1//===- FlatternCFG.cpp - Code to perform CFG flattening -------------------===//
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// Reduce conditional branches in CFG.
10//
11//===----------------------------------------------------------------------===//
12
13#include "llvm/ADT/SmallPtrSet.h"
14#include "llvm/Analysis/AliasAnalysis.h"
15#include "llvm/Transforms/Utils/Local.h"
16#include "llvm/Analysis/ValueTracking.h"
17#include "llvm/IR/BasicBlock.h"
18#include "llvm/IR/IRBuilder.h"
19#include "llvm/IR/InstrTypes.h"
20#include "llvm/IR/Instruction.h"
21#include "llvm/IR/Instructions.h"
22#include "llvm/IR/Value.h"
23#include "llvm/Support/Casting.h"
24#include "llvm/Support/Debug.h"
25#include "llvm/Support/raw_ostream.h"
26#include "llvm/Transforms/Utils/BasicBlockUtils.h"
27#include <cassert>
28
29using namespace llvm;
30
31#define DEBUG_TYPE "flatten-cfg"
32
33namespace {
34
35class FlattenCFGOpt {
36 AliasAnalysis *AA;
37
38 /// Use parallel-and or parallel-or to generate conditions for
39 /// conditional branches.
40 bool FlattenParallelAndOr(BasicBlock *BB, IRBuilder<> &Builder);
41
42 /// If \param BB is the merge block of an if-region, attempt to merge
43 /// the if-region with an adjacent if-region upstream if two if-regions
44 /// contain identical instructions.
45 bool MergeIfRegion(BasicBlock *BB, IRBuilder<> &Builder);
46
47 /// Compare a pair of blocks: \p Block1 and \p Block2, which
48 /// are from two if-regions, where \p Head2 is the entry block of the 2nd
49 /// if-region. \returns true if \p Block1 and \p Block2 contain identical
50 /// instructions, and have no memory reference alias with \p Head2.
51 /// This is used as a legality check for merging if-regions.
52 bool CompareIfRegionBlock(BasicBlock *Block1, BasicBlock *Block2,
53 BasicBlock *Head2);
54
55public:
56 FlattenCFGOpt(AliasAnalysis *AA) : AA(AA) {}
57
58 bool run(BasicBlock *BB);
59};
60
61} // end anonymous namespace
62
63/// If \param [in] BB has more than one predecessor that is a conditional
64/// branch, attempt to use parallel and/or for the branch condition. \returns
65/// true on success.
66///
67/// Before:
68/// ......
69/// %cmp10 = fcmp une float %tmp1, %tmp2
70/// br i1 %cmp10, label %if.then, label %lor.rhs
71///
72/// lor.rhs:
73/// ......
74/// %cmp11 = fcmp une float %tmp3, %tmp4
75/// br i1 %cmp11, label %if.then, label %ifend
76///
77/// if.end: // the merge block
78/// ......
79///
80/// if.then: // has two predecessors, both of them contains conditional branch.
81/// ......
82/// br label %if.end;
83///
84/// After:
85/// ......
86/// %cmp10 = fcmp une float %tmp1, %tmp2
87/// ......
88/// %cmp11 = fcmp une float %tmp3, %tmp4
89/// %cmp12 = or i1 %cmp10, %cmp11 // parallel-or mode.
90/// br i1 %cmp12, label %if.then, label %ifend
91///
92/// if.end:
93/// ......
94///
95/// if.then:
96/// ......
97/// br label %if.end;
98///
99/// Current implementation handles two cases.
100/// Case 1: BB is on the else-path.
101///
102/// BB1
103/// / |
104/// BB2 |
105/// / \ |
106/// BB3 \ | where, BB1, BB2 contain conditional branches.
107/// \ | / BB3 contains unconditional branch.
108/// \ | / BB4 corresponds to BB which is also the merge.
109/// BB => BB4
110///
111///
112/// Corresponding source code:
113///
114/// if (a == b && c == d)
115/// statement; // BB3
116///
117/// Case 2: BB is on the then-path.
118///
119/// BB1
120/// / |
121/// | BB2
122/// \ / | where BB1, BB2 contain conditional branches.
123/// BB => BB3 | BB3 contains unconditiona branch and corresponds
124/// \ / to BB. BB4 is the merge.
125/// BB4
126///
127/// Corresponding source code:
128///
129/// if (a == b || c == d)
130/// statement; // BB3
131///
132/// In both cases, BB is the common successor of conditional branches.
133/// In Case 1, BB (BB4) has an unconditional branch (BB3) as
134/// its predecessor. In Case 2, BB (BB3) only has conditional branches
135/// as its predecessors.
136bool FlattenCFGOpt::FlattenParallelAndOr(BasicBlock *BB, IRBuilder<> &Builder) {
137 PHINode *PHI = dyn_cast<PHINode>(Val: BB->begin());
138 if (PHI)
139 return false; // For simplicity, avoid cases containing PHI nodes.
140
141 BasicBlock *LastCondBlock = nullptr;
142 BasicBlock *FirstCondBlock = nullptr;
143 BasicBlock *UnCondBlock = nullptr;
144 int Idx = -1;
145
146 // Check predecessors of \param BB.
147 SmallPtrSet<BasicBlock *, 16> Preds(llvm::from_range, predecessors(BB));
148 for (BasicBlock *Pred : Preds) {
149 BranchInst *PBI = dyn_cast<BranchInst>(Val: Pred->getTerminator());
150
151 // All predecessors should terminate with a branch.
152 if (!PBI)
153 return false;
154
155 BasicBlock *PP = Pred->getSinglePredecessor();
156
157 if (PBI->isUnconditional()) {
158 // Case 1: Pred (BB3) is an unconditional block, it should
159 // have a single predecessor (BB2) that is also a predecessor
160 // of \param BB (BB4) and should not have address-taken.
161 // There should exist only one such unconditional
162 // branch among the predecessors.
163 if (UnCondBlock || !PP || !Preds.contains(Ptr: PP) ||
164 Pred->hasAddressTaken())
165 return false;
166
167 UnCondBlock = Pred;
168 continue;
169 }
170
171 // Only conditional branches are allowed beyond this point.
172 assert(PBI->isConditional());
173
174 // Condition's unique use should be the branch instruction.
175 Value *PC = PBI->getCondition();
176 if (!PC || !PC->hasOneUse())
177 return false;
178
179 if (PP && Preds.count(Ptr: PP)) {
180 // These are internal condition blocks to be merged from, e.g.,
181 // BB2 in both cases.
182 // Should not be address-taken.
183 if (Pred->hasAddressTaken())
184 return false;
185
186 // Instructions in the internal condition blocks should be safe
187 // to hoist up.
188 for (BasicBlock::iterator BI = Pred->begin(), BE = PBI->getIterator();
189 BI != BE;) {
190 Instruction *CI = &*BI++;
191 if (isa<PHINode>(Val: CI) || !isSafeToSpeculativelyExecute(I: CI))
192 return false;
193 }
194 } else {
195 // This is the condition block to be merged into, e.g. BB1 in
196 // both cases.
197 if (FirstCondBlock)
198 return false;
199 FirstCondBlock = Pred;
200 }
201
202 // Find whether BB is uniformly on the true (or false) path
203 // for all of its predecessors.
204 BasicBlock *PS1 = PBI->getSuccessor(i: 0);
205 BasicBlock *PS2 = PBI->getSuccessor(i: 1);
206 BasicBlock *PS = (PS1 == BB) ? PS2 : PS1;
207 int CIdx = (PS1 == BB) ? 0 : 1;
208
209 if (Idx == -1)
210 Idx = CIdx;
211 else if (CIdx != Idx)
212 return false;
213
214 // PS is the successor which is not BB. Check successors to identify
215 // the last conditional branch.
216 if (!Preds.contains(Ptr: PS)) {
217 // Case 2.
218 LastCondBlock = Pred;
219 } else {
220 // Case 1
221 BranchInst *BPS = dyn_cast<BranchInst>(Val: PS->getTerminator());
222 if (BPS && BPS->isUnconditional()) {
223 // Case 1: PS(BB3) should be an unconditional branch.
224 LastCondBlock = Pred;
225 }
226 }
227 }
228
229 if (!FirstCondBlock || !LastCondBlock || (FirstCondBlock == LastCondBlock))
230 return false;
231
232 Instruction *TBB = LastCondBlock->getTerminator();
233 BasicBlock *PS1 = TBB->getSuccessor(Idx: 0);
234 BasicBlock *PS2 = TBB->getSuccessor(Idx: 1);
235 BranchInst *PBI1 = dyn_cast<BranchInst>(Val: PS1->getTerminator());
236 BranchInst *PBI2 = dyn_cast<BranchInst>(Val: PS2->getTerminator());
237
238 // If PS1 does not jump into PS2, but PS2 jumps into PS1,
239 // attempt branch inversion.
240 if (!PBI1 || !PBI1->isUnconditional() ||
241 (PS1->getTerminator()->getSuccessor(Idx: 0) != PS2)) {
242 // Check whether PS2 jumps into PS1.
243 if (!PBI2 || !PBI2->isUnconditional() ||
244 (PS2->getTerminator()->getSuccessor(Idx: 0) != PS1))
245 return false;
246
247 // Do branch inversion.
248 BasicBlock *CurrBlock = LastCondBlock;
249 bool EverChanged = false;
250 for (; CurrBlock != FirstCondBlock;
251 CurrBlock = CurrBlock->getSinglePredecessor()) {
252 auto *BI = cast<BranchInst>(Val: CurrBlock->getTerminator());
253 auto *CI = dyn_cast<CmpInst>(Val: BI->getCondition());
254 if (!CI)
255 continue;
256
257 CmpInst::Predicate Predicate = CI->getPredicate();
258 // Canonicalize icmp_ne -> icmp_eq, fcmp_one -> fcmp_oeq
259 if ((Predicate == CmpInst::ICMP_NE) || (Predicate == CmpInst::FCMP_ONE)) {
260 CI->setPredicate(ICmpInst::getInversePredicate(pred: Predicate));
261 BI->swapSuccessors();
262 EverChanged = true;
263 }
264 }
265 return EverChanged;
266 }
267
268 // PS1 must have a conditional branch.
269 if (!PBI1 || !PBI1->isUnconditional())
270 return false;
271
272 // PS2 should not contain PHI node.
273 PHI = dyn_cast<PHINode>(Val: PS2->begin());
274 if (PHI)
275 return false;
276
277 // Do the transformation.
278 BasicBlock *CB;
279 BranchInst *PBI = cast<BranchInst>(Val: FirstCondBlock->getTerminator());
280 bool Iteration = true;
281 IRBuilder<>::InsertPointGuard Guard(Builder);
282 Value *PC = PBI->getCondition();
283
284 do {
285 CB = PBI->getSuccessor(i: 1 - Idx);
286 // Delete the conditional branch.
287 FirstCondBlock->back().eraseFromParent();
288 FirstCondBlock->splice(ToIt: FirstCondBlock->end(), FromBB: CB);
289 PBI = cast<BranchInst>(Val: FirstCondBlock->getTerminator());
290 Value *CC = PBI->getCondition();
291 // Merge conditions.
292 Builder.SetInsertPoint(PBI);
293 Value *NC;
294 if (Idx == 0)
295 // Case 2, use parallel or.
296 NC = Builder.CreateOr(LHS: PC, RHS: CC);
297 else
298 // Case 1, use parallel and.
299 NC = Builder.CreateAnd(LHS: PC, RHS: CC);
300
301 PBI->replaceUsesOfWith(From: CC, To: NC);
302 PC = NC;
303 if (CB == LastCondBlock)
304 Iteration = false;
305 // Remove internal conditional branches.
306 CB->dropAllReferences();
307 // make CB unreachable and let downstream to delete the block.
308 new UnreachableInst(CB->getContext(), CB);
309 } while (Iteration);
310
311 LLVM_DEBUG(dbgs() << "Use parallel and/or in:\n" << *FirstCondBlock);
312 return true;
313}
314
315/// Compare blocks from two if-regions, where \param Head2 is the entry of the
316/// 2nd if-region. \param Block1 is a block in the 1st if-region to compare.
317/// \param Block2 is a block in the 2nd if-region to compare. \returns true if
318/// Block1 and Block2 have identical instructions and do not have
319/// memory reference alias with Head2.
320bool FlattenCFGOpt::CompareIfRegionBlock(BasicBlock *Block1, BasicBlock *Block2,
321 BasicBlock *Head2) {
322 Instruction *PTI2 = Head2->getTerminator();
323 Instruction *PBI2 = &Head2->front();
324
325 // Check whether instructions in Block1 and Block2 are identical
326 // and do not alias with instructions in Head2.
327 BasicBlock::iterator iter1 = Block1->begin();
328 BasicBlock::iterator end1 = Block1->getTerminator()->getIterator();
329 BasicBlock::iterator iter2 = Block2->begin();
330 BasicBlock::iterator end2 = Block2->getTerminator()->getIterator();
331
332 while (true) {
333 if (iter1 == end1) {
334 if (iter2 != end2)
335 return false;
336 break;
337 }
338
339 if (!iter1->isIdenticalTo(I: &*iter2))
340 return false;
341
342 // Illegal to remove instructions with side effects except
343 // non-volatile stores.
344 if (iter1->mayHaveSideEffects()) {
345 Instruction *CurI = &*iter1;
346 StoreInst *SI = dyn_cast<StoreInst>(Val: CurI);
347 if (!SI || SI->isVolatile())
348 return false;
349 }
350
351 // For simplicity and speed, data dependency check can be
352 // avoided if read from memory doesn't exist.
353 if (iter1->mayReadFromMemory())
354 return false;
355
356 if (iter1->mayWriteToMemory()) {
357 for (BasicBlock::iterator BI(PBI2), BE(PTI2); BI != BE; ++BI) {
358 if (BI->mayReadFromMemory() || BI->mayWriteToMemory()) {
359 // Check whether iter1 and BI may access the same memory location.
360 if (!AA || AA->getModRefInfo(I1: &*iter1, I2: &*BI) != ModRefInfo::NoModRef)
361 return false;
362 }
363 }
364 }
365 ++iter1;
366 ++iter2;
367 }
368
369 return true;
370}
371
372/// Check whether \param BB is the merge block of a if-region. If yes, check
373/// whether there exists an adjacent if-region upstream, the two if-regions
374/// contain identical instructions and can be legally merged. \returns true if
375/// the two if-regions are merged.
376///
377/// From:
378/// if (a)
379/// statement;
380/// if (b)
381/// statement;
382///
383/// To:
384/// if (a || b)
385/// statement;
386///
387///
388/// And from:
389/// if (a)
390/// ;
391/// else
392/// statement;
393/// if (b)
394/// ;
395/// else
396/// statement;
397///
398/// To:
399/// if (a && b)
400/// ;
401/// else
402/// statement;
403///
404/// We always take the form of the first if-region. This means that if the
405/// statement in the first if-region, is in the "then-path", while in the second
406/// if-region it is in the "else-path", then we convert the second to the first
407/// form, by inverting the condition and the branch successors. The same
408/// approach goes for the opposite case.
409bool FlattenCFGOpt::MergeIfRegion(BasicBlock *BB, IRBuilder<> &Builder) {
410 // We cannot merge the if-region if the merge point has phi nodes.
411 if (isa<PHINode>(Val: BB->front()))
412 return false;
413
414 BasicBlock *IfTrue2, *IfFalse2;
415 BranchInst *DomBI2 = GetIfCondition(BB, IfTrue&: IfTrue2, IfFalse&: IfFalse2);
416 if (!DomBI2)
417 return false;
418 Instruction *CInst2 = dyn_cast<Instruction>(Val: DomBI2->getCondition());
419 if (!CInst2)
420 return false;
421
422 BasicBlock *SecondEntryBlock = CInst2->getParent();
423 if (SecondEntryBlock->hasAddressTaken())
424 return false;
425
426 BasicBlock *IfTrue1, *IfFalse1;
427 BranchInst *DomBI1 = GetIfCondition(BB: SecondEntryBlock, IfTrue&: IfTrue1, IfFalse&: IfFalse1);
428 if (!DomBI1)
429 return false;
430 Instruction *CInst1 = dyn_cast<Instruction>(Val: DomBI1->getCondition());
431 if (!CInst1)
432 return false;
433
434 BasicBlock *FirstEntryBlock = CInst1->getParent();
435 // Don't die trying to process degenerate/unreachable code.
436 if (FirstEntryBlock == SecondEntryBlock)
437 return false;
438
439 // Either then-path or else-path should be empty.
440 bool InvertCond2 = false;
441 BinaryOperator::BinaryOps CombineOp;
442 if (IfFalse1 == FirstEntryBlock) {
443 // The else-path is empty, so we must use "or" operation to combine the
444 // conditions.
445 CombineOp = BinaryOperator::Or;
446 if (IfFalse2 != SecondEntryBlock) {
447 if (IfTrue2 != SecondEntryBlock)
448 return false;
449
450 InvertCond2 = true;
451 std::swap(a&: IfTrue2, b&: IfFalse2);
452 }
453
454 if (!CompareIfRegionBlock(Block1: IfTrue1, Block2: IfTrue2, Head2: SecondEntryBlock))
455 return false;
456 } else if (IfTrue1 == FirstEntryBlock) {
457 // The then-path is empty, so we must use "and" operation to combine the
458 // conditions.
459 CombineOp = BinaryOperator::And;
460 if (IfTrue2 != SecondEntryBlock) {
461 if (IfFalse2 != SecondEntryBlock)
462 return false;
463
464 InvertCond2 = true;
465 std::swap(a&: IfTrue2, b&: IfFalse2);
466 }
467
468 if (!CompareIfRegionBlock(Block1: IfFalse1, Block2: IfFalse2, Head2: SecondEntryBlock))
469 return false;
470 } else
471 return false;
472
473 Instruction *PTI2 = SecondEntryBlock->getTerminator();
474 Instruction *PBI2 = &SecondEntryBlock->front();
475
476 // Check whether \param SecondEntryBlock has side-effect and is safe to
477 // speculate.
478 for (BasicBlock::iterator BI(PBI2), BE(PTI2); BI != BE; ++BI) {
479 Instruction *CI = &*BI;
480 if (isa<PHINode>(Val: CI) || CI->mayHaveSideEffects() ||
481 !isSafeToSpeculativelyExecute(I: CI))
482 return false;
483 }
484
485 // Merge \param SecondEntryBlock into \param FirstEntryBlock.
486 FirstEntryBlock->back().eraseFromParent();
487 FirstEntryBlock->splice(ToIt: FirstEntryBlock->end(), FromBB: SecondEntryBlock);
488 BranchInst *PBI = cast<BranchInst>(Val: FirstEntryBlock->getTerminator());
489 assert(PBI->getCondition() == CInst2);
490 BasicBlock *SaveInsertBB = Builder.GetInsertBlock();
491 BasicBlock::iterator SaveInsertPt = Builder.GetInsertPoint();
492 Builder.SetInsertPoint(PBI);
493 if (InvertCond2) {
494 InvertBranch(PBI, Builder);
495 }
496 Value *NC = Builder.CreateBinOp(Opc: CombineOp, LHS: CInst1, RHS: PBI->getCondition());
497 PBI->replaceUsesOfWith(From: PBI->getCondition(), To: NC);
498 Builder.SetInsertPoint(TheBB: SaveInsertBB, IP: SaveInsertPt);
499
500 // Remove IfTrue1
501 if (IfTrue1 != FirstEntryBlock) {
502 IfTrue1->dropAllReferences();
503 IfTrue1->eraseFromParent();
504 }
505
506 // Remove IfFalse1
507 if (IfFalse1 != FirstEntryBlock) {
508 IfFalse1->dropAllReferences();
509 IfFalse1->eraseFromParent();
510 }
511
512 // Remove \param SecondEntryBlock
513 SecondEntryBlock->dropAllReferences();
514 SecondEntryBlock->eraseFromParent();
515 LLVM_DEBUG(dbgs() << "If conditions merged into:\n" << *FirstEntryBlock);
516 return true;
517}
518
519bool FlattenCFGOpt::run(BasicBlock *BB) {
520 assert(BB && BB->getParent() && "Block not embedded in function!");
521 assert(BB->getTerminator() && "Degenerate basic block encountered!");
522
523 IRBuilder<> Builder(BB);
524
525 if (FlattenParallelAndOr(BB, Builder) || MergeIfRegion(BB, Builder))
526 return true;
527 return false;
528}
529
530/// FlattenCFG - This function is used to flatten a CFG. For
531/// example, it uses parallel-and and parallel-or mode to collapse
532/// if-conditions and merge if-regions with identical statements.
533bool llvm::FlattenCFG(BasicBlock *BB, AAResults *AA) {
534 return FlattenCFGOpt(AA).run(BB);
535}
536