Transforms/Utils/CodeExtractor.cpp

//===- CodeExtractor.cpp - Pull code region into a new function -----------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file was developed by the LLVM research group and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements the interface to tear out a code region, such as an
// individual loop or a parallel section, into a new function, replacing it with
// a call to the new function.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Utils/FunctionUtils.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Instructions.h"
#include "llvm/Module.h"
#include "llvm/Pass.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/Verifier.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "Support/Debug.h"
#include "Support/StringExtras.h"
#include <algorithm>
#include <set>
using namespace llvm;

namespace {
  class CodeExtractor {
    typedef std::vector<Value*> Values;
    std::set<BasicBlock*> BlocksToExtract;
    DominatorSet *DS;
  public:
    CodeExtractor(DominatorSet *ds = 0) : DS(ds) {}

    Function *ExtractCodeRegion(const std::vector<BasicBlock*> &code);

  private:
    void findInputsOutputs(Values &inputs, Values &outputs,
                           BasicBlock *newHeader,
                           BasicBlock *newRootNode);

    Function *constructFunction(const Values &inputs,
                                const Values &outputs,
                                BasicBlock *header,
                                BasicBlock *newRootNode, BasicBlock *newHeader,
                                Function *oldFunction, Module *M);

    void moveCodeToFunction(Function *newFunction);

    void emitCallAndSwitchStatement(Function *newFunction,
                                    BasicBlock *newHeader,
                                    Values &inputs,
                                    Values &outputs);

  };
}

void CodeExtractor::findInputsOutputs(Values &inputs, Values &outputs,
                                      BasicBlock *newHeader,
                                      BasicBlock *newRootNode) {
  for (std::set<BasicBlock*>::const_iterator ci = BlocksToExtract.begin(),
       ce = BlocksToExtract.end(); ci != ce; ++ci) {
    BasicBlock *BB = *ci;
    for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
      // If a used value is defined outside the region, it's an input.  If an
      // instruction is used outside the region, it's an output.
      if (PHINode *PN = dyn_cast<PHINode>(I)) {
        for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
          Value *V = PN->getIncomingValue(i);
          if (!BlocksToExtract.count(PN->getIncomingBlock(i)) &&
              (isa<Instruction>(V) || isa<Argument>(V)))
            inputs.push_back(V);
          else if (Instruction *opI = dyn_cast<Instruction>(V)) {
            if (!BlocksToExtract.count(opI->getParent()))
              inputs.push_back(opI);
          } else if (isa<Argument>(V))
            inputs.push_back(V);
        }
      } else {
        // All other instructions go through the generic input finder
        // Loop over the operands of each instruction (inputs)
        for (User::op_iterator op = I->op_begin(), opE = I->op_end();
             op != opE; ++op)
          if (Instruction *opI = dyn_cast<Instruction>(*op)) {
            // Check if definition of this operand is within the loop
            if (!BlocksToExtract.count(opI->getParent()))
              inputs.push_back(opI);
          } else if (isa<Argument>(*op)) {
            inputs.push_back(*op);
          }
      }

      // Consider uses of this instruction (outputs)
      for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
           UI != E; ++UI)
        if (!BlocksToExtract.count(cast<Instruction>(*UI)->getParent())) {
          outputs.push_back(I);
          break;
        }
    } // for: insts
  } // for: basic blocks
}

/// constructFunction - make a function based on inputs and outputs, as follows:
/// f(in0, ..., inN, out0, ..., outN)
///
Function *CodeExtractor::constructFunction(const Values &inputs,
                                           const Values &outputs,
                                           BasicBlock *header,
                                           BasicBlock *newRootNode,
                                           BasicBlock *newHeader,
                                           Function *oldFunction,
                                           Module *M) {
  DEBUG(std::cerr << "inputs: " << inputs.size() << "\n");
  DEBUG(std::cerr << "outputs: " << outputs.size() << "\n");

  // This function returns unsigned, outputs will go back by reference.
  Type *retTy = Type::UShortTy;
  std::vector<const Type*> paramTy;

  // Add the types of the input values to the function's argument list
  for (Values::const_iterator i = inputs.begin(),
         e = inputs.end(); i != e; ++i) {
    const Value *value = *i;
    DEBUG(std::cerr << "value used in func: " << value << "\n");
    paramTy.push_back(value->getType());
  }

  // Add the types of the output values to the function's argument list.
  for (Values::const_iterator I = outputs.begin(), E = outputs.end();
       I != E; ++I) {
    DEBUG(std::cerr << "instr used in func: " << *I << "\n");
    paramTy.push_back(PointerType::get((*I)->getType()));
  }

  DEBUG(std::cerr << "Function type: " << retTy << " f(");
  for (std::vector<const Type*>::iterator i = paramTy.begin(),
         e = paramTy.end(); i != e; ++i)
    DEBUG(std::cerr << *i << ", ");
  DEBUG(std::cerr << ")\n");

  const FunctionType *funcType = FunctionType::get(retTy, paramTy, false);

  // Create the new function
  Function *newFunction = new Function(funcType,
                                       GlobalValue::InternalLinkage,
                                       oldFunction->getName() + "_code", M);
  newFunction->getBasicBlockList().push_back(newRootNode);

  // Create an iterator to name all of the arguments we inserted.
  Function::aiterator AI = newFunction->abegin();

  // Rewrite all users of the inputs in the extracted region to use the
  // arguments instead.
  for (unsigned i = 0, e = inputs.size(); i != e; ++i, ++AI) {
    AI->setName(inputs[i]->getName());
    std::vector<User*> Users(inputs[i]->use_begin(), inputs[i]->use_end());
    for (std::vector<User*>::iterator use = Users.begin(), useE = Users.end();
         use != useE; ++use)
      if (Instruction* inst = dyn_cast<Instruction>(*use))
        if (BlocksToExtract.count(inst->getParent()))
          inst->replaceUsesOfWith(inputs[i], AI);
  }

  // Set names for all of the output arguments.
  for (unsigned i = 0, e = outputs.size(); i != e; ++i, ++AI)
    AI->setName(outputs[i]->getName()+".out");


  // Rewrite branches to basic blocks outside of the loop to new dummy blocks
  // within the new function. This must be done before we lose track of which
  // blocks were originally in the code region.
  std::vector<User*> Users(header->use_begin(), header->use_end());
  for (unsigned i = 0, e = Users.size(); i != e; ++i)
    // The BasicBlock which contains the branch is not in the region
    // modify the branch target to a new block
    if (TerminatorInst *TI = dyn_cast<TerminatorInst>(Users[i]))
      if (!BlocksToExtract.count(TI->getParent()) &&
          TI->getParent()->getParent() == oldFunction)
        TI->replaceUsesOfWith(header, newHeader);

  return newFunction;
}

void CodeExtractor::moveCodeToFunction(Function *newFunction) {
  Function *oldFunc = (*BlocksToExtract.begin())->getParent();
  Function::BasicBlockListType &oldBlocks = oldFunc->getBasicBlockList();
  Function::BasicBlockListType &newBlocks = newFunction->getBasicBlockList();

  for (std::set<BasicBlock*>::const_iterator i = BlocksToExtract.begin(),
         e = BlocksToExtract.end(); i != e; ++i) {
    // Delete the basic block from the old function, and the list of blocks
    oldBlocks.remove(*i);

    // Insert this basic block into the new function
    newBlocks.push_back(*i);
  }
}

void
CodeExtractor::emitCallAndSwitchStatement(Function *newFunction,
                                          BasicBlock *codeReplacer,
                                          Values &inputs,
                                          Values &outputs) {
  // Emit a call to the new function, passing allocated memory for outputs and
  // just plain inputs for non-scalars
  std::vector<Value*> params(inputs);

  // Get an iterator to the first output argument.
  Function::aiterator OutputArgBegin = newFunction->abegin();
  std::advance(OutputArgBegin, inputs.size());

  for (unsigned i = 0, e = outputs.size(); i != e; ++i) {
    Value *Output = outputs[i];
    // Create allocas for scalar outputs
    AllocaInst *alloca =
      new AllocaInst(outputs[i]->getType(), 0, Output->getName()+".loc",
                     codeReplacer->getParent()->begin()->begin());
    params.push_back(alloca);

    LoadInst *load = new LoadInst(alloca, Output->getName()+".reload");
    codeReplacer->getInstList().push_back(load);
    std::vector<User*> Users(outputs[i]->use_begin(), outputs[i]->use_end());
    for (unsigned u = 0, e = Users.size(); u != e; ++u) {
      Instruction *inst = cast<Instruction>(Users[u]);
      if (!BlocksToExtract.count(inst->getParent()))
        inst->replaceUsesOfWith(outputs[i], load);
    }
  }

  CallInst *call = new CallInst(newFunction, params, "targetBlock");
  codeReplacer->getInstList().push_front(call);

  // Now we can emit a switch statement using the call as a value.
  SwitchInst *TheSwitch = new SwitchInst(call, codeReplacer, codeReplacer);

  // Since there may be multiple exits from the original region, make the new
  // function return an unsigned, switch on that number.  This loop iterates
  // over all of the blocks in the extracted region, updating any terminator
  // instructions in the to-be-extracted region that branch to blocks that are
  // not in the region to be extracted.
  std::map<BasicBlock*, BasicBlock*> ExitBlockMap;

  unsigned switchVal = 0;
  for (std::set<BasicBlock*>::const_iterator i = BlocksToExtract.begin(),
         e = BlocksToExtract.end(); i != e; ++i) {
    TerminatorInst *TI = (*i)->getTerminator();
    for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
      if (!BlocksToExtract.count(TI->getSuccessor(i))) {
        BasicBlock *OldTarget = TI->getSuccessor(i);
        // add a new basic block which returns the appropriate value
        BasicBlock *&NewTarget = ExitBlockMap[OldTarget];
        if (!NewTarget) {
          // If we don't already have an exit stub for this non-extracted
          // destination, create one now!
          NewTarget = new BasicBlock(OldTarget->getName() + ".exitStub",
                                     newFunction);

          ConstantUInt *brVal = ConstantUInt::get(Type::UShortTy, switchVal++);
          ReturnInst *NTRet = new ReturnInst(brVal, NewTarget);

          // Update the switch instruction.
          TheSwitch->addCase(brVal, OldTarget);

          // Restore values just before we exit
          // FIXME: Use a GetElementPtr to bunch the outputs in a struct
          Function::aiterator OAI = OutputArgBegin;
          for (unsigned out = 0, e = outputs.size(); out != e; ++out, ++OAI)
            if (!DS ||
                DS->dominates(cast<Instruction>(outputs[out])->getParent(),
                              TI->getParent()))
              new StoreInst(outputs[out], OAI, NTRet);
        }

        // rewrite the original branch instruction with this new target
        TI->setSuccessor(i, NewTarget);
      }
  }

  // Now that we've done the deed, make the default destination of the switch
  // instruction be one of the exit blocks of the region.
  if (TheSwitch->getNumSuccessors() > 1) {
    // FIXME: this is broken w.r.t. PHI nodes, but the old code was more broken.
    // This edge is not traversable.
    TheSwitch->setSuccessor(0, TheSwitch->getSuccessor(1));
  }
}


/// ExtractRegion - Removes a loop from a function, replaces it with a call to
/// new function. Returns pointer to the new function.
///
/// algorithm:
///
/// find inputs and outputs for the region
///
/// for inputs: add to function as args, map input instr* to arg#
/// for outputs: add allocas for scalars,
///             add to func as args, map output instr* to arg#
///
/// rewrite func to use argument #s instead of instr*
///
/// for each scalar output in the function: at every exit, store intermediate
/// computed result back into memory.
///
Function *CodeExtractor::ExtractCodeRegion(const std::vector<BasicBlock*> &code)
{
  // 1) Find inputs, outputs
  // 2) Construct new function
  //  * Add allocas for defs, pass as args by reference
  //  * Pass in uses as args
  // 3) Move code region, add call instr to func
  //
  BlocksToExtract.insert(code.begin(), code.end());

  Values inputs, outputs;

  // Assumption: this is a single-entry code region, and the header is the first
  // block in the region.
  BasicBlock *header = code[0];
  for (unsigned i = 1, e = code.size(); i != e; ++i)
    for (pred_iterator PI = pred_begin(code[i]), E = pred_end(code[i]);
         PI != E; ++PI)
      assert(BlocksToExtract.count(*PI) &&
             "No blocks in this region may have entries from outside the region"
             " except for the first block!");

  Function *oldFunction = header->getParent();

  // This takes place of the original loop
  BasicBlock *codeReplacer = new BasicBlock("codeRepl", oldFunction);

  // The new function needs a root node because other nodes can branch to the
  // head of the loop, and the root cannot have predecessors
  BasicBlock *newFuncRoot = new BasicBlock("newFuncRoot");
  newFuncRoot->getInstList().push_back(new BranchInst(header));

  // Find inputs to, outputs from the code region
  //
  // If one of the inputs is coming from a different basic block and it's in a
  // phi node, we need to rewrite the phi node:
  //
  // * All the inputs which involve basic blocks OUTSIDE of this region go into
  //   a NEW phi node that takes care of finding which value really came in.
  //   The result of this phi is passed to the function as an argument.
  //
  // * All the other phi values stay.
  //
  // FIXME: PHI nodes' incoming blocks aren't being rewritten to accomodate for
  // blocks moving to a new function.
  // SOLUTION: move Phi nodes out of the loop header into the codeReplacer, pass
  // the values as parameters to the function
  findInputsOutputs(inputs, outputs, codeReplacer, newFuncRoot);

  // Step 2: Construct new function based on inputs/outputs,
  // Add allocas for all defs
  Function *newFunction = constructFunction(inputs, outputs, code[0],
                                            newFuncRoot,
                                            codeReplacer, oldFunction,
                                            oldFunction->getParent());

  emitCallAndSwitchStatement(newFunction, codeReplacer, inputs, outputs);

  moveCodeToFunction(newFunction);

  // Loop over all of the PHI nodes in the entry block (code[0]), and change any
  // references to the old incoming edge to be the new incoming edge.
  for (BasicBlock::iterator I = code[0]->begin();
       PHINode *PN = dyn_cast<PHINode>(I); ++I)
    for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
      if (!BlocksToExtract.count(PN->getIncomingBlock(i)))
        PN->setIncomingBlock(i, newFuncRoot);

  // Look at all successors of the codeReplacer block.  If any of these blocks
  // had PHI nodes in them, we need to update the "from" block to be the code
  // replacer, not the original block in the extracted region.
  std::vector<BasicBlock*> Succs(succ_begin(codeReplacer),
                                 succ_end(codeReplacer));
  for (unsigned i = 0, e = Succs.size(); i != e; ++i)
    for (BasicBlock::iterator I = Succs[i]->begin();
         PHINode *PN = dyn_cast<PHINode>(I); ++I)
      for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
        if (BlocksToExtract.count(PN->getIncomingBlock(i)))
          PN->setIncomingBlock(i, codeReplacer);


  DEBUG(if (verifyFunction(*newFunction)) abort());
  return newFunction;
}

/// ExtractCodeRegion - slurp a sequence of basic blocks into a brand new
/// function
///
Function* llvm::ExtractCodeRegion(DominatorSet &DS,
                                  const std::vector<BasicBlock*> &code) {
  return CodeExtractor(&DS).ExtractCodeRegion(code);
}

/// ExtractBasicBlock - slurp a natural loop into a brand new function
///
Function* llvm::ExtractLoop(DominatorSet &DS, Loop *L) {
  return CodeExtractor(&DS).ExtractCodeRegion(L->getBlocks());
}

/// ExtractBasicBlock - slurp a basic block into a brand new function
///
Function* llvm::ExtractBasicBlock(BasicBlock *BB) {
  std::vector<BasicBlock*> Blocks;
  Blocks.push_back(BB);
  return CodeExtractor().ExtractCodeRegion(Blocks);
}