path: root/lib/CodeGen/SimpleRegisterCoalescing.cpp

//===-- SimpleRegisterCoalescing.cpp - Register Coalescing ----------------===//
//
//                     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 a simple register coalescing pass that attempts to
// aggressively coalesce every register copy that it can.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "regcoalescing"
#include "llvm/CodeGen/SimpleRegisterCoalescing.h"
#include "llvm/CodeGen/LiveIntervalAnalysis.h"
#include "VirtRegMap.h"
#include "llvm/Value.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/CodeGen/LiveVariables.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/SSARegMap.h"
#include "llvm/Target/MRegisterInfo.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
#include <algorithm>
#include <cmath>
using namespace llvm;

STATISTIC(numJoins    , "Number of interval joins performed");
STATISTIC(numPeep     , "Number of identity moves eliminated after coalescing");
STATISTIC(numAborts   , "Number of times interval joining aborted");

char SimpleRegisterCoalescing::ID = 0;
namespace {
  static cl::opt<bool>
  EnableJoining("join-liveintervals",
                cl::desc("Coalesce copies (default=true)"),
                cl::init(true));

  RegisterPass<SimpleRegisterCoalescing> 
  X("simple-register-coalescing", "Simple Register Coalescing");
}

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

void SimpleRegisterCoalescing::getAnalysisUsage(AnalysisUsage &AU) const {
   //AU.addPreserved<LiveVariables>();
  AU.addPreserved<LiveIntervals>();
  AU.addPreservedID(PHIEliminationID);
  AU.addPreservedID(TwoAddressInstructionPassID);
  AU.addRequired<LiveVariables>();
  AU.addRequired<LiveIntervals>();
  AU.addRequired<LoopInfo>();
  MachineFunctionPass::getAnalysisUsage(AU);
}

/// AdjustCopiesBackFrom - We found a non-trivially-coalescable copy with IntA
/// being the source and IntB being the dest, thus this defines a value number
/// in IntB.  If the source value number (in IntA) is defined by a copy from B,
/// see if we can merge these two pieces of B into a single value number,
/// eliminating a copy.  For example:
///
///  A3 = B0
///    ...
///  B1 = A3      <- this copy
///
/// In this case, B0 can be extended to where the B1 copy lives, allowing the B1
/// value number to be replaced with B0 (which simplifies the B liveinterval).
///
/// This returns true if an interval was modified.
///
bool SimpleRegisterCoalescing::AdjustCopiesBackFrom(LiveInterval &IntA, LiveInterval &IntB,
                                         MachineInstr *CopyMI) {
  unsigned CopyIdx = li_->getDefIndex(li_->getInstructionIndex(CopyMI));

  // BValNo is a value number in B that is defined by a copy from A.  'B3' in
  // the example above.
  LiveInterval::iterator BLR = IntB.FindLiveRangeContaining(CopyIdx);
  unsigned BValNo = BLR->ValId;
  
  // Get the location that B is defined at.  Two options: either this value has
  // an unknown definition point or it is defined at CopyIdx.  If unknown, we 
  // can't process it.
  unsigned BValNoDefIdx = IntB.getDefForValNum(BValNo);
  if (!IntB.getSrcRegForValNum(BValNo)) return false;
  assert(BValNoDefIdx == CopyIdx &&
         "Copy doesn't define the value?");
  
  // AValNo is the value number in A that defines the copy, A0 in the example.
  LiveInterval::iterator AValLR = IntA.FindLiveRangeContaining(CopyIdx-1);
  unsigned AValNo = AValLR->ValId;
  
  // If AValNo is defined as a copy from IntB, we can potentially process this.
  
  // Get the instruction that defines this value number.
  unsigned SrcReg = IntA.getSrcRegForValNum(AValNo);
  if (!SrcReg) return false;  // Not defined by a copy.
    
  // If the value number is not defined by a copy instruction, ignore it.
    
  // If the source register comes from an interval other than IntB, we can't