RelLookupTableConverter.cpp

//===- RelLookupTableConverterPass - Rel Table Conv -----------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file implements relative lookup table converter that converts
// lookup tables to relative lookup tables to make them PIC-friendly.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Utils/RelLookupTableConverter.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Module.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"

using namespace llvm;

static bool shouldConvertToRelLookupTable(Module &M, GlobalVariable &GV) {
  // If lookup table has more than one user,
  // do not generate a relative lookup table.
  // This is to simplify the analysis that needs to be done for this pass.
  // TODO: Add support for lookup tables with multiple uses.
  // For ex, this can happen when a function that uses a lookup table gets
  // inlined into multiple call sites.
  if (!GV.hasInitializer() ||
      !GV.isConstant() ||
      !GV.hasOneUse())
    return false;

  GetElementPtrInst *GEP =
      dyn_cast<GetElementPtrInst>(GV.use_begin()->getUser());
  if (!GEP || !GEP->hasOneUse())
    return false;

  LoadInst *Load = dyn_cast<LoadInst>(GEP->use_begin()->getUser());
  if (!Load || !Load->hasOneUse())
    return false;

  // If the original lookup table does not have local linkage and is
  // not dso_local, do not generate a relative lookup table.
  // This optimization creates a relative lookup table that consists of
  // offsets between the start of the lookup table and its elements.
  // To be able to generate these offsets, relative lookup table and
  // its elements should have internal linkage and be dso_local, which means
  // that they should resolve to symbols within the same linkage unit.
  if (!GV.hasLocalLinkage() ||
      !GV.isDSOLocal() ||
      !GV.isImplicitDSOLocal())
    return false;

  ConstantArray *Array = dyn_cast<ConstantArray>(GV.getInitializer());
  // If values are not pointers, do not generate a relative lookup table.
  if (!Array || !Array->getType()->getElementType()->isPointerTy())
    return false;

  const DataLayout &DL = M.getDataLayout();
  for (const Use &Op : Array->operands()) {
    Constant *ConstOp = cast<Constant>(&Op);
    GlobalValue *GVOp;
    APInt Offset;

    // If an operand is not a constant offset from a lookup table,
    // do not generate a relative lookup table.
    if (!IsConstantOffsetFromGlobal(ConstOp, GVOp, Offset, DL))
      return false;

    // If operand is mutable, do not generate a relative lookup table.
    auto *GlovalVarOp = dyn_cast<GlobalVariable>(GVOp);
    if (!GlovalVarOp || !GlovalVarOp->isConstant())
      return false;

    if (!GlovalVarOp->hasLocalLinkage() ||
        !GlovalVarOp->isDSOLocal() ||
        !GlovalVarOp->isImplicitDSOLocal())
      return false;
  }

  return true;
}

static GlobalVariable *createRelLookupTable(Function &Func,
                                            GlobalVariable &LookupTable) {
  Module &M = *Func.getParent();
  ConstantArray *LookupTableArr =
      cast<ConstantArray>(LookupTable.getInitializer());
  unsigned NumElts = LookupTableArr->getType()->getNumElements();
  ArrayType *IntArrayTy =
      ArrayType::get(Type::getInt32Ty(M.getContext()), NumElts);

  GlobalVariable *RelLookupTable = new GlobalVariable(
    M, IntArrayTy, LookupTable.isConstant(), LookupTable.getLinkage(),
    nullptr, "reltable." + Func.getName(), &LookupTable,
    LookupTable.getThreadLocalMode(), LookupTable.getAddressSpace(),
    LookupTable.isExternallyInitialized());

  uint64_t Idx = 0;
  SmallVector<Constant *, 64> RelLookupTableContents(NumElts);

  for (Use &Operand : LookupTableArr->operands()) {
    Constant *Element = cast<Constant>(Operand);
    Type *IntPtrTy = M.getDataLayout().getIntPtrType(M.getContext());
    Constant *Base = llvm::ConstantExpr::getPtrToInt(RelLookupTable, IntPtrTy);
    Constant *Target = llvm::ConstantExpr::getPtrToInt(Element, IntPtrTy);
    Constant *Sub = llvm::ConstantExpr::getSub(Target, Base);
    Constant *RelOffset =
        llvm::ConstantExpr::getTrunc(Sub, Type::getInt32Ty(M.getContext()));
    RelLookupTableContents[Idx++] = RelOffset;
  }

  Constant *Initializer =
      ConstantArray::get(IntArrayTy, RelLookupTableContents);
  RelLookupTable->setInitializer(Initializer);
  RelLookupTable->setUnnamedAddr(GlobalValue::UnnamedAddr::Global);
  RelLookupTable->setAlignment(llvm::Align(4));
  return RelLookupTable;
}

static void convertToRelLookupTable(GlobalVariable &LookupTable) {
  GetElementPtrInst *GEP =
      cast<GetElementPtrInst>(LookupTable.use_begin()->getUser());
  LoadInst *Load = cast<LoadInst>(GEP->use_begin()->getUser());

  Module &M = *LookupTable.getParent();
  BasicBlock *BB = GEP->getParent();
  IRBuilder<> Builder(BB);
  Function &Func = *BB->getParent();

  // Generate an array that consists of relative offsets.
  GlobalVariable *RelLookupTable = createRelLookupTable(Func, LookupTable);

  // Place new instruction sequence before GEP.
  Builder.SetInsertPoint(GEP);
  Value *Index = GEP->getOperand(2);
  IntegerType *IntTy = cast<IntegerType>(Index->getType());
  Value *Offset =
      Builder.CreateShl(Index, ConstantInt::get(IntTy, 2), "reltable.shift");

  Function *LoadRelIntrinsic = llvm::Intrinsic::getDeclaration(
      &M, Intrinsic::load_relative, {Index->getType()});
  Value *Base = Builder.CreateBitCast(RelLookupTable, Builder.getInt8PtrTy());

  // Create a call to load.relative intrinsic that computes the target address
  // by adding base address (lookup table address) and relative offset.
  Value *Result = Builder.CreateCall(LoadRelIntrinsic, {Base, Offset},
                                     "reltable.intrinsic");

  // Create a bitcast instruction if necessary.
  if (Load->getType() != Builder.getInt8PtrTy())
    Result = Builder.CreateBitCast(Result, Load->getType(), "reltable.bitcast");

  // Replace load instruction with the new generated instruction sequence.
  Load->replaceAllUsesWith(Result);
  // Remove Load and GEP instructions.
  Load->eraseFromParent();
  GEP->eraseFromParent();
}

// Convert lookup tables to relative lookup tables in the module.
static bool convertToRelativeLookupTables(
    Module &M, function_ref<TargetTransformInfo &(Function &)> GetTTI) {
  Module::iterator FI = M.begin();
  if (FI == M.end())
    return false;

  // Check if we have a target that supports relative lookup tables.
  if (!GetTTI(*FI).shouldBuildRelLookupTables())
    return false;

  bool Changed = false;

  for (GlobalVariable &GV : llvm::make_early_inc_range(M.globals())) {
    if (!shouldConvertToRelLookupTable(M, GV))
      continue;

    convertToRelLookupTable(GV);

    // Remove the original lookup table.
    GV.eraseFromParent();

    Changed = true;
  }

  return Changed;
}

PreservedAnalyses RelLookupTableConverterPass::run(Module &M,
                                                   ModuleAnalysisManager &AM) {
  FunctionAnalysisManager &FAM =
      AM.getResult<FunctionAnalysisManagerModuleProxy>(M).getManager();

  auto GetTTI = [&](Function &F) -> TargetTransformInfo & {
    return FAM.getResult<TargetIRAnalysis>(F);
  };

  if (!convertToRelativeLookupTables(M, GetTTI))
    return PreservedAnalyses::all();

  PreservedAnalyses PA;
  PA.preserveSet<CFGAnalyses>();
  return PA;
}