path: root/lib/Transforms/Utils/PromoteMemoryToRegister.cpp

//===- PromoteMemoryToRegister.cpp - Convert memory refs to regs ----------===//
//
// This pass is used to promote memory references to be register references.  A
// simple example of the transformation performed by this pass is:
//
//        FROM CODE                           TO CODE
//   %X = alloca int, uint 1                 ret int 42
//   store int 42, int *%X
//   %Y = load int* %X
//   ret int %Y
//
// To do this transformation, a simple analysis is done to ensure it is safe.
// Currently this just loops over all alloca instructions, looking for
// instructions that are only used in simple load and stores.
//
// After this, the code is transformed by looping over all of the alloca
// instruction, calculating dominator frontiers, then inserting phi-nodes
// following the usual SSA construction algorithm.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Scalar.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/iMemory.h"
#include "llvm/iPHINode.h"
#include "llvm/iTerminators.h"
#include "llvm/Function.h"
#include "llvm/Constant.h"
#include "llvm/Type.h"
#include "Support/Statistic.h"

namespace {
  Statistic<> NumPromoted("mem2reg", "Number of alloca's promoted");

  struct PromotePass : public FunctionPass {
    std::vector<AllocaInst*>          Allocas;      // the alloca instruction..
    std::map<Instruction*, unsigned>  AllocaLookup; // reverse mapping of above
    
    std::vector<std::vector<BasicBlock*> > PhiNodes;// Idx corresponds 2 Allocas
    
    // List of instructions to remove at end of pass
    std::vector<Instruction *>        KillList;
    
    std::map<BasicBlock*,
             std::vector<PHINode*> >  NewPhiNodes; // the PhiNodes we're adding

  public:
    // runOnFunction - To run this pass, first we calculate the alloca
    // instructions that are safe for promotion, then we promote each one.
    //
    virtual bool runOnFunction(Function &F);

    // getAnalysisUsage - We need dominance frontiers
    //
    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.addRequired<DominanceFrontier>();
      AU.setPreservesCFG();
    }

  private:
    void RenamePass(BasicBlock *BB, BasicBlock *Pred,
                    std::vector<Value*> &IncVals,
                    std::set<BasicBlock*> &Visited);
    bool QueuePhiNode(BasicBlock *BB, unsigned AllocaIdx);
    void FindSafeAllocas(Function &F);
  };

  RegisterOpt<PromotePass> X("mem2reg", "Promote Memory to Register");
}  // end of anonymous namespace


// isSafeAlloca - This predicate controls what types of alloca instructions are
// allowed to be promoted...
//
static inline bool isSafeAlloca(const AllocaInst *AI) {
  if (AI->isArrayAllocation()) return false;

  // Only allow direct loads and stores...
  for (Value::use_const_iterator UI = AI->use_begin(), UE = AI->use_end();
       UI != UE; ++UI)     // Loop over all of the uses of the alloca
    if (!isa<LoadInst>(*UI))
      if (const StoreInst *SI = dyn_cast<StoreInst>(*UI)) {
        if (SI->getOperand(0) == AI)
          return false;   // Don't allow a store of the AI, only INTO the AI.
      } else {
        return false;   // Not a load or store?
      }
  
  return true;
}

// FindSafeAllocas - Find allocas that are safe to promote
//
void PromotePass::FindSafeAllocas(Function &F) {
  BasicBlock &BB = F.getEntryNode();  // Get the entry node for the function

  // Look at all instructions in the entry node
  for (BasicBlock::iterator I = BB.begin(), E = --BB.end(); I != E; ++I)
    if (AllocaInst *AI = dyn_cast<AllocaInst>(&*I))       // Is it an alloca?
      if (isSafeAlloca(AI)) {   // If safe alloca, add alloca to safe list
        AllocaLookup[AI] = Allocas.size();  // Keep reverse mapping
        Allocas.push_back(AI);
      }
}



bool PromotePass::runOnFunction(Function &F) {
  // Calculate the set of safe allocas
  FindSafeAllocas(F);

  // If there is nothing to do, bail out...
  if (Allocas.empty()) return false;

  // Add each alloca to the KillList.  Note: KillList is destroyed MOST recently
  // added to least recently.
  KillList.assign(Allocas.begin(), Allocas.end());

  // Calculate the set of write-locations for each alloca.  This is analogous to
  // counting the number of 'redefinitions' of each variable.
  std::vector<std::vector<BasicBlock*> > WriteSets;// Idx corresponds to Allocas
  WriteSets.resize(Allocas.size());
  for (unsigned i = 0; i != Allocas.size(); ++i) {
    AllocaInst *AI = Allocas[i];
    for (Value::use_iterator U =AI->use_begin(), E = AI->use_end(); U != E; ++U)
      if (StoreInst *SI = dyn_cast<StoreInst>(*U))
        // jot down the basic-block it came from
        WriteSets[i].push_back(SI->getParent());
  }

  // Get dominance frontier information...
  DominanceFrontier &DF = getAnalysis<DominanceFrontier>();

  // Compute the locations where PhiNodes need to be inserted.  Look at the
  // dominance frontier of EACH basic-block we have a write in
  //
  PhiNodes.resize(Allocas.size());
  for (unsigned i = 0; i != Allocas.size(); ++i) {
    for (unsigned j = 0; j != WriteSets[i].size(); j++) {
      // Look up the DF for this write, add it to PhiNodes
      DominanceFrontier::const_iterator it = DF.find(WriteSets[i][j]);
      DominanceFrontier::DomSetType     S = it->second;
      for (DominanceFrontier::DomSetType::iterator P = S.begin(), PE = S.end();
           P != PE; ++P)
        QueuePhiNode(*P, i);
    }
    
    // Perform iterative step
    for (unsigned k = 0; k != PhiNodes[i].size(); k++) {
      DominanceFrontier::const_iterator it = DF.find(PhiNodes[i][k]);
      DominanceFrontier::DomSetType     S = it->second;
      for (DominanceFrontier::DomSetType::iterator P = S.begin(), PE = S.end();
           P != PE; ++P)
        QueuePhiNode(*P, i);
    }
  }

  // Set the incoming values for the basic block to be null values for all of
  // the alloca's.  We do this in case there is a load of a value that has not
  // been stored yet.  In this case, it will get this null value.
  //
  std::vector<Value *> Values(Allocas.size());
  for (unsigned i = 0, e = Allocas.size(); i != e; ++i)
    Values[i] = Constant::getNullValue(Allocas[i]->getAllocatedType());

  // Walks all basic blocks in the function performing the SSA rename algorithm
  // and inserting the phi nodes we marked as necessary
  //
  std::set<BasicBlock*> Visited;      // The basic blocks we've already visited
  RenamePass(F.begin(), 0, Values, Visited);

  // Remove all instructions marked by being placed in the KillList...
  //
  while (!KillList.empty()) {
    Instruction *I = KillList.back();
    KillList.pop_back();

    I->getParent()->getInstList().erase(I);
  }

  NumPromoted += Allocas.size();

  // Purge data structurse so they are available the next iteration...
  Allocas.clear();
  AllocaLookup.clear();
  PhiNodes.clear();
  NewPhiNodes.clear();
  return true;
}


// QueuePhiNode - queues a phi-node to be added to a basic-block for a specific
// Alloca returns true if there wasn't already a phi-node for that variable
//
bool PromotePass::QueuePhiNode(BasicBlock *BB, unsigned AllocaNo) {
  // Look up the basic-block in question
  std::vector<PHINode*> &BBPNs = NewPhiNodes[BB];
  if (BBPNs.empty()) BBPNs.resize(Allocas.size());

  // If the BB already has a phi node added for the i'th alloca then we're done!
  if (BBPNs[AllocaNo]) return false;

  // Create a PhiNode using the dereferenced type... and add the phi-node to the
  // BasicBlock
  PHINode *PN = new PHINode(Allocas[AllocaNo]->getAllocatedType(),
                            Allocas[AllocaNo]->getName()+".mem2reg",
                            BB->begin());
  BBPNs[AllocaNo] = PN;
  PhiNodes[AllocaNo].push_back(BB);
  return true;
}

void PromotePass::RenamePass(BasicBlock *BB, BasicBlock *Pred,
                             std::vector<Value*> &IncomingVals,
                             std::set<BasicBlock*> &Visited) {
  // If this is a BB needing a phi node, lookup/create the phinode for each
  // variable we need phinodes for.
  std::vector<PHINode *> &BBPNs = NewPhiNodes[BB];
  for (unsigned k = 0; k != BBPNs.size(); ++k)
    if (PHINode *PN = BBPNs[k]) {
      // at this point we can assume that the array has phi nodes.. let's add
      // the incoming data
      PN->addIncoming(IncomingVals[k], Pred);

      // also note that the active variable IS designated by the phi node
      IncomingVals[k] = PN;
    }

  // don't revisit nodes
  if (Visited.count(BB)) return;
  
  // mark as visited
  Visited.insert(BB);

  // keep track of the value of each variable we're watching.. how?
  for (BasicBlock::iterator II = BB->begin(); II != BB->end(); ++II) {
    Instruction *I = II; // get the instruction

    if (LoadInst *LI = dyn_cast<