lib/CodeGen/RegAllocSimple.cpp

   1 //===-- RegAllocSimple.cpp - A simple generic register allocator ----------===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This file implements a simple register allocator. *Very* simple: It immediate
  11 // spills every value right after it is computed, and it reloads all used
  12 // operands from the spill area to temporary registers before each instruction.
  13 // It does not keep values in registers across instructions.
  14 //
  15 //===----------------------------------------------------------------------===//
  16
  17 #define DEBUG_TYPE "regalloc"
  18 #include "llvm/CodeGen/Passes.h"
  19 #include "llvm/CodeGen/MachineFunctionPass.h"
  20 #include "llvm/CodeGen/MachineInstr.h"
  21 #include "llvm/CodeGen/MachineFrameInfo.h"
  22 #include "llvm/CodeGen/MachineRegisterInfo.h"
  23 #include "llvm/CodeGen/RegAllocRegistry.h"
  24 #include "llvm/Target/TargetInstrInfo.h"
  25 #include "llvm/Target/TargetMachine.h"
  26 #include "llvm/Support/Compiler.h"
  27 #include "llvm/Support/Debug.h"
  28 #include "llvm/Support/raw_ostream.h"
  29 #include "llvm/ADT/Statistic.h"
  30 #include "llvm/ADT/STLExtras.h"
  31 #include <map>
  32 using namespace llvm;
  33
  34 STATISTIC(NumStores, "Number of stores added");
  35 STATISTIC(NumLoads , "Number of loads added");
  36
  37 namespace {
  38   static RegisterRegAlloc
  39     simpleRegAlloc("simple", "simple register allocator",
  40                    createSimpleRegisterAllocator);
  41
  42   class VISIBILITY_HIDDEN RegAllocSimple : public MachineFunctionPass {
  43   public:
  44     static char ID;
  45     RegAllocSimple() : MachineFunctionPass(&ID) {}
  46   private:
  47     MachineFunction *MF;
  48     const TargetMachine *TM;
  49     const TargetRegisterInfo *TRI;
  50     const TargetInstrInfo *TII;
  51
  52     // StackSlotForVirtReg - Maps SSA Regs => frame index on the stack where
  53     // these values are spilled
  54     std::map<unsigned, int> StackSlotForVirtReg;
  55
  56     // RegsUsed - Keep track of what registers are currently in use.  This is a
  57     // bitset.
  58     std::vector<bool> RegsUsed;
  59
  60     // RegClassIdx - Maps RegClass => which index we can take a register
  61     // from. Since this is a simple register allocator, when we need a register
  62     // of a certain class, we just take the next available one.
  63     std::map<const TargetRegisterClass*, unsigned> RegClassIdx;
  64
  65   public:
  66     virtual const char *getPassName() const {
  67       return "Simple Register Allocator";
  68     }
  69
  70     /// runOnMachineFunction - Register allocate the whole function
  71     bool runOnMachineFunction(MachineFunction &Fn);
  72
  73     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  74       AU.setPreservesCFG();
  75       AU.addRequiredID(PHIEliminationID);           // Eliminate PHI nodes
  76       MachineFunctionPass::getAnalysisUsage(AU);
  77     }
  78   private:
  79     /// AllocateBasicBlock - Register allocate the specified basic block.
  80     void AllocateBasicBlock(MachineBasicBlock &MBB);
  81
  82     /// getStackSpaceFor - This returns the offset of the specified virtual
  83     /// register on the stack, allocating space if necessary.
  84     int getStackSpaceFor(unsigned VirtReg, const TargetRegisterClass *RC);
  85
  86     /// Given a virtual register, return a compatible physical register that is
  87     /// currently unused.
  88     ///
  89     /// Side effect: marks that register as being used until manually cleared
  90     ///
  91     unsigned getFreeReg(unsigned virtualReg);
  92
  93     /// Moves value from memory into that register
  94     unsigned reloadVirtReg(MachineBasicBlock &MBB,
  95                            MachineBasicBlock::iterator I, unsigned VirtReg);
  96
  97     /// Saves reg value on the stack (maps virtual register to stack value)
  98     void spillVirtReg(MachineBasicBlock &MBB, MachineBasicBlock::iterator I,
  99                       unsigned VirtReg, unsigned PhysReg);
 100   };
 101   char RegAllocSimple::ID = 0;
 102 }
 103
 104 /// getStackSpaceFor - This allocates space for the specified virtual
 105 /// register to be held on the stack.
 106 int RegAllocSimple::getStackSpaceFor(unsigned VirtReg,
 107                                      const TargetRegisterClass *RC) {
 108   // Find the location VirtReg would belong...
 109   std::map<unsigned, int>::iterator I = StackSlotForVirtReg.find(VirtReg);
 110
 111   if (I != StackSlotForVirtReg.end())
 112     return I->second;          // Already has space allocated?
 113
 114   // Allocate a new stack object for this spill location...
 115   int FrameIdx = MF->getFrameInfo()->CreateStackObject(RC->getSize(),
 116                                                        RC->getAlignment());
 117
 118   // Assign the slot...
 119   StackSlotForVirtReg.insert(I, std::make_pair(VirtReg, FrameIdx));
 120
 121   return FrameIdx;
 122 }
 123
 124 unsigned RegAllocSimple::getFreeReg(unsigned virtualReg) {
 125   const TargetRegisterClass* RC = MF->getRegInfo().getRegClass(virtualReg);
 126   TargetRegisterClass::iterator RI = RC->allocation_order_begin(*MF);
 127 #ifndef NDEBUG
 128   TargetRegisterClass::iterator RE = RC->allocation_order_end(*MF);
 129 #endif
 130
 131   while (1) {
 132     unsigned regIdx = RegClassIdx[RC]++;
 133     assert(RI+regIdx != RE && "Not enough registers!");
 134     unsigned PhysReg = *(RI+regIdx);
 135
 136     if (!RegsUsed[PhysReg]) {
 137       MF->getRegInfo().setPhysRegUsed(PhysReg);
 138       return PhysReg;
 139     }
 140   }
 141 }
 142
 143 unsigned RegAllocSimple::reloadVirtReg(MachineBasicBlock &MBB,
 144                                        MachineBasicBlock::iterator I,
 145                                        unsigned VirtReg) {
 146   const TargetRegisterClass* RC = MF->getRegInfo().getRegClass(VirtReg);
 147   int FrameIdx = getStackSpaceFor(VirtReg, RC);
 148   unsigned PhysReg = getFreeReg(VirtReg);
 149
 150   // Add move instruction(s)
 151   ++NumLoads;
 152   TII->loadRegFromStackSlot(MBB, I, PhysReg, FrameIdx, RC);
 153   return PhysReg;
 154 }
 155
 156 void RegAllocSimple::spillVirtReg(MachineBasicBlock &MBB,
 157                                   MachineBasicBlock::iterator I,
 158                                   unsigned VirtReg, unsigned PhysReg) {
 159   const TargetRegisterClass* RC = MF->getRegInfo().getRegClass(VirtReg);
 160
 161   int FrameIdx = getStackSpaceFor(VirtReg, RC);
 162
 163   // Add move instruction(s)
 164   ++NumStores;
 165   TII->storeRegToStackSlot(MBB, I, PhysReg, true, FrameIdx, RC);
 166 }
 167
 168
 169 void RegAllocSimple::AllocateBasicBlock(MachineBasicBlock &MBB) {
 170   // loop over each instruction
 171   for (MachineBasicBlock::iterator MI = MBB.begin(); MI != MBB.end(); ++MI) {
 172     // Made to combat the incorrect allocation of r2 = add r1, r1
 173     std::map<unsigned, unsigned> Virt2PhysRegMap;
 174
 175     RegsUsed.resize(TRI->getNumRegs());
 176
 177     // This is a preliminary pass that will invalidate any registers that are
 178     // used by the instruction (including implicit uses).
 179     const TargetInstrDesc &Desc = MI->getDesc();
 180     const unsigned *Regs;
 181     if (Desc.ImplicitUses) {
 182       for (Regs = Desc.ImplicitUses; *Regs; ++Regs)
 183         RegsUsed[*Regs] = true;
 184     }
 185
 186     if (Desc.ImplicitDefs) {
 187       for (Regs = Desc.ImplicitDefs; *Regs; ++Regs) {
 188         RegsUsed[*Regs] = true;
 189         MF->getRegInfo().setPhysRegUsed(*Regs);
 190       }
 191     }
 192
 193     // Loop over uses, move from memory into registers.
 194     for (int i = MI->getNumOperands() - 1; i >= 0; --i) {
 195       MachineOperand &MO = MI->getOperand(i);
 196
 197       if (MO.isReg() && MO.getReg() &&
 198           TargetRegisterInfo::isVirtualRegister(MO.getReg())) {
 199         unsigned virtualReg = (unsigned) MO.getReg();
 200         DEBUG({
 201             errs() << "op: " << MO << "\n" << "\t inst[" << i << "]: ";
 202             MI->print(errs(), TM);
 203           });
 204
 205         // make sure the same virtual register maps to the same physical
 206         // register in any given instruction
 207         unsigned physReg = Virt2PhysRegMap[virtualReg];
 208         if (physReg == 0) {
 209           if (MO.isDef()) {
 210             unsigned TiedOp;
 211             if (!MI->isRegTiedToUseOperand(i, &TiedOp)) {
 212               physReg = getFreeReg(virtualReg);
 213             } else {
 214               // must be same register number as the source operand that is
 215               // tied to. This maps a = b + c into b = b + c, and saves b into
 216               // a's spot.
 217               assert(MI->getOperand(TiedOp).isReg()  &&
 218                      MI->getOperand(TiedOp).getReg() &&
 219                      MI->getOperand(TiedOp).isUse() &&
 220                      "Two address instruction invalid!");
 221
 222               physReg = MI->getOperand(TiedOp).getReg();
 223             }
 224             spillVirtReg(MBB, next(MI), virtualReg, physReg);
 225           } else {
 226             physReg = reloadVirtReg(MBB, MI, virtualReg);
 227             Virt2PhysRegMap[virtualReg] = physReg;
 228           }
 229         }
 230         MO.setReg(physReg);
 231         DEBUG(errs() << "virt: " << virtualReg
 232                      << ", phys: " << MO.getReg() << "\n");
 233       }
 234     }
 235     RegClassIdx.clear();
 236     RegsUsed.clear();
 237   }
 238 }
 239
 240
 241 /// runOnMachineFunction - Register allocate the whole function
 242 ///
 243 bool RegAllocSimple::runOnMachineFunction(MachineFunction &Fn) {
 244   DEBUG(errs() << "Machine Function\n");
 245   MF = &Fn;
 246   TM = &MF->getTarget();
 247   TRI = TM->getRegisterInfo();
 248   TII = TM->getInstrInfo();
 249
 250   // Loop over all of the basic blocks, eliminating virtual register references
 251   for (MachineFunction::iterator MBB = Fn.begin(), MBBe = Fn.end();
 252        MBB != MBBe; ++MBB)
 253     AllocateBasicBlock(*MBB);
 254
 255   StackSlotForVirtReg.clear();
 256   return true;
 257 }
 258
 259 FunctionPass *llvm::createSimpleRegisterAllocator() {
 260   return new RegAllocSimple();
 261 }