1//===- MachineSSAUpdater.cpp - Unstructured SSA Update Tool ---------------===//
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 file implements the MachineSSAUpdater class. It's based on SSAUpdater
10// class in lib/Transforms/Utils.
11//
12//===----------------------------------------------------------------------===//
13
14#include "llvm/CodeGen/MachineSSAUpdater.h"
15#include "llvm/ADT/DenseMap.h"
16#include "llvm/ADT/SmallVector.h"
17#include "llvm/CodeGen/MachineBasicBlock.h"
18#include "llvm/CodeGen/MachineFunction.h"
19#include "llvm/CodeGen/MachineInstr.h"
20#include "llvm/CodeGen/MachineInstrBuilder.h"
21#include "llvm/CodeGen/MachineOperand.h"
22#include "llvm/CodeGen/MachineRegisterInfo.h"
23#include "llvm/CodeGen/TargetInstrInfo.h"
24#include "llvm/CodeGen/TargetOpcodes.h"
25#include "llvm/CodeGen/TargetSubtargetInfo.h"
26#include "llvm/IR/DebugLoc.h"
27#include "llvm/Support/Debug.h"
28#include "llvm/Support/ErrorHandling.h"
29#include "llvm/Support/raw_ostream.h"
30#include "llvm/Transforms/Utils/SSAUpdaterImpl.h"
31#include <utility>
32
33using namespace llvm;
34
35#define DEBUG_TYPE "machine-ssaupdater"
36
37using AvailableValsTy = DenseMap<MachineBasicBlock *, Register>;
38
39static AvailableValsTy &getAvailableVals(void *AV) {
40 return *static_cast<AvailableValsTy*>(AV);
41}
42
43MachineSSAUpdater::MachineSSAUpdater(MachineFunction &MF,
44 SmallVectorImpl<MachineInstr*> *NewPHI)
45 : InsertedPHIs(NewPHI), TII(MF.getSubtarget().getInstrInfo()),
46 MRI(&MF.getRegInfo()) {}
47
48MachineSSAUpdater::~MachineSSAUpdater() {
49 delete static_cast<AvailableValsTy*>(AV);
50}
51
52/// Initialize - Reset this object to get ready for a new set of SSA
53/// updates.
54void MachineSSAUpdater::Initialize(Register V) {
55 if (!AV)
56 AV = new AvailableValsTy();
57 else
58 getAvailableVals(AV).clear();
59
60 RegAttrs = MRI->getVRegAttrs(Reg: V);
61}
62
63/// HasValueForBlock - Return true if the MachineSSAUpdater already has a value for
64/// the specified block.
65bool MachineSSAUpdater::HasValueForBlock(MachineBasicBlock *BB) const {
66 return getAvailableVals(AV).count(Val: BB);
67}
68
69/// AddAvailableValue - Indicate that a rewritten value is available in the
70/// specified block with the specified value.
71void MachineSSAUpdater::AddAvailableValue(MachineBasicBlock *BB, Register V) {
72 getAvailableVals(AV)[BB] = V;
73}
74
75/// GetValueAtEndOfBlock - Construct SSA form, materializing a value that is
76/// live at the end of the specified block.
77Register MachineSSAUpdater::GetValueAtEndOfBlock(MachineBasicBlock *BB) {
78 return GetValueAtEndOfBlockInternal(BB);
79}
80
81static
82Register LookForIdenticalPHI(MachineBasicBlock *BB,
83 SmallVectorImpl<std::pair<MachineBasicBlock *, Register>> &PredValues) {
84 if (BB->empty())
85 return Register();
86
87 MachineBasicBlock::iterator I = BB->begin();
88 if (!I->isPHI())
89 return Register();
90
91 AvailableValsTy AVals;
92 for (const auto &[SrcBB, SrcReg] : PredValues)
93 AVals[SrcBB] = SrcReg;
94 while (I != BB->end() && I->isPHI()) {
95 bool Same = true;
96 for (unsigned i = 1, e = I->getNumOperands(); i != e; i += 2) {
97 Register SrcReg = I->getOperand(i).getReg();
98 MachineBasicBlock *SrcBB = I->getOperand(i: i+1).getMBB();
99 if (AVals[SrcBB] != SrcReg) {
100 Same = false;
101 break;
102 }
103 }
104 if (Same)
105 return I->getOperand(i: 0).getReg();
106 ++I;
107 }
108 return Register();
109}
110
111/// InsertNewDef - Insert an empty PHI or IMPLICIT_DEF instruction which define
112/// a value of the given register class at the start of the specified basic
113/// block. It returns the virtual register defined by the instruction.
114static MachineInstrBuilder InsertNewDef(unsigned Opcode, MachineBasicBlock *BB,
115 MachineBasicBlock::iterator I,
116 MachineRegisterInfo::VRegAttrs RegAttrs,
117 MachineRegisterInfo *MRI,
118 const TargetInstrInfo *TII) {
119 Register NewVR = MRI->createVirtualRegister(RegAttr: RegAttrs);
120 return BuildMI(BB&: *BB, I, MIMD: DebugLoc(), MCID: TII->get(Opcode), DestReg: NewVR);
121}
122
123/// GetValueInMiddleOfBlock - Construct SSA form, materializing a value that
124/// is live in the middle of the specified block. If ExistingValueOnly is
125/// true then this will only return an existing value or $noreg; otherwise new
126/// instructions may be inserted to materialize a value.
127///
128/// GetValueInMiddleOfBlock is the same as GetValueAtEndOfBlock except in one
129/// important case: if there is a definition of the rewritten value after the
130/// 'use' in BB. Consider code like this:
131///
132/// X1 = ...
133/// SomeBB:
134/// use(X)
135/// X2 = ...
136/// br Cond, SomeBB, OutBB
137///
138/// In this case, there are two values (X1 and X2) added to the AvailableVals
139/// set by the client of the rewriter, and those values are both live out of
140/// their respective blocks. However, the use of X happens in the *middle* of
141/// a block. Because of this, we need to insert a new PHI node in SomeBB to
142/// merge the appropriate values, and this value isn't live out of the block.
143Register MachineSSAUpdater::GetValueInMiddleOfBlock(MachineBasicBlock *BB,
144 bool ExistingValueOnly) {
145 // If there is no definition of the renamed variable in this block, just use
146 // GetValueAtEndOfBlock to do our work.
147 if (!HasValueForBlock(BB))
148 return GetValueAtEndOfBlockInternal(BB, ExistingValueOnly);
149
150 // If there are no predecessors, just return undef.
151 if (BB->pred_empty()) {
152 // If we cannot insert new instructions, just return $noreg.
153 if (ExistingValueOnly)
154 return Register();
155 // Insert an implicit_def to represent an undef value.
156 MachineInstr *NewDef =
157 InsertNewDef(Opcode: TargetOpcode::IMPLICIT_DEF, BB, I: BB->getFirstTerminator(),
158 RegAttrs, MRI, TII);
159 return NewDef->getOperand(i: 0).getReg();
160 }
161
162 // Otherwise, we have the hard case. Get the live-in values for each
163 // predecessor.
164 SmallVector<std::pair<MachineBasicBlock*, Register>, 8> PredValues;
165 Register SingularValue;
166
167 bool isFirstPred = true;
168 for (MachineBasicBlock *PredBB : BB->predecessors()) {
169 Register PredVal = GetValueAtEndOfBlockInternal(BB: PredBB, ExistingValueOnly);
170 PredValues.push_back(Elt: std::make_pair(x&: PredBB, y&: PredVal));
171
172 // Compute SingularValue.
173 if (isFirstPred) {
174 SingularValue = PredVal;
175 isFirstPred = false;
176 } else if (PredVal != SingularValue)
177 SingularValue = Register();
178 }
179
180 // Otherwise, if all the merged values are the same, just use it.
181 if (SingularValue)
182 return SingularValue;
183
184 // If an identical PHI is already in BB, just reuse it.
185 Register DupPHI = LookForIdenticalPHI(BB, PredValues);
186 if (DupPHI)
187 return DupPHI;
188
189 // If we cannot create new instructions, return $noreg now.
190 if (ExistingValueOnly)
191 return Register();
192
193 // Otherwise, we do need a PHI: insert one now.
194 MachineBasicBlock::iterator Loc = BB->empty() ? BB->end() : BB->begin();
195 MachineInstrBuilder InsertedPHI =
196 InsertNewDef(Opcode: TargetOpcode::PHI, BB, I: Loc, RegAttrs, MRI, TII);
197
198 // Fill in all the predecessors of the PHI.
199 for (const auto &[SrcBB, SrcReg] : PredValues)
200 InsertedPHI.addReg(RegNo: SrcReg).addMBB(MBB: SrcBB);
201
202 // See if the PHI node can be merged to a single value. This can happen in
203 // loop cases when we get a PHI of itself and one other value.
204 if (Register ConstVal = InsertedPHI->isConstantValuePHI()) {
205 InsertedPHI->eraseFromParent();
206 return ConstVal;
207 }
208
209 // If the client wants to know about all new instructions, tell it.
210 if (InsertedPHIs) InsertedPHIs->push_back(Elt: InsertedPHI);
211
212 LLVM_DEBUG(dbgs() << " Inserted PHI: " << *InsertedPHI);
213 return InsertedPHI.getReg(Idx: 0);
214}
215
216static
217MachineBasicBlock *findCorrespondingPred(const MachineInstr *MI,
218 MachineOperand *U) {
219 for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) {
220 if (&MI->getOperand(i) == U)
221 return MI->getOperand(i: i+1).getMBB();
222 }
223
224 llvm_unreachable("MachineOperand::getParent() failure?");
225}
226
227/// RewriteUse - Rewrite a use of the symbolic value. This handles PHI nodes,
228/// which use their value in the corresponding predecessor.
229void MachineSSAUpdater::RewriteUse(MachineOperand &U) {
230 MachineInstr *UseMI = U.getParent();
231 Register NewVR;
232 if (UseMI->isPHI()) {
233 MachineBasicBlock *SourceBB = findCorrespondingPred(MI: UseMI, U: &U);
234 NewVR = GetValueAtEndOfBlockInternal(BB: SourceBB);
235 } else {
236 NewVR = GetValueInMiddleOfBlock(BB: UseMI->getParent());
237 }
238
239 // Insert a COPY if needed to satisfy register class constraints for the using
240 // MO. Or, if possible, just constrain the class for NewVR to avoid the need
241 // for a COPY.
242 if (NewVR) {
243 const TargetRegisterClass *UseRC =
244 dyn_cast_or_null<const TargetRegisterClass *>(Val&: RegAttrs.RCOrRB);
245 if (UseRC && !MRI->constrainRegClass(Reg: NewVR, RC: UseRC)) {
246 MachineBasicBlock *UseBB = UseMI->getParent();
247 MachineInstr *InsertedCopy =
248 InsertNewDef(Opcode: TargetOpcode::COPY, BB: UseBB, I: UseBB->getFirstNonPHI(),
249 RegAttrs, MRI, TII)
250 .addReg(RegNo: NewVR);
251 NewVR = InsertedCopy->getOperand(i: 0).getReg();
252 LLVM_DEBUG(dbgs() << " Inserted COPY: " << *InsertedCopy);
253 }
254 }
255 U.setReg(NewVR);
256}
257
258namespace llvm {
259
260/// SSAUpdaterTraits<MachineSSAUpdater> - Traits for the SSAUpdaterImpl
261/// template, specialized for MachineSSAUpdater.
262template<>
263class SSAUpdaterTraits<MachineSSAUpdater> {
264public:
265 using BlkT = MachineBasicBlock;
266 using ValT = Register;
267 using PhiT = MachineInstr;
268 using BlkSucc_iterator = MachineBasicBlock::succ_iterator;
269
270 static BlkSucc_iterator BlkSucc_begin(BlkT *BB) { return BB->succ_begin(); }
271 static BlkSucc_iterator BlkSucc_end(BlkT *BB) { return BB->succ_end(); }
272
273 /// Iterator for PHI operands.
274 class PHI_iterator {
275 private:
276 MachineInstr *PHI;
277 unsigned idx;
278
279 public:
280 explicit PHI_iterator(MachineInstr *P) // begin iterator
281 : PHI(P), idx(1) {}
282 PHI_iterator(MachineInstr *P, bool) // end iterator
283 : PHI(P), idx(PHI->getNumOperands()) {}
284
285 PHI_iterator &operator++() { idx += 2; return *this; }
286 bool operator==(const PHI_iterator& x) const { return idx == x.idx; }
287 bool operator!=(const PHI_iterator& x) const { return !operator==(x); }
288
289 Register getIncomingValue() { return PHI->getOperand(i: idx).getReg(); }
290
291 MachineBasicBlock *getIncomingBlock() {
292 return PHI->getOperand(i: idx+1).getMBB();
293 }
294 };
295
296 static inline PHI_iterator PHI_begin(PhiT *PHI) { return PHI_iterator(PHI); }
297
298 static inline PHI_iterator PHI_end(PhiT *PHI) {
299 return PHI_iterator(PHI, true);
300 }
301
302 /// FindPredecessorBlocks - Put the predecessors of BB into the Preds
303 /// vector.
304 static void FindPredecessorBlocks(MachineBasicBlock *BB,
305 SmallVectorImpl<MachineBasicBlock*> *Preds){
306 append_range(C&: *Preds, R: BB->predecessors());
307 }
308
309 /// GetPoisonVal - Create an IMPLICIT_DEF instruction with a new register.
310 /// Add it into the specified block and return the register.
311 static Register GetPoisonVal(MachineBasicBlock *BB,
312 MachineSSAUpdater *Updater) {
313 // Insert an implicit_def to represent a poison value.
314 MachineInstr *NewDef =
315 InsertNewDef(Opcode: TargetOpcode::IMPLICIT_DEF, BB, I: BB->getFirstNonPHI(),
316 RegAttrs: Updater->RegAttrs, MRI: Updater->MRI, TII: Updater->TII);
317 return NewDef->getOperand(i: 0).getReg();
318 }
319
320 /// CreateEmptyPHI - Create a PHI instruction that defines a new register.
321 /// Add it into the specified block and return the register.
322 static Register CreateEmptyPHI(MachineBasicBlock *BB, unsigned NumPreds,
323 MachineSSAUpdater *Updater) {
324 MachineBasicBlock::iterator Loc = BB->empty() ? BB->end() : BB->begin();
325 MachineInstr *PHI =
326 InsertNewDef(Opcode: TargetOpcode::PHI, BB, I: Loc, RegAttrs: Updater->RegAttrs,
327 MRI: Updater->MRI, TII: Updater->TII);
328 return PHI->getOperand(i: 0).getReg();
329 }
330
331 /// AddPHIOperand - Add the specified value as an operand of the PHI for
332 /// the specified predecessor block.
333 static void AddPHIOperand(MachineInstr *PHI, Register Val,
334 MachineBasicBlock *Pred) {
335 MachineInstrBuilder(*Pred->getParent(), PHI).addReg(RegNo: Val).addMBB(MBB: Pred);
336 }
337
338 /// InstrIsPHI - Check if an instruction is a PHI.
339 static MachineInstr *InstrIsPHI(MachineInstr *I) {
340 if (I && I->isPHI())
341 return I;
342 return nullptr;
343 }
344
345 /// ValueIsPHI - Check if the instruction that defines the specified register
346 /// is a PHI instruction.
347 static MachineInstr *ValueIsPHI(Register Val, MachineSSAUpdater *Updater) {
348 return InstrIsPHI(I: Updater->MRI->getVRegDef(Reg: Val));
349 }
350
351 /// ValueIsNewPHI - Like ValueIsPHI but also check if the PHI has no source
352 /// operands, i.e., it was just added.
353 static MachineInstr *ValueIsNewPHI(Register Val, MachineSSAUpdater *Updater) {
354 MachineInstr *PHI = ValueIsPHI(Val, Updater);
355 if (PHI && PHI->getNumOperands() <= 1)
356 return PHI;
357 return nullptr;
358 }
359
360 /// GetPHIValue - For the specified PHI instruction, return the register
361 /// that it defines.
362 static Register GetPHIValue(MachineInstr *PHI) {
363 return PHI->getOperand(i: 0).getReg();
364 }
365};
366
367} // end namespace llvm
368
369/// GetValueAtEndOfBlockInternal - Check to see if AvailableVals has an entry
370/// for the specified BB and if so, return it. If not, construct SSA form by
371/// first calculating the required placement of PHIs and then inserting new
372/// PHIs where needed.
373Register
374MachineSSAUpdater::GetValueAtEndOfBlockInternal(MachineBasicBlock *BB,
375 bool ExistingValueOnly) {
376 AvailableValsTy &AvailableVals = getAvailableVals(AV);
377 Register ExistingVal = AvailableVals.lookup(Val: BB);
378 if (ExistingVal || ExistingValueOnly)
379 return ExistingVal;
380
381 SSAUpdaterImpl<MachineSSAUpdater> Impl(this, &AvailableVals, InsertedPHIs);
382 return Impl.GetValue(BB);
383}
384