1 //==-- AArch64DeadRegisterDefinitions.cpp - Replace dead defs w/ zero reg --==//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
9 /// \file When allowed by the instruction, replace a dead definition of a GPR
10 /// with the zero register. This makes the code a bit friendlier towards the
11 /// hardware's register renamer.
12 //===----------------------------------------------------------------------===//
15 #include "AArch64RegisterInfo.h"
16 #include "AArch64Subtarget.h"
17 #include "llvm/ADT/Statistic.h"
18 #include "llvm/CodeGen/ISDOpcodes.h"
19 #include "llvm/CodeGen/MachineFunction.h"
20 #include "llvm/CodeGen/MachineFunctionPass.h"
21 #include "llvm/CodeGen/MachineInstr.h"
22 #include "llvm/CodeGen/MachineRegisterInfo.h"
23 #include "llvm/Support/Debug.h"
24 #include "llvm/Support/raw_ostream.h"
25 #include "llvm/Target/TargetInstrInfo.h"
26 #include "llvm/Target/TargetSubtargetInfo.h"
29 #define DEBUG_TYPE "aarch64-dead-defs"
31 STATISTIC(NumDeadDefsReplaced, "Number of dead definitions replaced");
33 #define AARCH64_DEAD_REG_DEF_NAME "AArch64 Dead register definitions"
36 class AArch64DeadRegisterDefinitions : public MachineFunctionPass {
38 const TargetRegisterInfo *TRI;
39 const MachineRegisterInfo *MRI;
40 const TargetInstrInfo *TII;
42 void processMachineBasicBlock(MachineBasicBlock &MBB);
44 static char ID; // Pass identification, replacement for typeid.
45 AArch64DeadRegisterDefinitions() : MachineFunctionPass(ID) {
46 initializeAArch64DeadRegisterDefinitionsPass(
47 *PassRegistry::getPassRegistry());
50 bool runOnMachineFunction(MachineFunction &F) override;
52 StringRef getPassName() const override { return AARCH64_DEAD_REG_DEF_NAME; }
54 void getAnalysisUsage(AnalysisUsage &AU) const override {
56 MachineFunctionPass::getAnalysisUsage(AU);
59 char AArch64DeadRegisterDefinitions::ID = 0;
60 } // end anonymous namespace
62 INITIALIZE_PASS(AArch64DeadRegisterDefinitions, "aarch64-dead-defs",
63 AARCH64_DEAD_REG_DEF_NAME, false, false)
65 static bool usesFrameIndex(const MachineInstr &MI) {
66 for (const MachineOperand &MO : MI.uses())
72 void AArch64DeadRegisterDefinitions::processMachineBasicBlock(
73 MachineBasicBlock &MBB) {
74 const MachineFunction &MF = *MBB.getParent();
75 for (MachineInstr &MI : MBB) {
76 if (usesFrameIndex(MI)) {
77 // We need to skip this instruction because while it appears to have a
78 // dead def it uses a frame index which might expand into a multi
79 // instruction sequence during EPI.
80 DEBUG(dbgs() << " Ignoring, operand is frame index\n");
83 if (MI.definesRegister(AArch64::XZR) || MI.definesRegister(AArch64::WZR)) {
84 // It is not allowed to write to the same register (not even the zero
85 // register) twice in a single instruction.
86 DEBUG(dbgs() << " Ignoring, XZR or WZR already used by the instruction\n");
89 if (MF.getSubtarget<AArch64Subtarget>().hasLSE()) {
90 // XZ/WZ for LSE can only be used when acquire semantics are not used,
91 // LDOPAL WZ is an invalid opcode.
92 switch (MI.getOpcode()) {
100 case AArch64::SWPALd:
101 case AArch64::LDADDALb:
102 case AArch64::LDADDALh:
103 case AArch64::LDADDALs:
104 case AArch64::LDADDALd:
105 case AArch64::LDCLRALb:
106 case AArch64::LDCLRALh:
107 case AArch64::LDCLRALs:
108 case AArch64::LDCLRALd:
109 case AArch64::LDEORALb:
110 case AArch64::LDEORALh:
111 case AArch64::LDEORALs:
112 case AArch64::LDEORALd:
113 case AArch64::LDSETALb:
114 case AArch64::LDSETALh:
115 case AArch64::LDSETALs:
116 case AArch64::LDSETALd:
117 case AArch64::LDSMINALb:
118 case AArch64::LDSMINALh:
119 case AArch64::LDSMINALs:
120 case AArch64::LDSMINALd:
121 case AArch64::LDSMAXALb:
122 case AArch64::LDSMAXALh:
123 case AArch64::LDSMAXALs:
124 case AArch64::LDSMAXALd:
125 case AArch64::LDUMINALb:
126 case AArch64::LDUMINALh:
127 case AArch64::LDUMINALs:
128 case AArch64::LDUMINALd:
129 case AArch64::LDUMAXALb:
130 case AArch64::LDUMAXALh:
131 case AArch64::LDUMAXALs:
132 case AArch64::LDUMAXALd:
138 const MCInstrDesc &Desc = MI.getDesc();
139 for (int I = 0, E = Desc.getNumDefs(); I != E; ++I) {
140 MachineOperand &MO = MI.getOperand(I);
141 if (!MO.isReg() || !MO.isDef())
143 // We should not have any relevant physreg defs that are replacable by
144 // zero before register allocation. So we just check for dead vreg defs.
145 unsigned Reg = MO.getReg();
146 if (!TargetRegisterInfo::isVirtualRegister(Reg) ||
147 (!MO.isDead() && !MRI->use_nodbg_empty(Reg)))
149 assert(!MO.isImplicit() && "Unexpected implicit def!");
150 DEBUG(dbgs() << " Dead def operand #" << I << " in:\n ";
152 // Be careful not to change the register if it's a tied operand.
153 if (MI.isRegTiedToUseOperand(I)) {
154 DEBUG(dbgs() << " Ignoring, def is tied operand.\n");
157 const TargetRegisterClass *RC = TII->getRegClass(Desc, I, TRI, MF);
160 DEBUG(dbgs() << " Ignoring, register is not a GPR.\n");
162 } else if (RC->contains(AArch64::WZR))
163 NewReg = AArch64::WZR;
164 else if (RC->contains(AArch64::XZR))
165 NewReg = AArch64::XZR;
167 DEBUG(dbgs() << " Ignoring, register is not a GPR.\n");
170 DEBUG(dbgs() << " Replacing with zero register. New:\n ");
173 DEBUG(MI.print(dbgs()));
174 ++NumDeadDefsReplaced;
176 // Only replace one dead register, see check for zero register above.
182 // Scan the function for instructions that have a dead definition of a
183 // register. Replace that register with the zero register when possible.
184 bool AArch64DeadRegisterDefinitions::runOnMachineFunction(MachineFunction &MF) {
185 if (skipFunction(*MF.getFunction()))
188 TRI = MF.getSubtarget().getRegisterInfo();
189 TII = MF.getSubtarget().getInstrInfo();
190 MRI = &MF.getRegInfo();
191 DEBUG(dbgs() << "***** AArch64DeadRegisterDefinitions *****\n");
194 processMachineBasicBlock(MBB);
198 FunctionPass *llvm::createAArch64DeadRegisterDefinitions() {
199 return new AArch64DeadRegisterDefinitions();