1 //===-- AArch64CondBrTuning.cpp --- Conditional branch tuning for AArch64 -===//
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 /// \file
9 /// This file contains a pass that transforms CBZ/CBNZ/TBZ/TBNZ instructions
10 /// into a conditional branch (B.cond), when the NZCV flags can be set for
11 /// "free". This is preferred on targets that have more flexibility when
12 /// scheduling B.cond instructions as compared to CBZ/CBNZ/TBZ/TBNZ (assuming
13 /// all other variables are equal). This can also reduce register pressure.
14 ///
15 /// A few examples:
16 ///
17 /// 1) add w8, w0, w1 -> cmn w0, w1 ; CMN is an alias of ADDS.
18 /// cbz w8, .LBB_2 -> b.eq .LBB0_2
19 ///
20 /// 2) add w8, w0, w1 -> adds w8, w0, w1 ; w8 has multiple uses.
21 /// cbz w8, .LBB1_2 -> b.eq .LBB1_2
22 ///
23 /// 3) sub w8, w0, w1 -> subs w8, w0, w1 ; w8 has multiple uses.
24 /// tbz w8, #31, .LBB6_2 -> b.pl .LBB6_2
25 ///
26 //===----------------------------------------------------------------------===//
27
28 #include "AArch64.h"
29 #include "AArch64Subtarget.h"
30 #include "llvm/CodeGen/MachineFunction.h"
31 #include "llvm/CodeGen/MachineFunctionPass.h"
32 #include "llvm/CodeGen/MachineInstrBuilder.h"
33 #include "llvm/CodeGen/MachineRegisterInfo.h"
34 #include "llvm/CodeGen/Passes.h"
35 #include "llvm/CodeGen/TargetInstrInfo.h"
36 #include "llvm/CodeGen/TargetRegisterInfo.h"
37 #include "llvm/CodeGen/TargetSubtargetInfo.h"
38 #include "llvm/Support/Debug.h"
39 #include "llvm/Support/raw_ostream.h"
40
41 using namespace llvm;
42
43 #define DEBUG_TYPE "aarch64-cond-br-tuning"
44 #define AARCH64_CONDBR_TUNING_NAME "AArch64 Conditional Branch Tuning"
45
46 namespace {
47 class AArch64CondBrTuning : public MachineFunctionPass {
48 const AArch64InstrInfo *TII;
49 const TargetRegisterInfo *TRI;
50
51 MachineRegisterInfo *MRI;
52
53 public:
54 static char ID;
AArch64CondBrTuning()55 AArch64CondBrTuning() : MachineFunctionPass(ID) {
56 initializeAArch64CondBrTuningPass(*PassRegistry::getPassRegistry());
57 }
58 void getAnalysisUsage(AnalysisUsage &AU) const override;
59 bool runOnMachineFunction(MachineFunction &MF) override;
getPassName() const60 StringRef getPassName() const override { return AARCH64_CONDBR_TUNING_NAME; }
61
62 private:
63 MachineInstr *getOperandDef(const MachineOperand &MO);
64 MachineInstr *convertToFlagSetting(MachineInstr &MI, bool IsFlagSetting,
65 bool Is64Bit);
66 MachineInstr *convertToCondBr(MachineInstr &MI);
67 bool tryToTuneBranch(MachineInstr &MI, MachineInstr &DefMI);
68 };
69 } // end anonymous namespace
70
71 char AArch64CondBrTuning::ID = 0;
72
73 INITIALIZE_PASS(AArch64CondBrTuning, "aarch64-cond-br-tuning",
74 AARCH64_CONDBR_TUNING_NAME, false, false)
75
getAnalysisUsage(AnalysisUsage & AU) const76 void AArch64CondBrTuning::getAnalysisUsage(AnalysisUsage &AU) const {
77 AU.setPreservesCFG();
78 MachineFunctionPass::getAnalysisUsage(AU);
79 }
80
getOperandDef(const MachineOperand & MO)81 MachineInstr *AArch64CondBrTuning::getOperandDef(const MachineOperand &MO) {
82 if (!MO.getReg().isVirtual())
83 return nullptr;
84 return MRI->getUniqueVRegDef(MO.getReg());
85 }
86
convertToFlagSetting(MachineInstr & MI,bool IsFlagSetting,bool Is64Bit)87 MachineInstr *AArch64CondBrTuning::convertToFlagSetting(MachineInstr &MI,
88 bool IsFlagSetting,
89 bool Is64Bit) {
90 // If this is already the flag setting version of the instruction (e.g., SUBS)
91 // just make sure the implicit-def of NZCV isn't marked dead.
92 if (IsFlagSetting) {
93 for (MachineOperand &MO : MI.implicit_operands())
94 if (MO.isReg() && MO.isDead() && MO.getReg() == AArch64::NZCV)
95 MO.setIsDead(false);
96 return &MI;
97 }
98 unsigned NewOpc = TII->convertToFlagSettingOpc(MI.getOpcode());
99 Register NewDestReg = MI.getOperand(0).getReg();
100 if (MRI->hasOneNonDBGUse(MI.getOperand(0).getReg()))
101 NewDestReg = Is64Bit ? AArch64::XZR : AArch64::WZR;
102
103 MachineInstrBuilder MIB = BuildMI(*MI.getParent(), MI, MI.getDebugLoc(),
104 TII->get(NewOpc), NewDestReg);
105 for (const MachineOperand &MO : llvm::drop_begin(MI.operands()))
106 MIB.add(MO);
107
108 return MIB;
109 }
110
convertToCondBr(MachineInstr & MI)111 MachineInstr *AArch64CondBrTuning::convertToCondBr(MachineInstr &MI) {
112 AArch64CC::CondCode CC;
113 MachineBasicBlock *TargetMBB = TII->getBranchDestBlock(MI);
114 switch (MI.getOpcode()) {
115 default:
116 llvm_unreachable("Unexpected opcode!");
117
118 case AArch64::CBZW:
119 case AArch64::CBZX:
120 CC = AArch64CC::EQ;
121 break;
122 case AArch64::CBNZW:
123 case AArch64::CBNZX:
124 CC = AArch64CC::NE;
125 break;
126 case AArch64::TBZW:
127 case AArch64::TBZX:
128 CC = AArch64CC::PL;
129 break;
130 case AArch64::TBNZW:
131 case AArch64::TBNZX:
132 CC = AArch64CC::MI;
133 break;
134 }
135 return BuildMI(*MI.getParent(), MI, MI.getDebugLoc(), TII->get(AArch64::Bcc))
136 .addImm(CC)
137 .addMBB(TargetMBB);
138 }
139
tryToTuneBranch(MachineInstr & MI,MachineInstr & DefMI)140 bool AArch64CondBrTuning::tryToTuneBranch(MachineInstr &MI,
141 MachineInstr &DefMI) {
142 // We don't want NZCV bits live across blocks.
143 if (MI.getParent() != DefMI.getParent())
144 return false;
145
146 bool IsFlagSetting = true;
147 unsigned MIOpc = MI.getOpcode();
148 MachineInstr *NewCmp = nullptr, *NewBr = nullptr;
149 switch (DefMI.getOpcode()) {
150 default:
151 return false;
152 case AArch64::ADDWri:
153 case AArch64::ADDWrr:
154 case AArch64::ADDWrs:
155 case AArch64::ADDWrx:
156 case AArch64::ANDWri:
157 case AArch64::ANDWrr:
158 case AArch64::ANDWrs:
159 case AArch64::BICWrr:
160 case AArch64::BICWrs:
161 case AArch64::SUBWri:
162 case AArch64::SUBWrr:
163 case AArch64::SUBWrs:
164 case AArch64::SUBWrx:
165 IsFlagSetting = false;
166 [[fallthrough]];
167 case AArch64::ADDSWri:
168 case AArch64::ADDSWrr:
169 case AArch64::ADDSWrs:
170 case AArch64::ADDSWrx:
171 case AArch64::ANDSWri:
172 case AArch64::ANDSWrr:
173 case AArch64::ANDSWrs:
174 case AArch64::BICSWrr:
175 case AArch64::BICSWrs:
176 case AArch64::SUBSWri:
177 case AArch64::SUBSWrr:
178 case AArch64::SUBSWrs:
179 case AArch64::SUBSWrx:
180 switch (MIOpc) {
181 default:
182 llvm_unreachable("Unexpected opcode!");
183
184 case AArch64::CBZW:
185 case AArch64::CBNZW:
186 case AArch64::TBZW:
187 case AArch64::TBNZW:
188 // Check to see if the TBZ/TBNZ is checking the sign bit.
189 if ((MIOpc == AArch64::TBZW || MIOpc == AArch64::TBNZW) &&
190 MI.getOperand(1).getImm() != 31)
191 return false;
192
193 // There must not be any instruction between DefMI and MI that clobbers or
194 // reads NZCV.
195 if (isNZCVTouchedInInstructionRange(DefMI, MI, TRI))
196 return false;
197 LLVM_DEBUG(dbgs() << " Replacing instructions:\n ");
198 LLVM_DEBUG(DefMI.print(dbgs()));
199 LLVM_DEBUG(dbgs() << " ");
200 LLVM_DEBUG(MI.print(dbgs()));
201
202 NewCmp = convertToFlagSetting(DefMI, IsFlagSetting, /*Is64Bit=*/false);
203 NewBr = convertToCondBr(MI);
204 break;
205 }
206 break;
207
208 case AArch64::ADDXri:
209 case AArch64::ADDXrr:
210 case AArch64::ADDXrs:
211 case AArch64::ADDXrx:
212 case AArch64::ANDXri:
213 case AArch64::ANDXrr:
214 case AArch64::ANDXrs:
215 case AArch64::BICXrr:
216 case AArch64::BICXrs:
217 case AArch64::SUBXri:
218 case AArch64::SUBXrr:
219 case AArch64::SUBXrs:
220 case AArch64::SUBXrx:
221 IsFlagSetting = false;
222 [[fallthrough]];
223 case AArch64::ADDSXri:
224 case AArch64::ADDSXrr:
225 case AArch64::ADDSXrs:
226 case AArch64::ADDSXrx:
227 case AArch64::ANDSXri:
228 case AArch64::ANDSXrr:
229 case AArch64::ANDSXrs:
230 case AArch64::BICSXrr:
231 case AArch64::BICSXrs:
232 case AArch64::SUBSXri:
233 case AArch64::SUBSXrr:
234 case AArch64::SUBSXrs:
235 case AArch64::SUBSXrx:
236 switch (MIOpc) {
237 default:
238 llvm_unreachable("Unexpected opcode!");
239
240 case AArch64::CBZX:
241 case AArch64::CBNZX:
242 case AArch64::TBZX:
243 case AArch64::TBNZX: {
244 // Check to see if the TBZ/TBNZ is checking the sign bit.
245 if ((MIOpc == AArch64::TBZX || MIOpc == AArch64::TBNZX) &&
246 MI.getOperand(1).getImm() != 63)
247 return false;
248 // There must not be any instruction between DefMI and MI that clobbers or
249 // reads NZCV.
250 if (isNZCVTouchedInInstructionRange(DefMI, MI, TRI))
251 return false;
252 LLVM_DEBUG(dbgs() << " Replacing instructions:\n ");
253 LLVM_DEBUG(DefMI.print(dbgs()));
254 LLVM_DEBUG(dbgs() << " ");
255 LLVM_DEBUG(MI.print(dbgs()));
256
257 NewCmp = convertToFlagSetting(DefMI, IsFlagSetting, /*Is64Bit=*/true);
258 NewBr = convertToCondBr(MI);
259 break;
260 }
261 }
262 break;
263 }
264 (void)NewCmp; (void)NewBr;
265 assert(NewCmp && NewBr && "Expected new instructions.");
266
267 LLVM_DEBUG(dbgs() << " with instruction:\n ");
268 LLVM_DEBUG(NewCmp->print(dbgs()));
269 LLVM_DEBUG(dbgs() << " ");
270 LLVM_DEBUG(NewBr->print(dbgs()));
271
272 // If this was a flag setting version of the instruction, we use the original
273 // instruction by just clearing the dead marked on the implicit-def of NCZV.
274 // Therefore, we should not erase this instruction.
275 if (!IsFlagSetting)
276 DefMI.eraseFromParent();
277 MI.eraseFromParent();
278 return true;
279 }
280
runOnMachineFunction(MachineFunction & MF)281 bool AArch64CondBrTuning::runOnMachineFunction(MachineFunction &MF) {
282 if (skipFunction(MF.getFunction()))
283 return false;
284
285 LLVM_DEBUG(
286 dbgs() << "********** AArch64 Conditional Branch Tuning **********\n"
287 << "********** Function: " << MF.getName() << '\n');
288
289 TII = static_cast<const AArch64InstrInfo *>(MF.getSubtarget().getInstrInfo());
290 TRI = MF.getSubtarget().getRegisterInfo();
291 MRI = &MF.getRegInfo();
292
293 bool Changed = false;
294 for (MachineBasicBlock &MBB : MF) {
295 bool LocalChange = false;
296 for (MachineInstr &MI : MBB.terminators()) {
297 switch (MI.getOpcode()) {
298 default:
299 break;
300 case AArch64::CBZW:
301 case AArch64::CBZX:
302 case AArch64::CBNZW:
303 case AArch64::CBNZX:
304 case AArch64::TBZW:
305 case AArch64::TBZX:
306 case AArch64::TBNZW:
307 case AArch64::TBNZX:
308 MachineInstr *DefMI = getOperandDef(MI.getOperand(0));
309 LocalChange = (DefMI && tryToTuneBranch(MI, *DefMI));
310 break;
311 }
312 // If the optimization was successful, we can't optimize any other
313 // branches because doing so would clobber the NZCV flags.
314 if (LocalChange) {
315 Changed = true;
316 break;
317 }
318 }
319 }
320 return Changed;
321 }
322
createAArch64CondBrTuning()323 FunctionPass *llvm::createAArch64CondBrTuning() {
324 return new AArch64CondBrTuning();
325 }
326