lib/CodeGen/DeadMachineInstructionElim.cpp

   1 //===- DeadMachineInstructionElim.cpp - Remove dead machine instructions --===//
   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 is an extremely simple MachineInstr-level dead-code-elimination pass.
  11 //
  12 //===----------------------------------------------------------------------===//
  13
  14 #include "llvm/CodeGen/Passes.h"
  15 #include "llvm/Pass.h"
  16 #include "llvm/CodeGen/MachineFunctionPass.h"
  17 #include "llvm/CodeGen/MachineRegisterInfo.h"
  18 #include "llvm/Support/Compiler.h"
  19 #include "llvm/Support/Debug.h"
  20 #include "llvm/Support/raw_ostream.h"
  21 #include "llvm/Target/TargetInstrInfo.h"
  22 #include "llvm/Target/TargetMachine.h"
  23 using namespace llvm;
  24
  25 namespace {
  26   class VISIBILITY_HIDDEN DeadMachineInstructionElim :
  27         public MachineFunctionPass {
  28     virtual bool runOnMachineFunction(MachineFunction &MF);
  29
  30     const TargetRegisterInfo *TRI;
  31     const MachineRegisterInfo *MRI;
  32     const TargetInstrInfo *TII;
  33     BitVector LivePhysRegs;
  34
  35   public:
  36     static char ID; // Pass identification, replacement for typeid
  37     DeadMachineInstructionElim() : MachineFunctionPass(&ID) {}
  38
  39   private:
  40     bool isDead(const MachineInstr *MI) const;
  41   };
  42 }
  43 char DeadMachineInstructionElim::ID = 0;
  44
  45 static RegisterPass<DeadMachineInstructionElim>
  46 Y("dead-mi-elimination",
  47   "Remove dead machine instructions");
  48
  49 FunctionPass *llvm::createDeadMachineInstructionElimPass() {
  50   return new DeadMachineInstructionElim();
  51 }
  52
  53 bool DeadMachineInstructionElim::isDead(const MachineInstr *MI) const {
  54   // Don't delete instructions with side effects.
  55   bool SawStore = false;
  56   if (!MI->isSafeToMove(TII, SawStore))
  57     return false;
  58
  59   // Examine each operand.
  60   for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
  61     const MachineOperand &MO = MI->getOperand(i);
  62     if (MO.isReg() && MO.isDef()) {
  63       unsigned Reg = MO.getReg();
  64       if (TargetRegisterInfo::isPhysicalRegister(Reg) ?
  65           LivePhysRegs[Reg] : !MRI->use_empty(Reg)) {
  66         // This def has a use. Don't delete the instruction!
  67         return false;
  68       }
  69     }
  70   }
  71
  72   // If there are no defs with uses, the instruction is dead.
  73   return true;
  74 }
  75
  76 bool DeadMachineInstructionElim::runOnMachineFunction(MachineFunction &MF) {
  77   bool AnyChanges = false;
  78   MRI = &MF.getRegInfo();
  79   TRI = MF.getTarget().getRegisterInfo();
  80   TII = MF.getTarget().getInstrInfo();
  81
  82   // Compute a bitvector to represent all non-allocatable physregs.
  83   BitVector NonAllocatableRegs = TRI->getAllocatableSet(MF);
  84   NonAllocatableRegs.flip();
  85
  86   // Loop over all instructions in all blocks, from bottom to top, so that it's
  87   // more likely that chains of dependent but ultimately dead instructions will
  88   // be cleaned up.
  89   for (MachineFunction::reverse_iterator I = MF.rbegin(), E = MF.rend();
  90        I != E; ++I) {
  91     MachineBasicBlock *MBB = &*I;
  92
  93     // Start out assuming that all non-allocatable registers are live
  94     // out of this block.
  95     LivePhysRegs = NonAllocatableRegs;
  96
  97     // Also add any explicit live-out physregs for this block.
  98     if (!MBB->empty() && MBB->back().getDesc().isReturn())
  99       for (MachineRegisterInfo::liveout_iterator LOI = MRI->liveout_begin(),
 100            LOE = MRI->liveout_end(); LOI != LOE; ++LOI) {
 101         unsigned Reg = *LOI;
 102         if (TargetRegisterInfo::isPhysicalRegister(Reg))
 103           LivePhysRegs.set(Reg);
 104       }
 105
 106     // Now scan the instructions and delete dead ones, tracking physreg
 107     // liveness as we go.
 108     for (MachineBasicBlock::reverse_iterator MII = MBB->rbegin(),
 109          MIE = MBB->rend(); MII != MIE; ) {
 110       MachineInstr *MI = &*MII;
 111
 112       // If the instruction is dead, delete it!
 113       if (isDead(MI)) {
 114         DEBUG(errs() << "DeadMachineInstructionElim: DELETING: " << *MI);
 115         AnyChanges = true;
 116         MI->eraseFromParent();
 117         MIE = MBB->rend();
 118         // MII is now pointing to the next instruction to process,
 119         // so don't increment it.
 120         continue;
 121       }
 122
 123       // Record the physreg defs.
 124       for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
 125         const MachineOperand &MO = MI->getOperand(i);
 126         if (MO.isReg() && MO.isDef()) {
 127           unsigned Reg = MO.getReg();
 128           if (Reg != 0 && TargetRegisterInfo::isPhysicalRegister(Reg)) {
 129             LivePhysRegs.reset(Reg);
 130             // Check the subreg set, not the alias set, because a def
 131             // of a super-register may still be partially live after
 132             // this def.
 133             for (const unsigned *SubRegs = TRI->getSubRegisters(Reg);
 134                  *SubRegs; ++SubRegs)
 135               LivePhysRegs.reset(*SubRegs);
 136           }
 137         }
 138       }
 139       // Record the physreg uses, after the defs, in case a physreg is
 140       // both defined and used in the same instruction.
 141       for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
 142         const MachineOperand &MO = MI->getOperand(i);
 143         if (MO.isReg() && MO.isUse()) {
 144           unsigned Reg = MO.getReg();
 145           if (Reg != 0 && TargetRegisterInfo::isPhysicalRegister(Reg)) {
 146             LivePhysRegs.set(Reg);
 147             for (const unsigned *AliasSet = TRI->getAliasSet(Reg);
 148                  *AliasSet; ++AliasSet)
 149               LivePhysRegs.set(*AliasSet);
 150           }
 151         }
 152       }
 153
 154       // We didn't delete the current instruction, so increment MII to
 155       // the next one.
 156       ++MII;
 157     }
 158   }
 159
 160   LivePhysRegs.clear();
 161   return AnyChanges;
 162 }