RegAllocLinearScan.cpp

//===-- RegAllocLinearScan.cpp - Linear Scan register allocator -----------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements a linear scan register allocator.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "regalloc"
#include "PhysRegTracker.h"
#include "VirtRegMap.h"
#include "llvm/Function.h"
#include "llvm/CodeGen/LiveIntervalAnalysis.h"
#include "llvm/CodeGen/LiveStackAnalysis.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/RegAllocRegistry.h"
#include "llvm/CodeGen/RegisterCoalescer.h"
#include "llvm/Target/TargetRegisterInfo.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/ADT/EquivalenceClasses.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/Compiler.h"
#include <algorithm>
#include <set>
#include <queue>
#include <memory>
#include <cmath>
using namespace llvm;

STATISTIC(NumIters     , "Number of iterations performed");
STATISTIC(NumBacktracks, "Number of times we had to backtrack");
STATISTIC(NumCoalesce,   "Number of copies coalesced");

static cl::opt<bool>
NewHeuristic("new-spilling-heuristic",
             cl::desc("Use new spilling heuristic"),
             cl::init(false), cl::Hidden);

static RegisterRegAlloc
linearscanRegAlloc("linearscan", "  linear scan register allocator",
                   createLinearScanRegisterAllocator);

namespace {
  struct VISIBILITY_HIDDEN RALinScan : public MachineFunctionPass {
    static char ID;
    RALinScan() : MachineFunctionPass(&ID) {}

    typedef std::pair<LiveInterval*, LiveInterval::iterator> IntervalPtr;
    typedef SmallVector<IntervalPtr, 32> IntervalPtrs;
  private:
    /// RelatedRegClasses - This structure is built the first time a function is
    /// compiled, and keeps track of which register classes have registers that
    /// belong to multiple classes or have aliases that are in other classes.
    EquivalenceClasses<const TargetRegisterClass*> RelatedRegClasses;
    DenseMap<unsigned, const TargetRegisterClass*> OneClassForEachPhysReg;

    MachineFunction* mf_;
    MachineRegisterInfo* mri_;
    const TargetMachine* tm_;
    const TargetRegisterInfo* tri_;
    const TargetInstrInfo* tii_;
    BitVector allocatableRegs_;
    LiveIntervals* li_;
    LiveStacks* ls_;
    const MachineLoopInfo *loopInfo;

    /// handled_ - Intervals are added to the handled_ set in the order of their
    /// start value.  This is uses for backtracking.
    std::vector<LiveInterval*> handled_;

    /// fixed_ - Intervals that correspond to machine registers.
    ///
    IntervalPtrs fixed_;

    /// active_ - Intervals that are currently being processed, and which have a
    /// live range active for the current point.
    IntervalPtrs active_;

    /// inactive_ - Intervals that are currently being processed, but which have
    /// a hold at the current point.
    IntervalPtrs inactive_;

    typedef std::priority_queue<LiveInterval*,
                                SmallVector<LiveInterval*, 64>,
                                greater_ptr<LiveInterval> > IntervalHeap;
    IntervalHeap unhandled_;
    std::auto_ptr<PhysRegTracker> prt_;
    std::auto_ptr<VirtRegMap> vrm_;
    std::auto_ptr<Spiller> spiller_;

  public:
    virtual const char* getPassName() const {
      return "Linear Scan Register Allocator";
    }

    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.addRequired<LiveIntervals>();
      // Make sure PassManager knows which analyses to make available
      // to coalescing and which analyses coalescing invalidates.
      AU.addRequiredTransitive<RegisterCoalescer>();
      AU.addRequired<LiveStacks>();
      AU.addPreserved<LiveStacks>();
      AU.addRequired<MachineLoopInfo>();
      AU.addPreserved<MachineLoopInfo>();
      AU.addPreservedID(MachineDominatorsID);
      MachineFunctionPass::getAnalysisUsage(AU);
    }

    /// runOnMachineFunction - register allocate the whole function
    bool runOnMachineFunction(MachineFunction&);

  private:
    /// linearScan - the linear scan algorithm
    void linearScan();

    /// initIntervalSets - initialize the interval sets.
    ///
    void initIntervalSets();

    /// processActiveIntervals - expire old intervals and move non-overlapping
    /// ones to the inactive list.
    void processActiveIntervals(unsigned CurPoint);

    /// processInactiveIntervals - expire old intervals and move overlapping
    /// ones to the active list.
    void processInactiveIntervals(unsigned CurPoint);

    /// assignRegOrStackSlotAtInterval - assign a register if one
    /// is available, or spill.
    void assignRegOrStackSlotAtInterval(LiveInterval* cur);

    /// findIntervalsToSpill - Determine the intervals to spill for the
    /// specified interval. It's passed the physical registers whose spill
    /// weight is the lowest among all the registers whose live intervals
    /// conflict with the interval.
    void findIntervalsToSpill(LiveInterval *cur,
                            std::vector<std::pair<unsigned,float> > &Candidates,
                            unsigned NumCands,
                            SmallVector<LiveInterval*, 8> &SpillIntervals);

    /// attemptTrivialCoalescing - If a simple interval is defined by a copy,
    /// try allocate the definition the same register as the source register
    /// if the register is not defined during live time of the interval. This
    /// eliminate a copy. This is used to coalesce copies which were not
    /// coalesced away before allocation either due to dest and src being in
    /// different register classes or because the coalescer was overly
    /// conservative.
    unsigned attemptTrivialCoalescing(LiveInterval &cur, unsigned Reg);

    ///
    /// register handling helpers
    ///

    /// getFreePhysReg - return a free physical register for this virtual
    /// register interval if we have one, otherwise return 0.
    unsigned getFreePhysReg(LiveInterval* cur);

    /// assignVirt2StackSlot - assigns this virtual register to a
    /// stack slot. returns the stack slot
    int assignVirt2StackSlot(unsigned virtReg);

    void ComputeRelatedRegClasses();

    template <typename ItTy>
    void printIntervals(const char* const str, ItTy i, ItTy e) const {
      if (str) DOUT << str << " intervals:\n";
      for (; i != e; ++i) {
        DOUT << "\t" << *i->first << " -> ";
        unsigned reg = i->first->reg;
        if (TargetRegisterInfo::isVirtualRegister(reg)) {
          reg = vrm_->getPhys(reg);
        }
        DOUT << tri_->getName(reg) << '\n';
      }
    }
  };
  char RALinScan::ID = 0;
}

static RegisterPass<RALinScan>
X("linearscan-regalloc", "Linear Scan Register Allocator");

void RALinScan::ComputeRelatedRegClasses() {
  const TargetRegisterInfo &TRI = *tri_;
  
  // First pass, add all reg classes to the union, and determine at least one
  // reg class that each register is in.
  bool HasAliases = false;