RegAllocFast.cpp

//===-- RegAllocFast.cpp - A fast register allocator for debug code -------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This register allocator allocates registers to a basic block at a time,
// attempting to keep values in registers and reusing registers as appropriate.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "regalloc"
#include "llvm/BasicBlock.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/RegAllocRegistry.h"
#include "llvm/Target/TargetInstrInfo.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/IndexedMap.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
#include <algorithm>
using namespace llvm;

STATISTIC(NumStores, "Number of stores added");
STATISTIC(NumLoads , "Number of loads added");
STATISTIC(NumCopies, "Number of copies coalesced");

static RegisterRegAlloc
  fastRegAlloc("fast", "fast register allocator", createFastRegisterAllocator);

namespace {
  class RAFast : public MachineFunctionPass {
  public:
    static char ID;
    RAFast() : MachineFunctionPass(ID), StackSlotForVirtReg(-1),
               isBulkSpilling(false) {}
  private:
    const TargetMachine *TM;
    MachineFunction *MF;
    MachineRegisterInfo *MRI;
    const TargetRegisterInfo *TRI;
    const TargetInstrInfo *TII;

    // Basic block currently being allocated.
    MachineBasicBlock *MBB;

    // StackSlotForVirtReg - Maps virtual regs to the frame index where these
    // values are spilled.
    IndexedMap<int, VirtReg2IndexFunctor> StackSlotForVirtReg;

    // Everything we know about a live virtual register.
    struct LiveReg {
      MachineInstr *LastUse;    // Last instr to use reg.
      unsigned PhysReg;         // Currently held here.
      unsigned short LastOpNum; // OpNum on LastUse.
      bool Dirty;               // Register needs spill.

      LiveReg(unsigned p=0) : LastUse(0), PhysReg(p), LastOpNum(0),
                              Dirty(false) {}
    };

    typedef DenseMap<unsigned, LiveReg> LiveRegMap;
    typedef LiveRegMap::value_type LiveRegEntry;

    // LiveVirtRegs - This map contains entries for each virtual register
    // that is currently available in a physical register.
    LiveRegMap LiveVirtRegs;

    DenseMap<unsigned, MachineInstr *> LiveDbgValueMap;

    // RegState - Track the state of a physical register.
    enum RegState {
      // A disabled register is not available for allocation, but an alias may
      // be in use. A register can only be moved out of the disabled state if
      // all aliases are disabled.
      regDisabled,

      // A free register is not currently in use and can be allocated
      // immediately without checking aliases.
      regFree,

      // A reserved register has been assigned expolicitly (e.g., setting up a
      // call parameter), and it remains reserved until it is used.
      regReserved

      // A register state may also be a virtual register number, indication that
      // the physical register is currently allocated to a virtual register. In
      // that case, LiveVirtRegs contains the inverse mapping.
    };

    // PhysRegState - One of the RegState enums, or a virtreg.
    std::vector<unsigned> PhysRegState;

    // UsedInInstr - BitVector of physregs that are used in the current
    // instruction, and so cannot be allocated.
    BitVector UsedInInstr;

    // Allocatable - vector of allocatable physical registers.
    BitVector Allocatable;

    // SkippedInstrs - Descriptors of instructions whose clobber list was
    // ignored because all registers were spilled. It is still necessary to
    // mark all the clobbered registers as used by the function.
    SmallPtrSet<const TargetInstrDesc*, 4> SkippedInstrs;

    // isBulkSpilling - This flag is set when LiveRegMap will be cleared
    // completely after spilling all live registers. LiveRegMap entries should
    // not be erased.
    bool isBulkSpilling;

    enum {
      spillClean = 1,
      spillDirty = 100,
      spillImpossible = ~0u
    };
  public:
    virtual const char *getPassName() const {
      return "Fast Register Allocator";
    }

    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.setPreservesCFG();
      AU.addRequiredID(PHIEliminationID);
      AU.addRequiredID(TwoAddressInstructionPassID);
      MachineFunctionPass::getAnalysisUsage(AU);
    }

  private:
    bool runOnMachineFunction(MachineFunction &Fn);
    void AllocateBasicBlock();
    void handleThroughOperands(MachineInstr *MI,
                               SmallVectorImpl<unsigned> &VirtDead);
    int getStackSpaceFor(unsigned VirtReg, const TargetRegisterClass *RC);
    bool isLastUseOfLocalReg(MachineOperand&);

    void addKillFlag(const LiveReg&);
    void killVirtReg(LiveRegMap::iterator);
    void killVirtReg(unsigned VirtReg);
    void spillVirtReg(MachineBasicBlock::iterator MI, LiveRegMap::iterator);
    void spillVirtReg(MachineBasicBlock::iterator MI, unsigned VirtReg);

    void usePhysReg(MachineOperand&);
    void definePhysReg(MachineInstr *MI, unsigned PhysReg, RegState NewState);
    unsigned calcSpillCost(unsigned PhysReg) const;
    void assignVirtToPhysReg(LiveRegEntry &LRE, unsigned PhysReg);
    void allocVirtReg(MachineInstr *MI, LiveRegEntry &LRE, unsigned Hint);
    LiveRegMap::iterator defineVirtReg(MachineInstr *MI, unsigned OpNum,
                                       unsigned VirtReg, unsigned Hint);
    LiveRegMap::iterator reloadVirtReg(MachineInstr *MI, unsigned OpNum,
                                       unsigned VirtReg, unsigned Hint);
    void spillAll(MachineInstr *MI);
    bool setPhysReg(MachineInstr *MI, unsigned OpNum, unsigned PhysReg);
  };
  char RAFast::ID = 0;
}

/// getStackSpaceFor - This allocates space for the specified virtual register
/// to be held on the stack.
int RAFast::getStackSpaceFor(unsigned VirtReg, const TargetRegisterClass *RC) {
  // Find the location Reg would belong...
  int SS = StackSlotForVirtReg[VirtReg];
  if (SS != -1)
    return SS;          // Already has space allocated?

  // Allocate a new stack object for this spill location...
  int FrameIdx = MF->getFrameInfo()->CreateSpillStackObject(RC->getSize(),
                                                            RC->getAlignment());

  // Assign the slot.
  StackSlotForVirtReg[VirtReg] = FrameIdx;
  return FrameIdx;
}

/// isLastUseOfLocalReg - Return true if MO is the only remaining reference to
/// its virtual register, and it is guaranteed to be a block-local register.
///
bool RAFast::isLastUseOfLocalReg(MachineOperand &MO) {
  // Check for non-debug uses or defs following MO.
  // This is the most likely way to fail - fast path it.
  MachineOperand *Next = &MO;
  while ((Next = Next->getNextOperandForReg()))
    if (!Next->isDebug())
      return false;

  // If the register has ever been spilled or reloaded, we conservatively assume