1 //===- DeadMachineInstructionElim.cpp - Remove dead machine instructions --===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This is an extremely simple MachineInstr-level dead-code-elimination pass.
10 //
11 //===----------------------------------------------------------------------===//
12 
13 #include "llvm/ADT/PostOrderIterator.h"
14 #include "llvm/ADT/Statistic.h"
15 #include "llvm/CodeGen/LiveRegUnits.h"
16 #include "llvm/CodeGen/MachineFunctionPass.h"
17 #include "llvm/CodeGen/MachineRegisterInfo.h"
18 #include "llvm/CodeGen/TargetSubtargetInfo.h"
19 #include "llvm/InitializePasses.h"
20 #include "llvm/Pass.h"
21 #include "llvm/Support/Debug.h"
22 #include "llvm/Support/raw_ostream.h"
23 
24 using namespace llvm;
25 
26 #define DEBUG_TYPE "dead-mi-elimination"
27 
28 STATISTIC(NumDeletes,          "Number of dead instructions deleted");
29 
30 namespace {
31   class DeadMachineInstructionElim : public MachineFunctionPass {
32     bool runOnMachineFunction(MachineFunction &MF) override;
33 
34     const MachineRegisterInfo *MRI = nullptr;
35     const TargetInstrInfo *TII = nullptr;
36     LiveRegUnits LivePhysRegs;
37 
38   public:
39     static char ID; // Pass identification, replacement for typeid
40     DeadMachineInstructionElim() : MachineFunctionPass(ID) {
41      initializeDeadMachineInstructionElimPass(*PassRegistry::getPassRegistry());
42     }
43 
44     void getAnalysisUsage(AnalysisUsage &AU) const override {
45       AU.setPreservesCFG();
46       MachineFunctionPass::getAnalysisUsage(AU);
47     }
48 
49   private:
50     bool isDead(const MachineInstr *MI) const;
51 
52     bool eliminateDeadMI(MachineFunction &MF);
53   };
54 }
55 char DeadMachineInstructionElim::ID = 0;
56 char &llvm::DeadMachineInstructionElimID = DeadMachineInstructionElim::ID;
57 
58 INITIALIZE_PASS(DeadMachineInstructionElim, DEBUG_TYPE,
59                 "Remove dead machine instructions", false, false)
60 
61 bool DeadMachineInstructionElim::isDead(const MachineInstr *MI) const {
62   // Technically speaking inline asm without side effects and no defs can still
63   // be deleted. But there is so much bad inline asm code out there, we should
64   // let them be.
65   if (MI->isInlineAsm())
66     return false;
67 
68   // Don't delete frame allocation labels.
69   if (MI->getOpcode() == TargetOpcode::LOCAL_ESCAPE)
70     return false;
71 
72   // Don't delete instructions with side effects.
73   bool SawStore = false;
74   if (!MI->isSafeToMove(nullptr, SawStore) && !MI->isPHI())
75     return false;
76 
77   // Examine each operand.
78   for (const MachineOperand &MO : MI->all_defs()) {
79     Register Reg = MO.getReg();
80     if (Reg.isPhysical()) {
81       // Don't delete live physreg defs, or any reserved register defs.
82       if (!LivePhysRegs.available(Reg) || MRI->isReserved(Reg))
83         return false;
84     } else {
85       if (MO.isDead()) {
86 #ifndef NDEBUG
87         // Basic check on the register. All of them should be 'undef'.
88         for (auto &U : MRI->use_nodbg_operands(Reg))
89           assert(U.isUndef() && "'Undef' use on a 'dead' register is found!");
90 #endif
91         continue;
92       }
93       for (const MachineInstr &Use : MRI->use_nodbg_instructions(Reg)) {
94         if (&Use != MI)
95           // This def has a non-debug use. Don't delete the instruction!
96           return false;
97       }
98     }
99   }
100 
101   // If there are no defs with uses, the instruction is dead.
102   return true;
103 }
104 
105 bool DeadMachineInstructionElim::runOnMachineFunction(MachineFunction &MF) {
106   if (skipFunction(MF.getFunction()))
107     return false;
108 
109   MRI = &MF.getRegInfo();
110 
111   const TargetSubtargetInfo &ST = MF.getSubtarget();
112   TII = ST.getInstrInfo();
113   LivePhysRegs.init(*ST.getRegisterInfo());
114 
115   bool AnyChanges = eliminateDeadMI(MF);
116   while (AnyChanges && eliminateDeadMI(MF))
117     ;
118   return AnyChanges;
119 }
120 
121 bool DeadMachineInstructionElim::eliminateDeadMI(MachineFunction &MF) {
122   bool AnyChanges = false;
123 
124   // Loop over all instructions in all blocks, from bottom to top, so that it's
125   // more likely that chains of dependent but ultimately dead instructions will
126   // be cleaned up.
127   for (MachineBasicBlock *MBB : post_order(&MF)) {
128     LivePhysRegs.addLiveOuts(*MBB);
129 
130     // Now scan the instructions and delete dead ones, tracking physreg
131     // liveness as we go.
132     for (MachineInstr &MI : make_early_inc_range(reverse(*MBB))) {
133       // If the instruction is dead, delete it!
134       if (isDead(&MI)) {
135         LLVM_DEBUG(dbgs() << "DeadMachineInstructionElim: DELETING: " << MI);
136         // It is possible that some DBG_VALUE instructions refer to this
137         // instruction. They will be deleted in the live debug variable
138         // analysis.
139         MI.eraseFromParent();
140         AnyChanges = true;
141         ++NumDeletes;
142         continue;
143       }
144 
145       LivePhysRegs.stepBackward(MI);
146     }
147   }
148 
149   LivePhysRegs.clear();
150   return AnyChanges;
151 }
152