//===- DeadMachineInstructionElim.cpp - Remove dead machine instructions --===// // // The LLVM Compiler Infrastructure // // This file is distributed under the University of Illinois Open Source // License. See LICENSE.TXT for details. // //===----------------------------------------------------------------------===// // // This is an extremely simple MachineInstr-level dead-code-elimination pass. // //===----------------------------------------------------------------------===// #define DEBUG_TYPE "codegen-dce" #include "llvm/CodeGen/Passes.h" #include "llvm/ADT/Statistic.h" #include "llvm/CodeGen/MachineFunctionPass.h" #include "llvm/CodeGen/MachineRegisterInfo.h" #include "llvm/Pass.h" #include "llvm/Support/Debug.h" #include "llvm/Support/raw_ostream.h" #include "llvm/Target/TargetInstrInfo.h" #include "llvm/Target/TargetMachine.h" using namespace llvm; STATISTIC(NumDeletes, "Number of dead instructions deleted"); namespace { class DeadMachineInstructionElim : public MachineFunctionPass { bool runOnMachineFunction(MachineFunction &MF) override; const TargetRegisterInfo *TRI; const MachineRegisterInfo *MRI; const TargetInstrInfo *TII; BitVector LivePhysRegs; public: static char ID; // Pass identification, replacement for typeid DeadMachineInstructionElim() : MachineFunctionPass(ID) { initializeDeadMachineInstructionElimPass(*PassRegistry::getPassRegistry()); } private: bool isDead(const MachineInstr *MI) const; }; } char DeadMachineInstructionElim::ID = 0; char &llvm::DeadMachineInstructionElimID = DeadMachineInstructionElim::ID; INITIALIZE_PASS(DeadMachineInstructionElim, "dead-mi-elimination", "Remove dead machine instructions", false, false) bool DeadMachineInstructionElim::isDead(const MachineInstr *MI) const { // Technically speaking inline asm without side effects and no defs can still // be deleted. But there is so much bad inline asm code out there, we should // let them be. if (MI->isInlineAsm()) return false; // Don't delete instructions with side effects. bool SawStore = false; if (!MI->isSafeToMove(TII, 0, SawStore) && !MI->isPHI()) return false; // Examine each operand. for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { const MachineOperand &MO = MI->getOperand(i); if (MO.isReg() && MO.isDef()) { unsigned Reg = MO.getReg(); if (TargetRegisterInfo::isPhysicalRegister(Reg)) { // Don't delete live physreg defs, or any reserved register defs. if (LivePhysRegs.test(Reg) || MRI->isReserved(Reg)) return false; } else { if (!MRI->use_nodbg_empty(Reg)) // This def has a non-debug use. Don't delete the instruction! return false; } } } // If there are no defs with uses, the instruction is dead. return true; } bool DeadMachineInstructionElim::runOnMachineFunction(MachineFunction &MF) { if (skipOptnoneFunction(*MF.getFunction())) return false; bool AnyChanges = false; MRI = &MF.getRegInfo(); TRI = MF.getTarget().getRegisterInfo(); TII = MF.getTarget().getInstrInfo(); // Loop over all instructions in all blocks, from bottom to top, so that it's // more likely that chains of dependent but ultimately dead instructions will // be cleaned up. for (MachineFunction::reverse_iterator I = MF.rbegin(), E = MF.rend(); I != E; ++I) { MachineBasicBlock *MBB = &*I; // Start out assuming that reserved registers are live out of this block. LivePhysRegs = MRI->getReservedRegs(); // Add live-ins from sucessors to LivePhysRegs. Normally, physregs are not // live across blocks, but some targets (x86) can have flags live out of a // block. for (MachineBasicBlock::succ_iterator S = MBB->succ_begin(), E = MBB->succ_end(); S != E; S++) for (MachineBasicBlock::livein_iterator LI = (*S)->livein_begin(); LI != (*S)->livein_end(); LI++) LivePhysRegs.set(*LI); // Now scan the instructions and delete dead ones, tracking physreg // liveness as we go. for (MachineBasicBlock::reverse_iterator MII = MBB->rbegin(), MIE = MBB->rend(); MII != MIE; ) { MachineInstr *MI = &*MII; // If the instruction is dead, delete it! if (isDead(MI)) { DEBUG(dbgs() << "DeadMachineInstructionElim: DELETING: " << *MI); // It is possible that some DBG_VALUE instructions refer to this // instruction. Examine each def operand for such references; // if found, mark the DBG_VALUE as undef (but don't delete it). for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { const MachineOperand &MO = MI->getOperand(i); if (!MO.isReg() || !MO.isDef()) continue; unsigned Reg = MO.getReg(); if (!TargetRegisterInfo::isVirtualRegister(Reg)) continue; MRI->markUsesInDebugValueAsUndef(Reg); } AnyChanges = true; MI->eraseFromParent(); ++NumDeletes; MIE = MBB->rend(); // MII is now pointing to the next instruction to process, // so don't increment it. continue; } // Record the physreg defs. for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { const MachineOperand &MO = MI->getOperand(i); if (MO.isReg() && MO.isDef()) { unsigned Reg = MO.getReg(); if (TargetRegisterInfo::isPhysicalRegister(Reg)) { // Check the subreg set, not the alias set, because a def // of a super-register may still be partially live after // this def. for (MCSubRegIterator SR(Reg, TRI,/*IncludeSelf=*/true); SR.isValid(); ++SR) LivePhysRegs.reset(*SR); } } else if (MO.isRegMask()) { // Register mask of preserved registers. All clobbers are dead. LivePhysRegs.clearBitsNotInMask(MO.getRegMask()); } } // Record the physreg uses, after the defs, in case a physreg is // both defined and used in the same instruction. for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) { const MachineOperand &MO = MI->getOperand(i); if (MO.isReg() && MO.isUse()) { unsigned Reg = MO.getReg(); if (TargetRegisterInfo::isPhysicalRegister(Reg)) { for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) LivePhysRegs.set(*AI); } } } // We didn't delete the current instruction, so increment MII to // the next one. ++MII; } } LivePhysRegs.clear(); return AnyChanges; }