1 //===------- LoopBoundSplit.cpp - Split Loop Bound --------------*- C++ -*-===//
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 #include "llvm/Transforms/Scalar/LoopBoundSplit.h"
10 #include "llvm/ADT/Sequence.h"
11 #include "llvm/Analysis/LoopAnalysisManager.h"
12 #include "llvm/Analysis/LoopInfo.h"
13 #include "llvm/Analysis/ScalarEvolution.h"
14 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
15 #include "llvm/IR/PatternMatch.h"
16 #include "llvm/Transforms/Scalar/LoopPassManager.h"
17 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
18 #include "llvm/Transforms/Utils/Cloning.h"
19 #include "llvm/Transforms/Utils/LoopSimplify.h"
20 #include "llvm/Transforms/Utils/ScalarEvolutionExpander.h"
21
22 #define DEBUG_TYPE "loop-bound-split"
23
24 namespace llvm {
25
26 using namespace PatternMatch;
27
28 namespace {
29 struct ConditionInfo {
30 /// Branch instruction with this condition
31 BranchInst *BI = nullptr;
32 /// ICmp instruction with this condition
33 ICmpInst *ICmp = nullptr;
34 /// Preciate info
35 ICmpInst::Predicate Pred = ICmpInst::BAD_ICMP_PREDICATE;
36 /// AddRec llvm value
37 Value *AddRecValue = nullptr;
38 /// Non PHI AddRec llvm value
39 Value *NonPHIAddRecValue;
40 /// Bound llvm value
41 Value *BoundValue = nullptr;
42 /// AddRec SCEV
43 const SCEVAddRecExpr *AddRecSCEV = nullptr;
44 /// Bound SCEV
45 const SCEV *BoundSCEV = nullptr;
46
47 ConditionInfo() = default;
48 };
49 } // namespace
50
analyzeICmp(ScalarEvolution & SE,ICmpInst * ICmp,ConditionInfo & Cond,const Loop & L)51 static void analyzeICmp(ScalarEvolution &SE, ICmpInst *ICmp,
52 ConditionInfo &Cond, const Loop &L) {
53 Cond.ICmp = ICmp;
54 if (match(ICmp, m_ICmp(Cond.Pred, m_Value(Cond.AddRecValue),
55 m_Value(Cond.BoundValue)))) {
56 const SCEV *AddRecSCEV = SE.getSCEV(Cond.AddRecValue);
57 const SCEV *BoundSCEV = SE.getSCEV(Cond.BoundValue);
58 const SCEVAddRecExpr *LHSAddRecSCEV = dyn_cast<SCEVAddRecExpr>(AddRecSCEV);
59 const SCEVAddRecExpr *RHSAddRecSCEV = dyn_cast<SCEVAddRecExpr>(BoundSCEV);
60 // Locate AddRec in LHSSCEV and Bound in RHSSCEV.
61 if (!LHSAddRecSCEV && RHSAddRecSCEV) {
62 std::swap(Cond.AddRecValue, Cond.BoundValue);
63 std::swap(AddRecSCEV, BoundSCEV);
64 Cond.Pred = ICmpInst::getSwappedPredicate(Cond.Pred);
65 }
66
67 Cond.AddRecSCEV = dyn_cast<SCEVAddRecExpr>(AddRecSCEV);
68 Cond.BoundSCEV = BoundSCEV;
69 Cond.NonPHIAddRecValue = Cond.AddRecValue;
70
71 // If the Cond.AddRecValue is PHI node, update Cond.NonPHIAddRecValue with
72 // value from backedge.
73 if (Cond.AddRecSCEV && isa<PHINode>(Cond.AddRecValue)) {
74 PHINode *PN = cast<PHINode>(Cond.AddRecValue);
75 Cond.NonPHIAddRecValue = PN->getIncomingValueForBlock(L.getLoopLatch());
76 }
77 }
78 }
79
calculateUpperBound(const Loop & L,ScalarEvolution & SE,ConditionInfo & Cond,bool IsExitCond)80 static bool calculateUpperBound(const Loop &L, ScalarEvolution &SE,
81 ConditionInfo &Cond, bool IsExitCond) {
82 if (IsExitCond) {
83 const SCEV *ExitCount = SE.getExitCount(&L, Cond.ICmp->getParent());
84 if (isa<SCEVCouldNotCompute>(ExitCount))
85 return false;
86
87 Cond.BoundSCEV = ExitCount;
88 return true;
89 }
90
91 // For non-exit condtion, if pred is LT, keep existing bound.
92 if (Cond.Pred == ICmpInst::ICMP_SLT || Cond.Pred == ICmpInst::ICMP_ULT)
93 return true;
94
95 // For non-exit condition, if pre is LE, try to convert it to LT.
96 // Range Range
97 // AddRec <= Bound --> AddRec < Bound + 1
98 if (Cond.Pred != ICmpInst::ICMP_ULE && Cond.Pred != ICmpInst::ICMP_SLE)
99 return false;
100
101 if (IntegerType *BoundSCEVIntType =
102 dyn_cast<IntegerType>(Cond.BoundSCEV->getType())) {
103 unsigned BitWidth = BoundSCEVIntType->getBitWidth();
104 APInt Max = ICmpInst::isSigned(Cond.Pred)
105 ? APInt::getSignedMaxValue(BitWidth)
106 : APInt::getMaxValue(BitWidth);
107 const SCEV *MaxSCEV = SE.getConstant(Max);
108 // Check Bound < INT_MAX
109 ICmpInst::Predicate Pred =
110 ICmpInst::isSigned(Cond.Pred) ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT;
111 if (SE.isKnownPredicate(Pred, Cond.BoundSCEV, MaxSCEV)) {
112 const SCEV *BoundPlusOneSCEV =
113 SE.getAddExpr(Cond.BoundSCEV, SE.getOne(BoundSCEVIntType));
114 Cond.BoundSCEV = BoundPlusOneSCEV;
115 Cond.Pred = Pred;
116 return true;
117 }
118 }
119
120 // ToDo: Support ICMP_NE/EQ.
121
122 return false;
123 }
124
hasProcessableCondition(const Loop & L,ScalarEvolution & SE,ICmpInst * ICmp,ConditionInfo & Cond,bool IsExitCond)125 static bool hasProcessableCondition(const Loop &L, ScalarEvolution &SE,
126 ICmpInst *ICmp, ConditionInfo &Cond,
127 bool IsExitCond) {
128 analyzeICmp(SE, ICmp, Cond, L);
129
130 // The BoundSCEV should be evaluated at loop entry.
131 if (!SE.isAvailableAtLoopEntry(Cond.BoundSCEV, &L))
132 return false;
133
134 // Allowed AddRec as induction variable.
135 if (!Cond.AddRecSCEV)
136 return false;
137
138 if (!Cond.AddRecSCEV->isAffine())
139 return false;
140
141 const SCEV *StepRecSCEV = Cond.AddRecSCEV->getStepRecurrence(SE);
142 // Allowed constant step.
143 if (!isa<SCEVConstant>(StepRecSCEV))
144 return false;
145
146 ConstantInt *StepCI = cast<SCEVConstant>(StepRecSCEV)->getValue();
147 // Allowed positive step for now.
148 // TODO: Support negative step.
149 if (StepCI->isNegative() || StepCI->isZero())
150 return false;
151
152 // Calculate upper bound.
153 if (!calculateUpperBound(L, SE, Cond, IsExitCond))
154 return false;
155
156 return true;
157 }
158
isProcessableCondBI(const ScalarEvolution & SE,const BranchInst * BI)159 static bool isProcessableCondBI(const ScalarEvolution &SE,
160 const BranchInst *BI) {
161 BasicBlock *TrueSucc = nullptr;
162 BasicBlock *FalseSucc = nullptr;
163 ICmpInst::Predicate Pred;
164 Value *LHS, *RHS;
165 if (!match(BI, m_Br(m_ICmp(Pred, m_Value(LHS), m_Value(RHS)),
166 m_BasicBlock(TrueSucc), m_BasicBlock(FalseSucc))))
167 return false;
168
169 if (!SE.isSCEVable(LHS->getType()))
170 return false;
171 assert(SE.isSCEVable(RHS->getType()) && "Expected RHS's type is SCEVable");
172
173 if (TrueSucc == FalseSucc)
174 return false;
175
176 return true;
177 }
178
canSplitLoopBound(const Loop & L,const DominatorTree & DT,ScalarEvolution & SE,ConditionInfo & Cond)179 static bool canSplitLoopBound(const Loop &L, const DominatorTree &DT,
180 ScalarEvolution &SE, ConditionInfo &Cond) {
181 // Skip function with optsize.
182 if (L.getHeader()->getParent()->hasOptSize())
183 return false;
184
185 // Split only innermost loop.
186 if (!L.isInnermost())
187 return false;
188
189 // Check loop is in simplified form.
190 if (!L.isLoopSimplifyForm())
191 return false;
192
193 // Check loop is in LCSSA form.
194 if (!L.isLCSSAForm(DT))
195 return false;
196
197 // Skip loop that cannot be cloned.
198 if (!L.isSafeToClone())
199 return false;
200
201 BasicBlock *ExitingBB = L.getExitingBlock();
202 // Assumed only one exiting block.
203 if (!ExitingBB)
204 return false;
205
206 BranchInst *ExitingBI = dyn_cast<BranchInst>(ExitingBB->getTerminator());
207 if (!ExitingBI)
208 return false;
209
210 // Allowed only conditional branch with ICmp.
211 if (!isProcessableCondBI(SE, ExitingBI))
212 return false;
213
214 // Check the condition is processable.
215 ICmpInst *ICmp = cast<ICmpInst>(ExitingBI->getCondition());
216 if (!hasProcessableCondition(L, SE, ICmp, Cond, /*IsExitCond*/ true))
217 return false;
218
219 Cond.BI = ExitingBI;
220 return true;
221 }
222
isProfitableToTransform(const Loop & L,const BranchInst * BI)223 static bool isProfitableToTransform(const Loop &L, const BranchInst *BI) {
224 // If the conditional branch splits a loop into two halves, we could
225 // generally say it is profitable.
226 //
227 // ToDo: Add more profitable cases here.
228
229 // Check this branch causes diamond CFG.
230 BasicBlock *Succ0 = BI->getSuccessor(0);
231 BasicBlock *Succ1 = BI->getSuccessor(1);
232
233 BasicBlock *Succ0Succ = Succ0->getSingleSuccessor();
234 BasicBlock *Succ1Succ = Succ1->getSingleSuccessor();
235 if (!Succ0Succ || !Succ1Succ || Succ0Succ != Succ1Succ)
236 return false;
237
238 // ToDo: Calculate each successor's instruction cost.
239
240 return true;
241 }
242
findSplitCandidate(const Loop & L,ScalarEvolution & SE,ConditionInfo & ExitingCond,ConditionInfo & SplitCandidateCond)243 static BranchInst *findSplitCandidate(const Loop &L, ScalarEvolution &SE,
244 ConditionInfo &ExitingCond,
245 ConditionInfo &SplitCandidateCond) {
246 for (auto *BB : L.blocks()) {
247 // Skip condition of backedge.
248 if (L.getLoopLatch() == BB)
249 continue;
250
251 auto *BI = dyn_cast<BranchInst>(BB->getTerminator());
252 if (!BI)
253 continue;
254
255 // Check conditional branch with ICmp.
256 if (!isProcessableCondBI(SE, BI))
257 continue;
258
259 // Skip loop invariant condition.
260 if (L.isLoopInvariant(BI->getCondition()))
261 continue;
262
263 // Check the condition is processable.
264 ICmpInst *ICmp = cast<ICmpInst>(BI->getCondition());
265 if (!hasProcessableCondition(L, SE, ICmp, SplitCandidateCond,
266 /*IsExitCond*/ false))
267 continue;
268
269 if (ExitingCond.BoundSCEV->getType() !=
270 SplitCandidateCond.BoundSCEV->getType())
271 continue;
272
273 // After transformation, we assume the split condition of the pre-loop is
274 // always true. In order to guarantee it, we need to check the start value
275 // of the split cond AddRec satisfies the split condition.
276 if (!SE.isLoopEntryGuardedByCond(&L, SplitCandidateCond.Pred,
277 SplitCandidateCond.AddRecSCEV->getStart(),
278 SplitCandidateCond.BoundSCEV))
279 continue;
280
281 SplitCandidateCond.BI = BI;
282 return BI;
283 }
284
285 return nullptr;
286 }
287
splitLoopBound(Loop & L,DominatorTree & DT,LoopInfo & LI,ScalarEvolution & SE,LPMUpdater & U)288 static bool splitLoopBound(Loop &L, DominatorTree &DT, LoopInfo &LI,
289 ScalarEvolution &SE, LPMUpdater &U) {
290 ConditionInfo SplitCandidateCond;
291 ConditionInfo ExitingCond;
292
293 // Check we can split this loop's bound.
294 if (!canSplitLoopBound(L, DT, SE, ExitingCond))
295 return false;
296
297 if (!findSplitCandidate(L, SE, ExitingCond, SplitCandidateCond))
298 return false;
299
300 if (!isProfitableToTransform(L, SplitCandidateCond.BI))
301 return false;
302
303 // Now, we have a split candidate. Let's build a form as below.
304 // +--------------------+
305 // | preheader |
306 // | set up newbound |
307 // +--------------------+
308 // | /----------------\
309 // +--------v----v------+ |
310 // | header |---\ |
311 // | with true condition| | |
312 // +--------------------+ | |
313 // | | |
314 // +--------v-----------+ | |
315 // | if.then.BB | | |
316 // +--------------------+ | |
317 // | | |
318 // +--------v-----------<---/ |
319 // | latch >----------/
320 // | with newbound |
321 // +--------------------+
322 // |
323 // +--------v-----------+
324 // | preheader2 |--------------\
325 // | if (AddRec i != | |
326 // | org bound) | |
327 // +--------------------+ |
328 // | /----------------\ |
329 // +--------v----v------+ | |
330 // | header2 |---\ | |
331 // | conditional branch | | | |
332 // |with false condition| | | |
333 // +--------------------+ | | |
334 // | | | |
335 // +--------v-----------+ | | |
336 // | if.then.BB2 | | | |
337 // +--------------------+ | | |
338 // | | | |
339 // +--------v-----------<---/ | |
340 // | latch2 >----------/ |
341 // | with org bound | |
342 // +--------v-----------+ |
343 // | |
344 // | +---------------+ |
345 // +--> exit <-------/
346 // +---------------+
347
348 // Let's create post loop.
349 SmallVector<BasicBlock *, 8> PostLoopBlocks;
350 Loop *PostLoop;
351 ValueToValueMapTy VMap;
352 BasicBlock *PreHeader = L.getLoopPreheader();
353 BasicBlock *SplitLoopPH = SplitEdge(PreHeader, L.getHeader(), &DT, &LI);
354 PostLoop = cloneLoopWithPreheader(L.getExitBlock(), SplitLoopPH, &L, VMap,
355 ".split", &LI, &DT, PostLoopBlocks);
356 remapInstructionsInBlocks(PostLoopBlocks, VMap);
357
358 BasicBlock *PostLoopPreHeader = PostLoop->getLoopPreheader();
359 IRBuilder<> Builder(&PostLoopPreHeader->front());
360
361 // Update phi nodes in header of post-loop.
362 bool isExitingLatch =
363 (L.getExitingBlock() == L.getLoopLatch()) ? true : false;
364 Value *ExitingCondLCSSAPhi = nullptr;
365 for (PHINode &PN : L.getHeader()->phis()) {
366 // Create LCSSA phi node in preheader of post-loop.
367 PHINode *LCSSAPhi =
368 Builder.CreatePHI(PN.getType(), 1, PN.getName() + ".lcssa");
369 LCSSAPhi->setDebugLoc(PN.getDebugLoc());
370 // If the exiting block is loop latch, the phi does not have the update at
371 // last iteration. In this case, update lcssa phi with value from backedge.
372 LCSSAPhi->addIncoming(
373 isExitingLatch ? PN.getIncomingValueForBlock(L.getLoopLatch()) : &PN,
374 L.getExitingBlock());
375
376 // Update the start value of phi node in post-loop with the LCSSA phi node.
377 PHINode *PostLoopPN = cast<PHINode>(VMap[&PN]);
378 PostLoopPN->setIncomingValueForBlock(PostLoopPreHeader, LCSSAPhi);
379
380 // Find PHI with exiting condition from pre-loop. The PHI should be
381 // SCEVAddRecExpr and have same incoming value from backedge with
382 // ExitingCond.
383 if (!SE.isSCEVable(PN.getType()))
384 continue;
385
386 const SCEVAddRecExpr *PhiSCEV = dyn_cast<SCEVAddRecExpr>(SE.getSCEV(&PN));
387 if (PhiSCEV && ExitingCond.NonPHIAddRecValue ==
388 PN.getIncomingValueForBlock(L.getLoopLatch()))
389 ExitingCondLCSSAPhi = LCSSAPhi;
390 }
391
392 // Add conditional branch to check we can skip post-loop in its preheader.
393 Instruction *OrigBI = PostLoopPreHeader->getTerminator();
394 ICmpInst::Predicate Pred = ICmpInst::ICMP_NE;
395 Value *Cond =
396 Builder.CreateICmp(Pred, ExitingCondLCSSAPhi, ExitingCond.BoundValue);
397 Builder.CreateCondBr(Cond, PostLoop->getHeader(), PostLoop->getExitBlock());
398 OrigBI->eraseFromParent();
399
400 // Create new loop bound and add it into preheader of pre-loop.
401 const SCEV *NewBoundSCEV = ExitingCond.BoundSCEV;
402 const SCEV *SplitBoundSCEV = SplitCandidateCond.BoundSCEV;
403 NewBoundSCEV = ICmpInst::isSigned(ExitingCond.Pred)
404 ? SE.getSMinExpr(NewBoundSCEV, SplitBoundSCEV)
405 : SE.getUMinExpr(NewBoundSCEV, SplitBoundSCEV);
406
407 SCEVExpander Expander(
408 SE, L.getHeader()->getDataLayout(), "split");
409 Instruction *InsertPt = SplitLoopPH->getTerminator();
410 Value *NewBoundValue =
411 Expander.expandCodeFor(NewBoundSCEV, NewBoundSCEV->getType(), InsertPt);
412 NewBoundValue->setName("new.bound");
413
414 // Replace exiting bound value of pre-loop NewBound.
415 ExitingCond.ICmp->setOperand(1, NewBoundValue);
416
417 // Replace SplitCandidateCond.BI's condition of pre-loop by True.
418 LLVMContext &Context = PreHeader->getContext();
419 SplitCandidateCond.BI->setCondition(ConstantInt::getTrue(Context));
420
421 // Replace cloned SplitCandidateCond.BI's condition in post-loop by False.
422 BranchInst *ClonedSplitCandidateBI =
423 cast<BranchInst>(VMap[SplitCandidateCond.BI]);
424 ClonedSplitCandidateBI->setCondition(ConstantInt::getFalse(Context));
425
426 // Replace exit branch target of pre-loop by post-loop's preheader.
427 if (L.getExitBlock() == ExitingCond.BI->getSuccessor(0))
428 ExitingCond.BI->setSuccessor(0, PostLoopPreHeader);
429 else
430 ExitingCond.BI->setSuccessor(1, PostLoopPreHeader);
431
432 // Update phi node in exit block of post-loop.
433 Builder.SetInsertPoint(PostLoopPreHeader, PostLoopPreHeader->begin());
434 for (PHINode &PN : PostLoop->getExitBlock()->phis()) {
435 for (auto i : seq<int>(0, PN.getNumOperands())) {
436 // Check incoming block is pre-loop's exiting block.
437 if (PN.getIncomingBlock(i) == L.getExitingBlock()) {
438 Value *IncomingValue = PN.getIncomingValue(i);
439
440 // Create LCSSA phi node for incoming value.
441 PHINode *LCSSAPhi =
442 Builder.CreatePHI(PN.getType(), 1, PN.getName() + ".lcssa");
443 LCSSAPhi->setDebugLoc(PN.getDebugLoc());
444 LCSSAPhi->addIncoming(IncomingValue, PN.getIncomingBlock(i));
445
446 // Replace pre-loop's exiting block by post-loop's preheader.
447 PN.setIncomingBlock(i, PostLoopPreHeader);
448 // Replace incoming value by LCSSAPhi.
449 PN.setIncomingValue(i, LCSSAPhi);
450 // Add a new incoming value with post-loop's exiting block.
451 PN.addIncoming(VMap[IncomingValue], PostLoop->getExitingBlock());
452 }
453 }
454 }
455
456 // Update dominator tree.
457 DT.changeImmediateDominator(PostLoopPreHeader, L.getExitingBlock());
458 DT.changeImmediateDominator(PostLoop->getExitBlock(), PostLoopPreHeader);
459
460 // Invalidate cached SE information.
461 SE.forgetLoop(&L);
462
463 // Canonicalize loops.
464 simplifyLoop(&L, &DT, &LI, &SE, nullptr, nullptr, true);
465 simplifyLoop(PostLoop, &DT, &LI, &SE, nullptr, nullptr, true);
466
467 // Add new post-loop to loop pass manager.
468 U.addSiblingLoops(PostLoop);
469
470 return true;
471 }
472
run(Loop & L,LoopAnalysisManager & AM,LoopStandardAnalysisResults & AR,LPMUpdater & U)473 PreservedAnalyses LoopBoundSplitPass::run(Loop &L, LoopAnalysisManager &AM,
474 LoopStandardAnalysisResults &AR,
475 LPMUpdater &U) {
476 Function &F = *L.getHeader()->getParent();
477 (void)F;
478
479 LLVM_DEBUG(dbgs() << "Spliting bound of loop in " << F.getName() << ": " << L
480 << "\n");
481
482 if (!splitLoopBound(L, AR.DT, AR.LI, AR.SE, U))
483 return PreservedAnalyses::all();
484
485 assert(AR.DT.verify(DominatorTree::VerificationLevel::Fast));
486 AR.LI.verify(AR.DT);
487
488 return getLoopPassPreservedAnalyses();
489 }
490
491 } // end namespace llvm
492