1//=- AArch64RedundantCopyElimination.cpp - Remove useless copy 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// This pass removes unnecessary copies/moves in BBs based on a dominating
8// condition.
9//
10// We handle three cases:
11// 1. For BBs that are targets of CBZ/CBNZ instructions, we know the value of
12// the CBZ/CBNZ source register is zero on the taken/not-taken path. For
13// instance, the copy instruction in the code below can be removed because
14// the CBZW jumps to %bb.2 when w0 is zero.
15//
16// %bb.1:
17// cbz w0, .LBB0_2
18// .LBB0_2:
19// mov w0, wzr ; <-- redundant
20//
21// 2. If the flag setting instruction defines a register other than WZR/XZR, we
22// can remove a zero copy in some cases.
23//
24// %bb.0:
25// subs w0, w1, w2
26// str w0, [x1]
27// b.ne .LBB0_2
28// %bb.1:
29// mov w0, wzr ; <-- redundant
30// str w0, [x2]
31// .LBB0_2
32//
33// 3. Finally, if the flag setting instruction is a comparison against a
34// constant (i.e., ADDS[W|X]ri, SUBS[W|X]ri), we can remove a mov immediate
35// in some cases.
36//
37// %bb.0:
38// subs xzr, x0, #1
39// b.eq .LBB0_1
40// .LBB0_1:
41// orr x0, xzr, #0x1 ; <-- redundant
42//
43// This pass should be run after register allocation.
44//
45// FIXME: This could also be extended to check the whole dominance subtree below
46// the comparison if the compile time regression is acceptable.
47//
48// FIXME: Add support for handling CCMP instructions.
49// FIXME: If the known register value is zero, we should be able to rewrite uses
50// to use WZR/XZR directly in some cases.
51//===----------------------------------------------------------------------===//
52#include "AArch64.h"
53#include "llvm/ADT/SetVector.h"
54#include "llvm/ADT/Statistic.h"
55#include "llvm/ADT/iterator_range.h"
56#include "llvm/CodeGen/LiveRegUnits.h"
57#include "llvm/CodeGen/MachineFunctionPass.h"
58#include "llvm/CodeGen/MachineRegisterInfo.h"
59#include "llvm/Support/Debug.h"
60
61using namespace llvm;
62
63#define DEBUG_TYPE "aarch64-copyelim"
64
65STATISTIC(NumCopiesRemoved, "Number of copies removed.");
66
67namespace {
68class AArch64RedundantCopyElimination : public MachineFunctionPass {
69 const MachineRegisterInfo *MRI;
70 const TargetRegisterInfo *TRI;
71
72 // DomBBClobberedRegs is used when computing known values in the dominating
73 // BB.
74 LiveRegUnits DomBBClobberedRegs, DomBBUsedRegs;
75
76 // OptBBClobberedRegs is used when optimizing away redundant copies/moves.
77 LiveRegUnits OptBBClobberedRegs, OptBBUsedRegs;
78
79public:
80 static char ID;
81 AArch64RedundantCopyElimination() : MachineFunctionPass(ID) {
82 initializeAArch64RedundantCopyEliminationPass(
83 *PassRegistry::getPassRegistry());
84 }
85
86 struct RegImm {
87 MCPhysReg Reg;
88 int32_t Imm;
89 RegImm(MCPhysReg Reg, int32_t Imm) : Reg(Reg), Imm(Imm) {}
90 };
91
92 bool knownRegValInBlock(MachineInstr &CondBr, MachineBasicBlock *MBB,
93 SmallVectorImpl<RegImm> &KnownRegs,
94 MachineBasicBlock::iterator &FirstUse);
95 bool optimizeBlock(MachineBasicBlock *MBB);
96 bool runOnMachineFunction(MachineFunction &MF) override;
97 MachineFunctionProperties getRequiredProperties() const override {
98 return MachineFunctionProperties().set(
99 MachineFunctionProperties::Property::NoVRegs);
100 }
101 StringRef getPassName() const override {
102 return "AArch64 Redundant Copy Elimination";
103 }
104};
105char AArch64RedundantCopyElimination::ID = 0;
106}
107
108INITIALIZE_PASS(AArch64RedundantCopyElimination, "aarch64-copyelim",
109 "AArch64 redundant copy elimination pass", false, false)
110
111/// It's possible to determine the value of a register based on a dominating
112/// condition. To do so, this function checks to see if the basic block \p MBB
113/// is the target of a conditional branch \p CondBr with an equality comparison.
114/// If the branch is a CBZ/CBNZ, we know the value of its source operand is zero
115/// in \p MBB for some cases. Otherwise, we find and inspect the NZCV setting
116/// instruction (e.g., SUBS, ADDS). If this instruction defines a register
117/// other than WZR/XZR, we know the value of the destination register is zero in
118/// \p MMB for some cases. In addition, if the NZCV setting instruction is
119/// comparing against a constant we know the other source register is equal to
120/// the constant in \p MBB for some cases. If we find any constant values, push
121/// a physical register and constant value pair onto the KnownRegs vector and
122/// return true. Otherwise, return false if no known values were found.
123bool AArch64RedundantCopyElimination::knownRegValInBlock(
124 MachineInstr &CondBr, MachineBasicBlock *MBB,
125 SmallVectorImpl<RegImm> &KnownRegs, MachineBasicBlock::iterator &FirstUse) {
126 unsigned Opc = CondBr.getOpcode();
127
128 // Check if the current basic block is the target block to which the
129 // CBZ/CBNZ instruction jumps when its Wt/Xt is zero.
130 if (((Opc == AArch64::CBZW || Opc == AArch64::CBZX) &&
131 MBB == CondBr.getOperand(i: 1).getMBB()) ||
132 ((Opc == AArch64::CBNZW || Opc == AArch64::CBNZX) &&
133 MBB != CondBr.getOperand(i: 1).getMBB())) {
134 FirstUse = CondBr;
135 KnownRegs.push_back(Elt: RegImm(CondBr.getOperand(i: 0).getReg(), 0));
136 return true;
137 }
138
139 // Otherwise, must be a conditional branch.
140 if (Opc != AArch64::Bcc)
141 return false;
142
143 // Must be an equality check (i.e., == or !=).
144 AArch64CC::CondCode CC = (AArch64CC::CondCode)CondBr.getOperand(i: 0).getImm();
145 if (CC != AArch64CC::EQ && CC != AArch64CC::NE)
146 return false;
147
148 MachineBasicBlock *BrTarget = CondBr.getOperand(i: 1).getMBB();
149 if ((CC == AArch64CC::EQ && BrTarget != MBB) ||
150 (CC == AArch64CC::NE && BrTarget == MBB))
151 return false;
152
153 // Stop if we get to the beginning of PredMBB.
154 MachineBasicBlock *PredMBB = *MBB->pred_begin();
155 assert(PredMBB == CondBr.getParent() &&
156 "Conditional branch not in predecessor block!");
157 if (CondBr == PredMBB->begin())
158 return false;
159
160 // Registers clobbered in PredMBB between CondBr instruction and current
161 // instruction being checked in loop.
162 DomBBClobberedRegs.clear();
163 DomBBUsedRegs.clear();
164
165 // Find compare instruction that sets NZCV used by CondBr.
166 MachineBasicBlock::reverse_iterator RIt = CondBr.getReverseIterator();
167 for (MachineInstr &PredI : make_range(x: std::next(x: RIt), y: PredMBB->rend())) {
168
169 bool IsCMN = false;
170 switch (PredI.getOpcode()) {
171 default:
172 break;
173
174 // CMN is an alias for ADDS with a dead destination register.
175 case AArch64::ADDSWri:
176 case AArch64::ADDSXri:
177 IsCMN = true;
178 [[fallthrough]];
179 // CMP is an alias for SUBS with a dead destination register.
180 case AArch64::SUBSWri:
181 case AArch64::SUBSXri: {
182 // Sometimes the first operand is a FrameIndex. Bail if tht happens.
183 if (!PredI.getOperand(i: 1).isReg())
184 return false;
185 MCPhysReg DstReg = PredI.getOperand(i: 0).getReg();
186 MCPhysReg SrcReg = PredI.getOperand(i: 1).getReg();
187
188 bool Res = false;
189 // If we're comparing against a non-symbolic immediate and the source
190 // register of the compare is not modified (including a self-clobbering
191 // compare) between the compare and conditional branch we known the value
192 // of the 1st source operand.
193 if (PredI.getOperand(i: 2).isImm() && DomBBClobberedRegs.available(Reg: SrcReg) &&
194 SrcReg != DstReg) {
195 // We've found the instruction that sets NZCV.
196 int32_t KnownImm = PredI.getOperand(i: 2).getImm();
197 int32_t Shift = PredI.getOperand(i: 3).getImm();
198 KnownImm <<= Shift;
199 if (IsCMN)
200 KnownImm = -KnownImm;
201 FirstUse = PredI;
202 KnownRegs.push_back(Elt: RegImm(SrcReg, KnownImm));
203 Res = true;
204 }
205
206 // If this instructions defines something other than WZR/XZR, we know it's
207 // result is zero in some cases.
208 if (DstReg == AArch64::WZR || DstReg == AArch64::XZR)
209 return Res;
210
211 // The destination register must not be modified between the NZCV setting
212 // instruction and the conditional branch.
213 if (!DomBBClobberedRegs.available(Reg: DstReg))
214 return Res;
215
216 FirstUse = PredI;
217 KnownRegs.push_back(Elt: RegImm(DstReg, 0));
218 return true;
219 }
220
221 // Look for NZCV setting instructions that define something other than
222 // WZR/XZR.
223 case AArch64::ADCSWr:
224 case AArch64::ADCSXr:
225 case AArch64::ADDSWrr:
226 case AArch64::ADDSWrs:
227 case AArch64::ADDSWrx:
228 case AArch64::ADDSXrr:
229 case AArch64::ADDSXrs:
230 case AArch64::ADDSXrx:
231 case AArch64::ADDSXrx64:
232 case AArch64::ANDSWri:
233 case AArch64::ANDSWrr:
234 case AArch64::ANDSWrs:
235 case AArch64::ANDSXri:
236 case AArch64::ANDSXrr:
237 case AArch64::ANDSXrs:
238 case AArch64::BICSWrr:
239 case AArch64::BICSWrs:
240 case AArch64::BICSXrs:
241 case AArch64::BICSXrr:
242 case AArch64::SBCSWr:
243 case AArch64::SBCSXr:
244 case AArch64::SUBSWrr:
245 case AArch64::SUBSWrs:
246 case AArch64::SUBSWrx:
247 case AArch64::SUBSXrr:
248 case AArch64::SUBSXrs:
249 case AArch64::SUBSXrx:
250 case AArch64::SUBSXrx64: {
251 MCPhysReg DstReg = PredI.getOperand(i: 0).getReg();
252 if (DstReg == AArch64::WZR || DstReg == AArch64::XZR)
253 return false;
254
255 // The destination register of the NZCV setting instruction must not be
256 // modified before the conditional branch.
257 if (!DomBBClobberedRegs.available(Reg: DstReg))
258 return false;
259
260 // We've found the instruction that sets NZCV whose DstReg == 0.
261 FirstUse = PredI;
262 KnownRegs.push_back(Elt: RegImm(DstReg, 0));
263 return true;
264 }
265 }
266
267 // Bail if we see an instruction that defines NZCV that we don't handle.
268 if (PredI.definesRegister(AArch64::Reg: NZCV, /*TRI=*/nullptr))
269 return false;
270
271 // Track clobbered and used registers.
272 LiveRegUnits::accumulateUsedDefed(MI: PredI, ModifiedRegUnits&: DomBBClobberedRegs, UsedRegUnits&: DomBBUsedRegs,
273 TRI);
274 }
275 return false;
276}
277
278bool AArch64RedundantCopyElimination::optimizeBlock(MachineBasicBlock *MBB) {
279 // Check if the current basic block has a single predecessor.
280 if (MBB->pred_size() != 1)
281 return false;
282
283 // Check if the predecessor has two successors, implying the block ends in a
284 // conditional branch.
285 MachineBasicBlock *PredMBB = *MBB->pred_begin();
286 if (PredMBB->succ_size() != 2)
287 return false;
288
289 MachineBasicBlock::iterator CondBr = PredMBB->getLastNonDebugInstr();
290 if (CondBr == PredMBB->end())
291 return false;
292
293 // Keep track of the earliest point in the PredMBB block where kill markers
294 // need to be removed if a COPY is removed.
295 MachineBasicBlock::iterator FirstUse;
296 // After calling knownRegValInBlock, FirstUse will either point to a CBZ/CBNZ
297 // or a compare (i.e., SUBS). In the latter case, we must take care when
298 // updating FirstUse when scanning for COPY instructions. In particular, if
299 // there's a COPY in between the compare and branch the COPY should not
300 // update FirstUse.
301 bool SeenFirstUse = false;
302 // Registers that contain a known value at the start of MBB.
303 SmallVector<RegImm, 4> KnownRegs;
304
305 MachineBasicBlock::iterator Itr = std::next(x: CondBr);
306 do {
307 --Itr;
308
309 if (!knownRegValInBlock(CondBr&: *Itr, MBB, KnownRegs, FirstUse))
310 continue;
311
312 // Reset the clobbered and used register units.
313 OptBBClobberedRegs.clear();
314 OptBBUsedRegs.clear();
315
316 // Look backward in PredMBB for COPYs from the known reg to find other
317 // registers that are known to be a constant value.
318 for (auto PredI = Itr;; --PredI) {
319 if (FirstUse == PredI)
320 SeenFirstUse = true;
321
322 if (PredI->isCopy()) {
323 MCPhysReg CopyDstReg = PredI->getOperand(i: 0).getReg();
324 MCPhysReg CopySrcReg = PredI->getOperand(i: 1).getReg();
325 for (auto &KnownReg : KnownRegs) {
326 if (!OptBBClobberedRegs.available(Reg: KnownReg.Reg))
327 continue;
328 // If we have X = COPY Y, and Y is known to be zero, then now X is
329 // known to be zero.
330 if (CopySrcReg == KnownReg.Reg &&
331 OptBBClobberedRegs.available(Reg: CopyDstReg)) {
332 KnownRegs.push_back(Elt: RegImm(CopyDstReg, KnownReg.Imm));
333 if (SeenFirstUse)
334 FirstUse = PredI;
335 break;
336 }
337 // If we have X = COPY Y, and X is known to be zero, then now Y is
338 // known to be zero.
339 if (CopyDstReg == KnownReg.Reg &&
340 OptBBClobberedRegs.available(Reg: CopySrcReg)) {
341 KnownRegs.push_back(Elt: RegImm(CopySrcReg, KnownReg.Imm));
342 if (SeenFirstUse)
343 FirstUse = PredI;
344 break;
345 }
346 }
347 }
348
349 // Stop if we get to the beginning of PredMBB.
350 if (PredI == PredMBB->begin())
351 break;
352
353 LiveRegUnits::accumulateUsedDefed(MI: *PredI, ModifiedRegUnits&: OptBBClobberedRegs,
354 UsedRegUnits&: OptBBUsedRegs, TRI);
355 // Stop if all of the known-zero regs have been clobbered.
356 if (all_of(Range&: KnownRegs, P: [&](RegImm KnownReg) {
357 return !OptBBClobberedRegs.available(Reg: KnownReg.Reg);
358 }))
359 break;
360 }
361 break;
362
363 } while (Itr != PredMBB->begin() && Itr->isTerminator());
364
365 // We've not found a registers with a known value, time to bail out.
366 if (KnownRegs.empty())
367 return false;
368
369 bool Changed = false;
370 // UsedKnownRegs is the set of KnownRegs that have had uses added to MBB.
371 SmallSetVector<unsigned, 4> UsedKnownRegs;
372 MachineBasicBlock::iterator LastChange = MBB->begin();
373 // Remove redundant copy/move instructions unless KnownReg is modified.
374 for (MachineBasicBlock::iterator I = MBB->begin(), E = MBB->end(); I != E;) {
375 MachineInstr *MI = &*I;
376 ++I;
377 bool RemovedMI = false;
378 bool IsCopy = MI->isCopy();
379 bool IsMoveImm = MI->isMoveImmediate();
380 if (IsCopy || IsMoveImm) {
381 Register DefReg = MI->getOperand(i: 0).getReg();
382 Register SrcReg = IsCopy ? MI->getOperand(i: 1).getReg() : Register();
383 int64_t SrcImm = IsMoveImm ? MI->getOperand(i: 1).getImm() : 0;
384 if (!MRI->isReserved(PhysReg: DefReg) &&
385 ((IsCopy && (SrcReg == AArch64::XZR || SrcReg == AArch64::WZR)) ||
386 IsMoveImm)) {
387 for (RegImm &KnownReg : KnownRegs) {
388 if (KnownReg.Reg != DefReg &&
389 !TRI->isSuperRegister(RegA: DefReg, RegB: KnownReg.Reg))
390 continue;
391
392 // For a copy, the known value must be a zero.
393 if (IsCopy && KnownReg.Imm != 0)
394 continue;
395
396 if (IsMoveImm) {
397 // For a move immediate, the known immediate must match the source
398 // immediate.
399 if (KnownReg.Imm != SrcImm)
400 continue;
401
402 // Don't remove a move immediate that implicitly defines the upper
403 // bits when only the lower 32 bits are known.
404 MCPhysReg CmpReg = KnownReg.Reg;
405 if (any_of(Range: MI->implicit_operands(), P: [CmpReg](MachineOperand &O) {
406 return !O.isDead() && O.isReg() && O.isDef() &&
407 O.getReg() != CmpReg;
408 }))
409 continue;
410
411 // Don't remove a move immediate that implicitly defines the upper
412 // bits as different.
413 if (TRI->isSuperRegister(RegA: DefReg, RegB: KnownReg.Reg) && KnownReg.Imm < 0)
414 continue;
415 }
416
417 if (IsCopy)
418 LLVM_DEBUG(dbgs() << "Remove redundant Copy : " << *MI);
419 else
420 LLVM_DEBUG(dbgs() << "Remove redundant Move : " << *MI);
421
422 MI->eraseFromParent();
423 Changed = true;
424 LastChange = I;
425 NumCopiesRemoved++;
426 UsedKnownRegs.insert(X: KnownReg.Reg);
427 RemovedMI = true;
428 break;
429 }
430 }
431 }
432
433 // Skip to the next instruction if we removed the COPY/MovImm.
434 if (RemovedMI)
435 continue;
436
437 // Remove any regs the MI clobbers from the KnownConstRegs set.
438 for (unsigned RI = 0; RI < KnownRegs.size();)
439 if (MI->modifiesRegister(Reg: KnownRegs[RI].Reg, TRI)) {
440 std::swap(a&: KnownRegs[RI], b&: KnownRegs[KnownRegs.size() - 1]);
441 KnownRegs.pop_back();
442 // Don't increment RI since we need to now check the swapped-in
443 // KnownRegs[RI].
444 } else {
445 ++RI;
446 }
447
448 // Continue until the KnownRegs set is empty.
449 if (KnownRegs.empty())
450 break;
451 }
452
453 if (!Changed)
454 return false;
455
456 // Add newly used regs to the block's live-in list if they aren't there
457 // already.
458 for (MCPhysReg KnownReg : UsedKnownRegs)
459 if (!MBB->isLiveIn(Reg: KnownReg))
460 MBB->addLiveIn(PhysReg: KnownReg);
461
462 // Clear kills in the range where changes were made. This is conservative,
463 // but should be okay since kill markers are being phased out.
464 LLVM_DEBUG(dbgs() << "Clearing kill flags.\n\tFirstUse: " << *FirstUse
465 << "\tLastChange: ";
466 if (LastChange == MBB->end()) dbgs() << "<end>\n";
467 else dbgs() << *LastChange);
468 for (MachineInstr &MMI : make_range(x: FirstUse, y: PredMBB->end()))
469 MMI.clearKillInfo();
470 for (MachineInstr &MMI : make_range(x: MBB->begin(), y: LastChange))
471 MMI.clearKillInfo();
472
473 return true;
474}
475
476bool AArch64RedundantCopyElimination::runOnMachineFunction(
477 MachineFunction &MF) {
478 if (skipFunction(F: MF.getFunction()))
479 return false;
480 TRI = MF.getSubtarget().getRegisterInfo();
481 MRI = &MF.getRegInfo();
482
483 // Resize the clobbered and used register unit trackers. We do this once per
484 // function.
485 DomBBClobberedRegs.init(TRI: *TRI);
486 DomBBUsedRegs.init(TRI: *TRI);
487 OptBBClobberedRegs.init(TRI: *TRI);
488 OptBBUsedRegs.init(TRI: *TRI);
489
490 bool Changed = false;
491 for (MachineBasicBlock &MBB : MF)
492 Changed |= optimizeBlock(MBB: &MBB);
493 return Changed;
494}
495
496FunctionPass *llvm::createAArch64RedundantCopyEliminationPass() {
497 return new AArch64RedundantCopyElimination();
498}
499

source code of llvm/lib/Target/AArch64/AArch64RedundantCopyElimination.cpp