lib/CodeGen/StrongPHIElimination.cpp

//===- StrongPhiElimination.cpp - Eliminate PHI nodes by inserting copies -===//
//
//                     The LLVM Compiler Infrastructure
//
// This file was developed by Owen Anderson and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass eliminates machine instruction PHI nodes by inserting copy
// instructions, using an intelligent copy-folding technique based on
// dominator information.  This is technique is derived from:
// 
//    Budimlic, et al. Fast copy coalescing and live-range identification.
//    In Proceedings of the ACM SIGPLAN 2002 Conference on Programming Language
//    Design and Implementation (Berlin, Germany, June 17 - 19, 2002).
//    PLDI '02. ACM, New York, NY, 25-32.
//    DOI= http://doi.acm.org/10.1145/512529.512534
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "strongphielim"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/BreakCriticalMachineEdge.h"
#include "llvm/CodeGen/LiveVariables.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/SSARegMap.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Support/Compiler.h"
using namespace llvm;


namespace {
  struct VISIBILITY_HIDDEN StrongPHIElimination : public MachineFunctionPass {
    static char ID; // Pass identification, replacement for typeid
    StrongPHIElimination() : MachineFunctionPass((intptr_t)&ID) {}

    DenseMap<MachineBasicBlock*,
             SmallVector<std::pair<unsigned, unsigned>, 2> > Waiting;

    bool runOnMachineFunction(MachineFunction &Fn);
    
    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.addPreserved<LiveVariables>();
      AU.addPreservedID(PHIEliminationID);
      AU.addRequired<MachineDominatorTree>();
      AU.addRequired<LiveVariables>();
      AU.setPreservesAll();
      MachineFunctionPass::getAnalysisUsage(AU);
    }
    
    virtual void releaseMemory() {
      preorder.clear();
      maxpreorder.clear();
      
      waiting.clear();
    }

  private:
    struct DomForestNode {
    private:
      std::vector<DomForestNode*> children;
      unsigned reg;
      
      void addChild(DomForestNode* DFN) { children.push_back(DFN); }
      
    public:
      typedef std::vector<DomForestNode*>::iterator iterator;
      
      DomForestNode(unsigned r, DomForestNode* parent) : reg(r) {
        if (parent)
          parent->addChild(this);
      }
      
      ~DomForestNode() {
        for (iterator I = begin(), E = end(); I != E; ++I)
          delete *I;
      }
      
      inline unsigned getReg() { return reg; }
      
      inline DomForestNode::iterator begin() { return children.begin(); }
      inline DomForestNode::iterator end() { return children.end(); }
    };
    
    DenseMap<MachineBasicBlock*, unsigned> preorder;
    DenseMap<MachineBasicBlock*, unsigned> maxpreorder;
    
    DenseMap<MachineBasicBlock*, std::vector<MachineInstr*> > waiting;
    
    
    void computeDFS(MachineFunction& MF);
    void processBlock(MachineBasicBlock* MBB);
    
    std::vector<DomForestNode*> computeDomForest(std::set<unsigned>& instrs);
    void processPHIUnion(MachineInstr* Inst,
                         std::set<unsigned>& PHIUnion,
                         std::vector<StrongPHIElimination::DomForestNode*>& DF);
    void breakCriticalEdges(MachineFunction &Fn);
    
  };

  char StrongPHIElimination::ID = 0;
  RegisterPass<StrongPHIElimination> X("strong-phi-node-elimination",
                  "Eliminate PHI nodes for register allocation, intelligently");
}

const PassInfo *llvm::StrongPHIEliminationID = X.getPassInfo();

/// computeDFS - Computes the DFS-in and DFS-out numbers of the dominator tree
/// of the given MachineFunction.  These numbers are then used in other parts
/// of the PHI elimination process.
void StrongPHIElimination::computeDFS(MachineFunction& MF) {
  SmallPtrSet<MachineDomTreeNode*, 8> frontier;
  SmallPtrSet<MachineDomTreeNode*, 8> visited;
  
  unsigned time = 0;
  
  MachineDominatorTree& DT = getAnalysis<MachineDominatorTree>();
  
  MachineDomTreeNode* node = DT.getRootNode();
  
  std::vector<MachineDomTreeNode*> worklist;
  worklist.push_back(node);
  
  while (!worklist.empty()) {
    MachineDomTreeNode* currNode = worklist.back();
    
    if (!frontier.count(currNode)) {
      frontier.insert(currNode);
      ++time;
      preorder.insert(std::make_pair(currNode->getBlock(), time));
    }
    
    bool inserted = false;
    for (MachineDomTreeNode::iterator I = node->begin(), E = node->end();
         I != E; ++I)
      if (!frontier.count(*I) && !visited.count(*I)) {
        worklist.push_back(*I);
        inserted = true;
        break;
      }
    
    if (!inserted) {
      frontier.erase(currNode);
      visited.insert(currNode);
      maxpreorder.insert(std::make_pair(currNode->getBlock(), time));
      
      worklist.pop_back();
    }
  }
}

/// PreorderSorter - a helper class that is used to sort registers
/// according to the preorder number of their defining blocks
class PreorderSorter {
private:
  DenseMap<MachineBasicBlock*, unsigned>& preorder;
  LiveVariables& LV;
  
public:
  PreorderSorter(DenseMap<MachineBasicBlock*, unsigned>& p,
                LiveVariables& L) : preorder(p), LV(L) { }
  
  bool operator()(unsigned A, unsigned B) {
    if (A == B)
      return false;
    
    MachineBasicBlock* ABlock = LV.getVarInfo(A).DefInst->getParent();
    MachineBasicBlock* BBlock = LV.getVarInfo(A).DefInst->getParent();
    
    if (preorder[ABlock] < preorder[BBlock])
      return true;
    else if (preorder[ABlock] > preorder[BBlock])
      return false;
    
    assert(0 && "Error sorting by dominance!");
    return false;
  }
};

/// computeDomForest - compute the subforest of the DomTree corresponding
/// to the defining blocks of the registers in question
std::vector<StrongPHIElimination::DomForestNode*>
StrongPHIElimination::computeDomForest(std::set<unsigned>& regs) {
  LiveVariables& LV = getAnalysis<LiveVariables>();
  
  DomForestNode* VirtualRoot = new DomForestNode(0, 0);
  maxpreorder.insert(std::make_pair((MachineBasicBlock*)0, ~0UL));
  
  std::vector<unsigned> worklist;
  worklist.reserve(regs.size());
  for (std::set<unsigned>::iterator I = regs.begin(), E = regs.end();
       I != E; ++I)
    worklist.push_back(*I);
  
  PreorderSorter PS(preorder, LV);
  std::sort(worklist.begin(), worklist.end(), PS);
  
  DomForestNode* CurrentParent = VirtualRoot;
  std::vector<DomForestNode*> stack;
  stack.push_back(VirtualRoot);
  
  for (std::vector<unsigned>::iterator I = worklist.begin(), E = worklist.end();
       I != E; ++I) {
    unsigned pre = preorder[LV.getVarInfo(*I).DefInst->getParent()];
    MachineBasicBlock* parentBlock =
      LV.getVarInfo(CurrentParent->getReg()).DefInst->getParent();
    
    while (pre > maxpreorder[parentBlock]) {
      stack.pop_back();
      CurrentParent = stack.back();
      
      parentBlock = LV.getVarInfo(CurrentParent->getReg()).DefInst->getParent();
    }
    
    DomForestNode* child = new DomForestNode(*I, CurrentParent);
    stack.push_back(child);
    CurrentParent = child;
  }
  
  std::vector<DomForestNode*> ret;
  ret.insert(ret.end(), VirtualRoot->begin(), VirtualRoot->end());
  return ret;
}

/// isLiveIn - helper method that determines, from a VarInfo, if a register
/// is live into a block
bool isLiveIn(LiveVariables::VarInfo& V, MachineBasicBlock* MBB) {
  if (V.AliveBlocks.test(MBB->getNumber()))
    return true;
  
  if (V.DefInst->getParent() != MBB &&
      V.UsedBlocks.test(MBB->getNumber()))
    return true;
  
  return false;
}

/// isLiveOut - help method that determines, from a VarInfo, if a register is
/// live out of a block.
bool isLiveOut(LiveVariables::VarInfo& V, MachineBasicBlock* MBB) {
  if (MBB == V.DefInst->getParent() ||
      V.UsedBlocks.test(MBB->getNumber())) {
    for (std::vector<MachineInstr*>::iterator I = V.Kills.begin(), 
         E = V.Kills.end(); I != E; ++I)
      if ((*I)->getParent() == MBB)
        return false;
    
    return true;
  }
  
  return false;
}

/// processBlock - Eliminate PHIs in the given block
void StrongPHIElimination::processBlock(MachineBasicBlock* MBB) {
  LiveVariables& LV = getAnalysis<LiveVariables>();
  
  // Holds names that have been added to a set in any PHI within this block
  // before the current one.
  std::set<unsigned> ProcessedNames;
  
  MachineBasicBlock::iterator P = MBB->begin();
  while (P->getOpcode() == TargetInstrInfo::PHI) {
    LiveVariables::VarInfo& PHIInfo = LV.getVarInfo(P->getOperand(0).getReg());

    unsigned DestReg = P->getOperand(0).getReg();

    // Hold the names that are currently in the candidate set.
    std::set<unsigned> PHIUnion;
    std::set<MachineBasicBlock*> UnionedBlocks;
  
    for (int i = P->getNumOperands() - 1; i >= 2; i-=2) {
      unsigned SrcReg = P->getOperand(i-1).getReg();
      LiveVariables::VarInfo& SrcInfo = LV.getVarInfo(SrcReg);
    
      // Check for trivial interferences
      if (isLiveIn(SrcInfo, P->getParent()) ||
          isLiveOut(PHIInfo, SrcInfo.DefInst->getParent()) ||
          ( PHIInfo.DefInst->getOpcode() == TargetInstrInfo::PHI &&
            isLiveIn(PHIInfo, SrcInfo.DefInst->getParent()) ) ||
          ProcessedNames.count(SrcReg) ||
          UnionedBlocks.count(SrcInfo.DefInst->getParent())) {
        
        // add a copy from a_i to p in Waiting[From[a_i]]
        MachineBasicBlock* From = P->getOperand(i).getMachineBasicBlock();
        Waiting[From].push_back(std::make_pair(SrcReg, DestReg));
      } else {
        PHIUnion.insert(SrcReg);
        UnionedBlocks.insert(SrcInfo.DefInst->getParent());
      }
    }
    
    std::vector<StrongPHIElimination::DomForestNode*> DF = 
                                                     computeDomForest(PHIUnion);
    
    // Walk DomForest to resolve interferences
    
    ProcessedNames.insert(PHIUnion.begin(), PHIUnion.end());
    ++P;
  }
}

void StrongPHIElimination::processPHIUnion(MachineInstr* Inst,
                                           std::set<unsigned>& PHIUnion,
                        std::vector<StrongPHIElimination::DomForestNode*>& DF) {
  
  std::vector<DomForestNode*> worklist(DF.begin(), DF.end());
  SmallPtrSet<DomForestNode*, 4> visited;
  
  LiveVariables& LV = getAnalysis<LiveVariables>();
  unsigned DestReg = Inst->getOperand(0).getReg();
  
  while (!worklist.empty()) {
    DomForestNode* DFNode = worklist.back();
    
    LiveVariables::VarInfo& Info = LV.getVarInfo(DFNode->getReg());
    visited.insert(DFNode);
    
    bool inserted = false;
    SmallPtrSet<DomForestNode*, 4> interferences;
    for (DomForestNode::iterator CI = DFNode->begin(), CE = DFNode->end();
         CI != CE; ++CI) {
      DomForestNode* child = *CI;   
      LiveVariables::VarInfo& CInfo = LV.getVarInfo(child->getReg());
        
      if (isLiveOut(Info, CInfo.DefInst->getParent())) {
        interferences.insert(child);
      } else if (isLiveIn(Info, CInfo.DefInst->getParent()) ||
                 Info.DefInst->getParent() == CInfo.DefInst->getParent()) {
        // FIXME: Add (p, c) to possible local interferences
      }
        
      if (!visited.count(child)) {
        worklist.push_back(child);
        inserted = true;
      }
    }
    
    if (interferences.size() == 1) {
      DomForestNode* child = *interferences.begin();
      
      unsigned numParentCopies = 0;
      unsigned numChildCopies = 0;
      for (int i = Inst->getNumOperands() - 1; i >= 2; i-=2) {
        unsigned SrcReg = Inst->getOperand(i-1).getReg();
        if (SrcReg == DFNode->getReg()) numParentCopies++;
        else if (SrcReg == child->getReg()) numChildCopies++;
      }
      
      if (numParentCopies < numChildCopies) {
        // Insert copies for child
        for (int i = Inst->getNumOperands() - 1; i >= 2; i-=2) {
          if (Inst->getOperand(i-1).getReg() == child->getReg()) {
            unsigned SrcReg = Inst->getOperand(i-1).getReg();
            MachineBasicBlock* From = Inst->getOperand(i).getMBB();
            
            Waiting[From].push_back(std::make_pair(SrcReg, DestReg));
          }
        }
        
        // FIXME: Make child's children parent's children
      } else {
        // Insert copies for parent
        for (int i = Inst->getNumOperands() - 1; i >= 2; i-=2) {
          if (Inst->getOperand(i-1).getReg() == DFNode->getReg()) {
            unsigned SrcReg = Inst->getOperand(i-1).getReg();
            MachineBasicBlock* From = Inst->getOperand(i).getMBB();
            
            Waiting[From].push_back(std::make_pair(SrcReg, DestReg));
          }
        }
      }
    } else if (interferences.size() > 1) {
      // Insert copies for parent
      for (int i = Inst->getNumOperands() - 1; i >= 2; i-=2) {
        if (Inst->getOperand(i-1).getReg() == DFNode->getReg()) {
          unsigned SrcReg = Inst->getOperand(i-1).getReg();
          MachineBasicBlock* From = Inst->getOperand(i).getMBB();
          
          Waiting[From].push_back(std::make_pair(SrcReg, DestReg));
        }
      }
    }
    
    if (!inserted) worklist.pop_back();
  }
}

/// breakCriticalEdges - Break critical edges coming into blocks with PHI
/// nodes, preserving dominator and livevariable info.
void StrongPHIElimination::breakCriticalEdges(MachineFunction &Fn) {
  typedef std::pair<MachineBasicBlock*, MachineBasicBlock*> MBB_pair;
  
  MachineDominatorTree& MDT = getAnalysis<MachineDominatorTree>();
  LiveVariables& LV = getAnalysis<LiveVariables>();
  
  // Find critical edges
  std::vector<MBB_pair> criticals;
  for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I)
    if (!I->empty() &&
        I->begin()->getOpcode() == TargetInstrInfo::PHI &&
        I->pred_size() > 1)
      for (MachineBasicBlock::pred_iterator PI = I->pred_begin(),
           PE = I->pred_end(); PI != PE; ++PI)
        if ((*PI)->succ_size() > 1)
          criticals.push_back(std::make_pair(*PI, I));
  
  for (std::vector<MBB_pair>::iterator I = criticals.begin(),
       E = criticals.end(); I != E; ++I) {
    // Split the edge
    MachineBasicBlock* new_bb = SplitCriticalMachineEdge(I->first, I->second);
    
    // Update dominators
    MDT.splitBlock(I->first);
    
    // Update livevariables
    for (unsigned var = 1024; var < Fn.getSSARegMap()->getLastVirtReg(); ++var)
      if (isLiveOut(LV.getVarInfo(var), I->first))
        LV.getVarInfo(var).AliveBlocks.set(new_bb->getNumber());
  }
}

bool StrongPHIElimination::runOnMachineFunction(MachineFunction &Fn) {
  breakCriticalEdges(Fn);
  computeDFS(Fn);
  
  for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I)
    if (!I->empty() &&
        I->begin()->getOpcode() == TargetInstrInfo::PHI)
      processBlock(I);
  
  return false;
}