<LoadPOPPair> &Loads) {
+  assert(Loads.size() >= 2 && "Insufficient loads!");
+  LoadInst *BaseLoad = nullptr;
+  SmallVector<LoadPOPPair, 8> AggregateLoads;
+  bool Combined = false;
+  uint64_t PrevOffset = -1ull;
+  uint64_t PrevSize = 0;
+  for (auto &L : Loads) {
+    if (PrevOffset == -1ull) {
+      BaseLoad = L.Load;
+      PrevOffset = L.POP.Offset;
+      PrevSize = DL->getTypeStoreSize(L.Load->getType());
+      AggregateLoads.push_back(L);
+      continue;
+    }
+    if (L.Load->getAlignment() > BaseLoad->getAlignment())
+      continue;
+    if (L.POP.Offset > PrevOffset + PrevSize) {
+      // No other load will be combinable
+      if (combineLoads(AggregateLoads))
+        Combined = true;
+      AggregateLoads.clear();
+      PrevOffset = -1;
+      continue;
+    }
+    if (L.POP.Offset != PrevOffset + PrevSize)
+      // This load is offset less than the size of the last load.
+      // FIXME: We may want to handle this case.
+      continue;
+    PrevOffset = L.POP.Offset;
+    PrevSize = DL->getTypeStoreSize(L.Load->getType());
+    AggregateLoads.push_back(L);
+  }
+  if (combineLoads(AggregateLoads))
+    Combined = true;
+  return Combined;
+}
+
+/// \brief Given a list of combinable load. Combine the maximum number of them.
+bool LoadCombine::combineLoads(SmallVectorImpl<LoadPOPPair> &Loads) {
+  // Remove loads from the end while the size is not a power of 2.
+  unsigned TotalSize = 0;
+  for (const auto &L : Loads)
+    TotalSize += L.Load->getType()->getPrimitiveSizeInBits();
+  while (TotalSize != 0 && !isPowerOf2_32(TotalSize))
+    TotalSize -= Loads.pop_back_val().Load->getType()->getPrimitiveSizeInBits();
+  if (Loads.size() < 2)
+    return false;
+
+  DEBUG({
+    dbgs() << "***** Combining Loads ******\n";
+    for (const auto &L : Loads) {
+      dbgs() << L.POP.Offset << ": " << *L.Load << "\n";
+    }
+  });
+
+  // Find first load. This is where we put the new load.
+  LoadPOPPair FirstLP;
+  FirstLP.InsertOrder = -1u;
+  for (const auto &L : Loads)
+    if (L.InsertOrder < FirstLP.InsertOrder)
+      FirstLP = L;
+
+  unsigned AddressSpace =
+      FirstLP.POP.Pointer->getType()->getPointerAddressSpace();
+
+  Builder->SetInsertPoint(FirstLP.Load);
+  Value *Ptr = Builder->CreateConstGEP1_64(
+      Builder->CreatePointerCast(Loads[0].POP.Pointer,
+                                 Builder->getInt8PtrTy(AddressSpace)),
+      Loads[0].POP.Offset);
+  LoadInst *NewLoad = new LoadInst(
+      Builder->CreatePointerCast(
+          Ptr, PointerType::get(IntegerType::get(Ptr->getContext(), TotalSize),
+                                Ptr->getType()->getPointerAddressSpace())),
+      Twine(Loads[0].Load->getName()) + ".combined", false,
+      Loads[0].Load->getAlignment(), FirstLP.Load);
+
+  for (const auto &L : Loads) {
+    Builder->SetInsertPoint(L.Load);
+    Value *V = Builder->CreateExtractInteger(
+        *DL, NewLoad, cast<IntegerType>(L.Load->getType()),
+        L.POP.Offset - Loads[0].POP.Offset, "combine.extract");
+    L.Load->replaceAllUsesWith(V);
+  }
+
+  NumLoadsCombined = NumLoadsCombined + Loads.size();
+  return true;
+}
+
+bool LoadCombine::runOnBasicBlock(BasicBlock &BB) {
+  if (skipOptnoneFunction(BB) || !DL)
+    return false;
+
+  IRBuilder<true, TargetFolder>
+  TheBuilder(BB.getContext(), TargetFolder(DL));
+  Builder = &TheBuilder;
+
+  DenseMap<const Value *, SmallVector<LoadPOPPair, 8>> LoadMap;
+
+  bool Combined = false;
+  unsigned Index = 0;
+  for (auto &I : BB) {
+    if (I.mayWriteToMemory() || I.mayThrow()) {
+      if (combineLoads(LoadMap))
+        Combined = true;
+      LoadMap.clear();
+      continue;
+    }
+    LoadInst *LI = dyn_cast<LoadInst>(&I);
+    if (!LI)
+      continue;
+    ++NumLoadsAnalyzed;
+    if (!LI->isSimple() || !LI->getType()->isIntegerTy())
+      continue;
+    auto POP = getPointerOffsetPair(*LI);
+    if (!POP.Pointer)
+      continue;
+    LoadMap[POP.Pointer].push_back(LoadPOPPair(LI, POP, Index++));
+  }
+  if (combineLoads(LoadMap))
+    Combined = true;
+  return Combined;
+}
+
+void LoadCombine::getAnalysisUsage(AnalysisUsage &AU) const {
+  AU.setPreservesCFG();
+}
+
+char LoadCombine::ID = 0;
+
+BasicBlockPass *llvm::createLoadCombinePass() {
+  return new LoadCombine();
+}
+
+INITIALIZE_PASS(LoadCombine, "load-combine", "Combine Adjacent Loads", false,
+                false)