llvm/lib/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/BasicBlock.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Instructions.h"
#include "llvm/Module.h"
#include "llvm/Pass.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/FunctionUtils.h"
#include "Support/Debug.h"
#include "Support/StringExtras.h"
#include <algorithm>
#include <map>
#include <vector>
using namespace llvm;

namespace {

  inline bool contains(const std::vector<BasicBlock*> &V, const BasicBlock *BB){
    return std::find(V.begin(), V.end(), BB) != V.end();
  }

  /// getFunctionArg - Return a pointer to F's ARGNOth argument.
  ///
  Argument *getFunctionArg(Function *F, unsigned argno) {
    Function::aiterator ai = F->abegin();
    while (argno) { ++ai; --argno; }
    return &*ai;
  }

  struct CodeExtractor {
    typedef std::vector<Value*> Values;
    typedef std::vector<std::pair<unsigned, unsigned> > PhiValChangesTy;
    typedef std::map<PHINode*, PhiValChangesTy> PhiVal2ArgTy;
    PhiVal2ArgTy PhiVal2Arg;

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

  private:
    void findInputsOutputs(const std::vector<BasicBlock*> &code,
                           Values &inputs,
                           Values &outputs,
                           BasicBlock *newHeader,
                           BasicBlock *newRootNode);

    void processPhiNodeInputs(PHINode *Phi,
                              const std::vector<BasicBlock*> &code,
                              Values &inputs,
                              BasicBlock *newHeader,
                              BasicBlock *newRootNode);

    void rewritePhiNodes(Function *F, BasicBlock *newFuncRoot);

    Function *constructFunction(const Values &inputs,
                                const Values &outputs,
                                BasicBlock *newRootNode, BasicBlock *newHeader,
                                const std::vector<BasicBlock*> &code,
                                Function *oldFunction, Module *M);

    void moveCodeToFunction(const std::vector<BasicBlock*> &code,
                            Function *newFunction);

    void emitCallAndSwitchStatement(Function *newFunction,
                                    BasicBlock *newHeader,
                                    const std::vector<BasicBlock*> &code,
                                    Values &inputs,
                                    Values &outputs);

  };
}

void CodeExtractor::processPhiNodeInputs(PHINode *Phi,
                                         const std::vector<BasicBlock*> &code,
                                         Values &inputs,
                                         BasicBlock *codeReplacer,
                                         BasicBlock *newFuncRoot)
{
  // Separate incoming values and BasicBlocks as internal/external. We ignore
  // the case where both the value and BasicBlock are internal, because we don't
  // need to do a thing.
  std::vector<unsigned> EValEBB;
  std::vector<unsigned> EValIBB;
  std::vector<unsigned> IValEBB;

  for (unsigned i = 0, e = Phi->getNumIncomingValues(); i != e; ++i) {
    Value *phiVal = Phi->getIncomingValue(i);
    if (Instruction *Inst = dyn_cast<Instruction>(phiVal)) {
      if (contains(code, Inst->getParent())) {
        if (!contains(code, Phi->getIncomingBlock(i)))
          IValEBB.push_back(i);
      } else {
        if (contains(code, Phi->getIncomingBlock(i)))
          EValIBB.push_back(i);
        else
          EValEBB.push_back(i);
      }
    } else if (Constant *Const = dyn_cast<Constant>(phiVal)) {
      // Constants are internal, but considered `external' if they are coming
      // from an external block.
      if (!contains(code, Phi->getIncomingBlock(i)))
        EValEBB.push_back(i);
    } else if (Argument *Arg = dyn_cast<Argument>(phiVal)) {
      // arguments are external
      if (contains(code, Phi->getIncomingBlock(i)))
        EValIBB.push_back(i);
      else
        EValEBB.push_back(i);
    } else {
      phiVal->dump();
      assert(0 && "Unhandled input in a Phi node");
    }
  }

  // Both value and block are external. Need to group all of
  // these, have an external phi, pass the result as an
  // argument, and have THIS phi use that result.
  if (EValEBB.size() > 0) {
    if (EValEBB.size() == 1) {
      // Now if it's coming from the newFuncRoot, it's that funky input
      unsigned phiIdx = EValEBB[0];
      if (!dyn_cast<Constant>(Phi->getIncomingValue(phiIdx)))
      {
        PhiVal2Arg[Phi].push_back(std::make_pair(phiIdx, inputs.size()));
        // We can just pass this value in as argument
        inputs.push_back(Phi->getIncomingValue(phiIdx));
      }
      Phi->setIncomingBlock(phiIdx, newFuncRoot);
    } else {
      PHINode *externalPhi = new PHINode(Phi->getType(), "extPhi");
      codeReplacer->getInstList().insert(codeReplacer->begin(), externalPhi);
      for (std::vector<unsigned>::iterator i = EValEBB.begin(),
             e = EValEBB.end(); i != e; ++i)
      {
        externalPhi->addIncoming(Phi->getIncomingValue(*i),
                                 Phi->getIncomingBlock(*i));

        // We make these values invalid instead of deleting them because that
        // would shift the indices of other values... The fixPhiNodes should
        // clean these phi nodes up later.
        Phi->setIncomingValue(*i, 0);
        Phi->setIncomingBlock(*i, 0);
      }
      PhiVal2Arg[Phi].push_back(std::make_pair(Phi->getNumIncomingValues(),
                                               inputs.size()));
      // We can just pass this value in as argument
      inputs.push_back(externalPhi);
    }
  }

  // When the value is external, but block internal...
  // just pass it in as argument, no change to phi node
  for (std::vector<unsigned>::iterator i = EValIBB.begin(),
         e = EValIBB.end(); i != e; ++i)
  {
    // rewrite the phi input node to be an argument
    PhiVal2Arg[Phi].push_back(std::make_pair(*i, inputs.size()));
    inputs.push_back(Phi->getIncomingValue(*i));
  }

  // Value internal, block external
  // this can happen if we are extracting a part of a loop
  for (std::vector<unsigned>::iterator i = IValEBB.begin(),
         e = IValEBB.end(); i != e; ++i)
  {
    assert(0 && "Cannot (YET) handle internal values via external blocks");
  }
}


void CodeExtractor::findInputsOutputs(const std::vector<BasicBlock*> &code,
                                      Values &inputs,
                                      Values &outputs,
                                      BasicBlock *newHeader,
                                      BasicBlock *newRootNode)
{
  for (std::vector<BasicBlock*>::const_iterator ci = code.begin(), 
       ce = code.end(); ci != ce; ++ci) {
    BasicBlock *BB = *ci;
    for (BasicBlock::iterator BBi = BB->begin(), BBe = BB->end();
         BBi != BBe; ++BBi) {
      // If a use is defined outside the region, it's an input.
      // If a def is used outside the region, it's an output.
      if (Instruction *I = dyn_cast<Instruction>(&*BBi)) {
        // If it's a phi node
        if (PHINode *Phi = dyn_cast<PHINode>(I)) {
          processPhiNodeInputs(Phi, code, inputs, newHeader, newRootNode);
        } 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->get())) {
              // Check if definition of this operand is within the loop
              if (!contains(code, opI->getParent())) {
                // add this operand to the inputs
                inputs.push_back(opI);
              }
            }
          }
        }

        // Consider uses of this instruction (outputs)
        for (Value::use_iterator use = I->use_begin(), useE = I->use_end();
             use != useE; ++use) {
          if (Instruction* inst = dyn_cast<Instruction>(*use)) {
            if (!contains(code, inst->getParent())) {
              // add this op to the outputs
              outputs.push_back(I);
            }
          }
        }
      } /* if */
    } /* for: insts */
  } /* for: basic blocks */
}

void CodeExtractor::rewritePhiNodes(Function *F,
                                    BasicBlock *newFuncRoot) {
  // Write any changes that were saved before: use function arguments as inputs
  for (PhiVal2ArgTy::iterator i = PhiVal2Arg.begin(), e = PhiVal2Arg.end();
       i != e; ++i)
  {
    PHINode *phi = (*i).first;
    PhiValChangesTy &values = (*i).second;
    for (unsigned cIdx = 0, ce = values.size(); cIdx != ce; ++cIdx)
    {
      unsigned phiValueIdx = values[cIdx].first, argNum = values[cIdx].second;
      if (phiValueIdx < phi->getNumIncomingValues())
        phi->setIncomingValue(phiValueIdx, getFunctionArg(F, argNum));
      else
        phi->addIncoming(getFunctionArg(F, argNum), newFuncRoot);
    }
  }

  // Delete any invalid Phi node inputs that were marked as NULL previously
  for (PhiVal2ArgTy::iterator i = PhiVal2Arg.begin(), e = PhiVal2Arg.end();
       i != e; ++i)
  {
    PHINode *phi = (*i).first;
    for (unsigned idx = 0, end = phi->getNumIncomingValues(); idx != end; ++idx)
    {
      if (phi->getIncomingValue(idx) == 0 && phi->getIncomingBlock(idx) == 0) {
        phi->removeIncomingValue(idx);
        --idx;
        --end;
      }
    }
  }

  // We are done with the saved values
  PhiVal2Arg.clear();
}


/// 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 *newRootNode,
                                           BasicBlock *newHeader,
                                           const std::vector<BasicBlock*> &code,
                                           Function *oldFunction, Module *M) {
  DEBUG(std::cerr << "inputs: " << inputs.size() << "\n");
  DEBUG(std::cerr << "outputs: " << outputs.size() << "\n");
  BasicBlock *header = code[0];

  // 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, but
  // make them pointer types for scalars
  for (Values::const_iterator i = outputs.begin(),
         e = outputs.end(); i != e; ++i) {
    const Value *value = *i;
    DEBUG(std::cerr << "instr used in func: " << value << "\n");
    const Type *valueType = value->getType();
    // Convert scalar types into a pointer of that type
    if (valueType->isPrimitiveType()) {
      valueType = PointerType::get(valueType);
    }
    paramTy.push_back(valueType);
  }

  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);

  for (unsigned i = 0, e = inputs.size(); i != e; ++i) {
    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 (contains(code, inst->getParent())) {
          inst->replaceUsesOfWith(inputs[i], getFunctionArg(newFunction, i));
        }
      }
    }
  }

  // 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 (std::vector<User*>::iterator i = Users.begin(), e = Users.end();
       i != e; ++i) {
    if (BranchInst *inst = dyn_cast<BranchInst>(*i)) {
      BasicBlock *BB = inst->getParent();
      if (!contains(code, BB) && BB->getParent() == oldFunction) {
        // The BasicBlock which contains the branch is not in the region
        // modify the branch target to a new block
        inst->replaceUsesOfWith(header, newHeader);
      }
    }
  }

  return newFunction;
}

void CodeExtractor::moveCodeToFunction(const std::vector<BasicBlock*> &code,
                                       Function *newFunction)
{
  for (std::vector<BasicBlock*>::const_iterator i = code.begin(), e =code.end();
       i != e; ++i) {
    BasicBlock *BB = *i;
    Function *oldFunc = BB->getParent();
    Function::BasicBlockListType &oldBlocks = oldFunc->getBasicBlockList();

    // Delete the basic block from the old function, and the list of blocks
    oldBlocks.remove(BB);

    // Insert this basic block into the new function
    Function::BasicBlockListType &newBlocks = newFunction->getBasicBlockList();
    newBlocks.push_back(BB);
  }
}

void
CodeExtractor::emitCallAndSwitchStatement(Function *newFunction,
                                          BasicBlock *codeReplacer,
                                          const std::vector<BasicBlock*> &code,
                                          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;
  BasicBlock *codeReplacerTail = new BasicBlock("codeReplTail",
                                                codeReplacer->getParent());
  for (Values::const_iterator i = inputs.begin(),
         e = inputs.end(); i != e; ++i)
    params.push_back(*i);
  for (Values::const_iterator i = outputs.begin(), 
         e = outputs.end(); i != e; ++i) {
    // Create allocas for scalar outputs
    if ((*i)->getType()->isPrimitiveType()) {
      Constant *one = ConstantUInt::get(Type::UIntTy, 1);
      AllocaInst *alloca = new AllocaInst((*i)->getType(), one);
      codeReplacer->getInstList().push_back(alloca);
      params.push_back(alloca);

      LoadInst *load = new LoadInst(alloca, "alloca");
      codeReplacerTail->getInstList().push_back(load);
      std::vector<User*> Users((*i)->use_begin(), (*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 (!contains(code, inst->getParent())) {
            inst->replaceUsesOfWith(*i, load);
          }
        }
      }
    } else {
      params.push_back(*i);
    }
  }
  CallInst *call = new CallInst(newFunction, params, "targetBlock");
  codeReplacer->getInstList().push_back(call);
  codeReplacer->getInstList().push_back(new BranchInst(codeReplacerTail));

  // Now we can emit a switch statement using the call as a value.
  // FIXME: perhaps instead of default being self BB, it should be a second
  // dummy block which asserts that the value is not within the range...?
  //BasicBlock *defaultBlock = new BasicBlock("defaultBlock", oldF);
  //insert abort() ?
  //defaultBlock->getInstList().push_back(new BranchInst(codeReplacer));

  SwitchInst *switchInst = new SwitchInst(call, codeReplacerTail,
                                          codeReplacerTail);

  // Since there may be multiple exits from the original region, make the new
  // function return an unsigned, switch on that number
  unsigned switchVal = 0;
  for (std::vector<BasicBlock*>::const_iterator i =code.begin(), e = code.end();
       i != e; ++i) {
    BasicBlock *BB = *i;

    // rewrite the terminator of the original BasicBlock
    Instruction *term = BB->getTerminator();
    if (BranchInst *brInst = dyn_cast<BranchInst>(term)) {

      // Restore values just before we exit
      // FIXME: Use a GetElementPtr to bunch the outputs in a struct
      for (unsigned outIdx = 0, outE = outputs.size(); outIdx != outE; ++outIdx)
      {
        new StoreInst(outputs[outIdx],
                      getFunctionArg(newFunction, outIdx),
                      brInst);
      }

      // Rewrite branches into exists which return a value based on which
      // exit we take from this function
      if (brInst->isUnconditional()) {
        if (!contains(code, brInst->getSuccessor(0))) {
          ConstantUInt *brVal = ConstantUInt::get(Type::UShortTy, switchVal);
          ReturnInst *newRet = new ReturnInst(brVal);
          // add a new target to the switch
          switchInst->addCase(brVal, brInst->getSuccessor(0));
          ++switchVal;
          // rewrite the branch with a return
          BasicBlock::iterator ii(brInst);
          ReplaceInstWithInst(BB->getInstList(), ii, newRet);
          delete brInst;
        }
      } else {
        // Replace the conditional branch to branch
        // to two new blocks, each of which returns a different code.
        for (unsigned idx = 0; idx < 2; ++idx) {
          BasicBlock *oldTarget = brInst->getSuccessor(idx);
          if (!contains(code, oldTarget)) {
            // add a new basic block which returns the appropriate value
            BasicBlock *newTarget = new BasicBlock("newTarget", newFunction);
            ConstantUInt *brVal = ConstantUInt::get(Type::UShortTy, switchVal);
            ReturnInst *newRet = new ReturnInst(brVal);
            newTarget->getInstList().push_back(newRet);
            // rewrite the original branch instruction with this new target
            brInst->setSuccessor(idx, newTarget);
            // the switch statement knows what to do with this value
            switchInst->addCase(brVal, oldTarget);
            ++switchVal;
          }
        }
      }
    } else if (ReturnInst *retTerm = dyn_cast<ReturnInst>(term)) {
      assert(0 && "Cannot handle return instructions just yet.");
      // FIXME: what if the terminator is a return!??!
      // Need to rewrite: add new basic block, move the return there
      // treat the original as an unconditional branch to that basicblock
    } else if (SwitchInst *swTerm = dyn_cast<SwitchInst>(term)) {
      assert(0 && "Cannot handle switch instructions just yet.");
    } else if (InvokeInst *invInst = dyn_cast<InvokeInst>(term)) {
      assert(0 && "Cannot handle invoke instructions just yet.");
    } else {
      assert(0 && "Unrecognized terminator, or badly-formed BasicBlock.");
    }
  }
}


/// 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
  // 

  Values inputs, outputs;

  // Assumption: this is a single-entry code region, and the header is the first
  // block in the region. FIXME: is this true for a list of blocks from a
  // natural function?
  BasicBlock *header = code[0];
  Function *oldFunction = header->getParent();
  Module *module = oldFunction->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(code, inputs, outputs, codeReplacer, newFuncRoot);

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

  rewritePhiNodes(newFunction, newFuncRoot);

  emitCallAndSwitchStatement(newFunction, codeReplacer, code, inputs, outputs);

  moveCodeToFunction(code, newFunction);

  return newFunction;
}

Function* llvm::ExtractLoop(Loop *L) {
  CodeExtractor CE;
  return CE.ExtractCodeRegion(L->getBlocks());
}