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