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/MachineDominators.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.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) {}

    bool runOnMachineFunction(MachineFunction &Fn) {
      computeDFS(Fn);
      
      
      return false;
    }

    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.addRequired<MachineDominatorTree>();
      MachineFunctionPass::getAnalysisUsage(AU);
    }
    
    virtual void releaseMemory() {
      preorder.clear();
      maxpreorder.clear();
    }

  private:
    DenseMap<MachineBasicBlock*, unsigned> preorder;
    DenseMap<MachineBasicBlock*, unsigned> maxpreorder;
    
    void computeDFS(MachineFunction& MF);
  };

  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();
    }
  }
}