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

include/llvm/IR/IRBuilder.h

@@ -1464,6 +1464,30 @@ public:
     Value *Zeros = ConstantAggregateZero::get(VectorType::get(I32Ty, NumElts));
     return CreateShuffleVector(V, Undef, Zeros, Name + ".splat");
   }
+
+  /// \brief Return a value that has been extracted from a larger integer type.
+  Value *CreateExtractInteger(const DataLayout &DL, Value *From,
+                              IntegerType *ExtractedTy, uint64_t Offset,
+                              const Twine &Name) {
+    IntegerType *IntTy = cast<IntegerType>(From->getType());
+    assert(DL.getTypeStoreSize(ExtractedTy) + Offset <=
+               DL.getTypeStoreSize(IntTy) &&
+           "Element extends past full value");
+    uint64_t ShAmt = 8 * Offset;
+    Value *V = From;
+    if (DL.isBigEndian())
+      ShAmt = 8 * (DL.getTypeStoreSize(IntTy) -
+                   DL.getTypeStoreSize(ExtractedTy) - Offset);
+    if (ShAmt) {
+      V = CreateLShr(V, ShAmt, Name + ".shift");
+    }
+    assert(ExtractedTy->getBitWidth() <= IntTy->getBitWidth() &&
+           "Cannot extract to a larger integer!");
+    if (ExtractedTy != IntTy) {
+      V = CreateTrunc(V, ExtractedTy, Name + ".trunc");
+    }
+    return V;
+  }
 };
 
 // Create wrappers for C Binding types (see CBindingWrapping.h).

include/llvm/InitializePasses.h

@@ -272,6 +272,7 @@ void initializeSLPVectorizerPass(PassRegistry&);
 void initializeBBVectorizePass(PassRegistry&);
 void initializeMachineFunctionPrinterPassPass(PassRegistry&);
 void initializeStackMapLivenessPass(PassRegistry&);
+void initializeLoadCombinePass(PassRegistry&);
 }
 
 #endif

include/llvm/Transforms/Scalar.h

@@ -19,6 +19,7 @@
 
 namespace llvm {
 
+class BasicBlockPass;
 class FunctionPass;
 class Pass;
 class GetElementPtrInst;
@@ -381,6 +382,12 @@ FunctionPass *createAddDiscriminatorsPass();
 //
 FunctionPass *createSeparateConstOffsetFromGEPPass();
 
+//===----------------------------------------------------------------------===//
+//
+// LoadCombine - Combine loads into bigger loads.
+//
+BasicBlockPass *createLoadCombinePass();
+
 } // End llvm namespace
 
 #endif

lib/Transforms/IPO/PassManagerBuilder.cpp

@@ -53,6 +53,10 @@ static cl::opt<bool>
 RunLoopRerolling("reroll-loops", cl::Hidden,
                  cl::desc("Run the loop rerolling pass"));
 
+static cl::opt<bool> RunLoadCombine("combine-loads", cl::init(false),
+                                    cl::Hidden,
+                                    cl::desc("Run the load combining pass"));
+
 PassManagerBuilder::PassManagerBuilder() {
     OptLevel = 2;
     SizeLevel = 0;
@@ -65,6 +69,7 @@ PassManagerBuilder::PassManagerBuilder() {
     SLPVectorize = RunSLPVectorization;
     LoopVectorize = RunLoopVectorization;
     RerollLoops = RunLoopRerolling;
+    LoadCombine = RunLoadCombine;
 }
 
 PassManagerBuilder::~PassManagerBuilder() {
@@ -236,6 +241,9 @@ void PassManagerBuilder::populateModulePassManager(PassManagerBase &MPM) {
       MPM.add(createLoopUnrollPass());
   }
 
+  if (LoadCombine)
+    MPM.add(createLoadCombinePass());
+
   MPM.add(createAggressiveDCEPass());         // Delete dead instructions
   MPM.add(createCFGSimplificationPass()); // Merge & remove BBs
   MPM.add(createInstructionCombiningPass());  // Clean up after everything.
@@ -352,6 +360,9 @@ void PassManagerBuilder::populateLTOPassManager(PassManagerBase &PM,
   // More scalar chains could be vectorized due to more alias information
   PM.add(createSLPVectorizerPass()); // Vectorize parallel scalar chains.
 
+  if (LoadCombine)
+    PM.add(createLoadCombinePass());
+
   // Cleanup and simplify the code after the scalar optimizations.
   PM.add(createInstructionCombiningPass());
   addExtensionsToPM(EP_Peephole, PM);

lib/Transforms/Scalar/CMakeLists.txt

@@ -12,6 +12,7 @@ add_llvm_library(LLVMScalarOpts
   IndVarSimplify.cpp
   JumpThreading.cpp
   LICM.cpp
+  LoadCombine.cpp
   LoopDeletion.cpp
   LoopIdiomRecognize.cpp
   LoopInstSimplify.cpp

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)

lib/Transforms/Scalar/Scalar.cpp

@@ -65,6 +65,7 @@ void llvm::initializeScalarOpts(PassRegistry &Registry) {
   initializeSinkingPass(Registry);
   initializeTailCallElimPass(Registry);
   initializeSeparateConstOffsetFromGEPPass(Registry);
+  initializeLoadCombinePass(Registry);
 }
 
 void LLVMInitializeScalarOpts(LLVMPassRegistryRef R) {

test/Transforms/LoadCombine/load-combine.ll

@@ -0,0 +1,190 @@
+; RUN: opt < %s -load-combine -instcombine -S | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; Combine read from char* idiom.
+define i64 @LoadU64_x64_0(i64* %pData) {
+  %1 = bitcast i64* %pData to i8*
+  %2 = load i8* %1, align 1
+  %3 = zext i8 %2 to i64
+  %4 = shl nuw i64 %3, 56
+  %5 = getelementptr inbounds i8* %1, i64 1
+  %6 = load i8* %5, align 1
+  %7 = zext i8 %6 to i64
+  %8 = shl nuw nsw i64 %7, 48
+  %9 = or i64 %8, %4
+  %10 = getelementptr inbounds i8* %1, i64 2
+  %11 = load i8* %10, align 1
+  %12 = zext i8 %11 to i64
+  %13 = shl nuw nsw i64 %12, 40
+  %14 = or i64 %9, %13
+  %15 = getelementptr inbounds i8* %1, i64 3
+  %16 = load i8* %15, align 1
+  %17 = zext i8 %16 to i64
+  %18 = shl nuw nsw i64 %17, 32
+  %19 = or i64 %14, %18
+  %20 = getelementptr inbounds i8* %1, i64 4
+  %21 = load i8* %20, align 1
+  %22 = zext i8 %21 to i64
+  %23 = shl nuw nsw i64 %22, 24
+  %24 = or i64 %19, %23
+  %25 = getelementptr inbounds i8* %1, i64 5
+  %26 = load i8* %25, align 1
+  %27 = zext i8 %26 to i64
+  %28 = shl nuw nsw i64 %27, 16
+  %29 = or i64 %24, %28
+  %30 = getelementptr inbounds i8* %1, i64 6
+  %31 = load i8* %30, align 1
+  %32 = zext i8 %31 to i64
+  %33 = shl nuw nsw i64 %32, 8
+  %34 = or i64 %29, %33
+  %35 = getelementptr inbounds i8* %1, i64 7
+  %36 = load i8* %35, align 1
+  %37 = zext i8 %36 to i64
+  %38 = or i64 %34, %37
+  ret i64 %38
+; CHECK-LABEL: @LoadU64_x64_0(
+; CHECK: load i64* %{{.*}}, align 1
+; CHECK-NOT: load
+}
+
+; Combine simple adjacent loads.
+define i32 @"2xi16_i32"(i16* %x) {
+  %1 = load i16* %x, align 2
+  %2 = getelementptr inbounds i16* %x, i64 1
+  %3 = load i16* %2, align 2
+  %4 = zext i16 %3 to i32
+  %5 = shl nuw i32 %4, 16
+  %6 = zext i16 %1 to i32
+  %7 = or i32 %5, %6
+  ret i32 %7
+; CHECK-LABEL: @"2xi16_i32"(
+; CHECK: load i32* %{{.*}}, align 2
+; CHECK-NOT: load
+}
+
+; Don't combine loads across stores.
+define i32 @"2xi16_i32_store"(i16* %x, i16* %y) {
+  %1 = load i16* %x, align 2
+  store i16 0, i16* %y, align 2
+  %2 = getelementptr inbounds i16* %x, i64 1
+  %3 = load i16* %2, align 2
+  %4 = zext i16 %3 to i32
+  %5 = shl nuw i32 %4, 16
+  %6 = zext i16 %1 to i32
+  %7 = or i32 %5, %6
+  ret i32 %7
+; CHECK-LABEL: @"2xi16_i32_store"(
+; CHECK: load i16* %{{.*}}, align 2
+; CHECK: store
+; CHECK: load i16* %{{.*}}, align 2
+}
+
+; Don't combine loads with a gap.
+define i32 @"2xi16_i32_gap"(i16* %x) {
+  %1 = load i16* %x, align 2
+  %2 = getelementptr inbounds i16* %x, i64 2
+  %3 = load i16* %2, align 2
+  %4 = zext i16 %3 to i32
+  %5 = shl nuw i32 %4, 16
+  %6 = zext i16 %1 to i32
+  %7 = or i32 %5, %6
+  ret i32 %7
+; CHECK-LABEL: @"2xi16_i32_gap"(
+; CHECK: load i16* %{{.*}}, align 2
+; CHECK: load i16* %{{.*}}, align 2
+}
+
+; Combine out of order loads.
+define i32 @"2xi16_i32_order"(i16* %x) {
+  %1 = getelementptr inbounds i16* %x, i64 1
+  %2 = load i16* %1, align 2
+  %3 = zext i16 %2 to i32
+  %4 = load i16* %x, align 2
+  %5 = shl nuw i32 %3, 16
+  %6 = zext i16 %4 to i32
+  %7 = or i32 %5, %6
+  ret i32 %7
+; CHECK-LABEL: @"2xi16_i32_order"(
+; CHECK: load i32* %{{.*}}, align 2
+; CHECK-NOT: load
+}
+
+; Overlapping loads.
+define i32 @"2xi16_i32_overlap"(i8* %x) {
+  %1 = bitcast i8* %x to i16*
+  %2 = load i16* %1, align 2
+  %3 = getelementptr inbounds i8* %x, i64 1
+  %4 = bitcast i8* %3 to i16*
+  %5 = load i16* %4, align 2
+  %6 = zext i16 %5 to i32
+  %7 = shl nuw i32 %6, 16
+  %8 = zext i16 %2 to i32
+  %9 = or i32 %7, %8
+  ret i32 %9
+; CHECK-LABEL: @"2xi16_i32_overlap"(
+; CHECK: load i16* %{{.*}}, align 2
+; CHECK: load i16* %{{.*}}, align 2
+}
+
+; Combine valid alignments.
+define i64 @"2xi16_i64_align"(i8* %x) {
+  %1 = bitcast i8* %x to i32*
+  %2 = load i32* %1, align 4
+  %3 = getelementptr inbounds i8* %x, i64 4
+  %4 = bitcast i8* %3 to i16*
+  %5 = load i16* %4, align 2
+  %6 = getelementptr inbounds i8* %x, i64 6
+  %7 = bitcast i8* %6 to i16*
+  %8 = load i16* %7, align 2
+  %9 = zext i16 %8 to i64
+  %10 = shl nuw i64 %9, 48
+  %11 = zext i16 %5 to i64
+  %12 = shl nuw nsw i64 %11, 32
+  %13 = zext i32 %2 to i64
+  %14 = or i64 %12, %13
+  %15 = or i64 %14, %10
+  ret i64 %15
+; CHECK-LABEL: @"2xi16_i64_align"(
+; CHECK: load i64* %{{.*}}, align 4
+}
+
+; Non power of two.
+define i64 @"2xi16_i64_npo2"(i8* %x) {
+  %1 = load i8* %x, align 1
+  %2 = zext i8 %1 to i64
+  %3 = getelementptr inbounds i8* %x, i64 1
+  %4 = load i8* %3, align 1
+  %5 = zext i8 %4 to i64
+  %6 = shl nuw nsw i64 %5, 8
+  %7 = or i64 %6, %2
+  %8 = getelementptr inbounds i8* %x, i64 2
+  %9 = load i8* %8, align 1
+  %10 = zext i8 %9 to i64
+  %11 = shl nuw nsw i64 %10, 16
+  %12 = or i64 %11, %7
+  %13 = getelementptr inbounds i8* %x, i64 3
+  %14 = load i8* %13, align 1
+  %15 = zext i8 %14 to i64
+  %16 = shl nuw nsw i64 %15, 24
+  %17 = or i64 %16, %12
+  %18 = getelementptr inbounds i8* %x, i64 4
+  %19 = load i8* %18, align 1
+  %20 = zext i8 %19 to i64
+  %21 = shl nuw nsw i64 %20, 32
+  %22 = or i64 %21, %17
+  %23 = getelementptr inbounds i8* %x, i64 5
+  %24 = load i8* %23, align 1
+  %25 = zext i8 %24 to i64
+  %26 = shl nuw nsw i64 %25, 40
+  %27 = or i64 %26, %22
+  %28 = getelementptr inbounds i8* %x, i64 6
+  %29 = load i8* %28, align 1
+  %30 = zext i8 %29 to i64
+  %31 = shl nuw nsw i64 %30, 48
+  %32 = or i64 %31, %27
+  ret i64 %32
+; CHECK-LABEL: @"2xi16_i64_npo2"(
+; CHECK: load i32* %{{.*}}, align 1
+}