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