1 //===-- LoopUnswitch.cpp - Hoist loop-invariant conditionals in loop ------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass transforms loops that contain branches on loop-invariant conditions
11 // to have multiple loops. For example, it turns the left into the right code:
20 // This can increase the size of the code exponentially (doubling it every time
21 // a loop is unswitched) so we only unswitch if the resultant code will be
22 // smaller than a threshold.
24 // This pass expects LICM to be run before it to hoist invariant conditions out
25 // of the loop, to make the unswitching opportunity obvious.
27 //===----------------------------------------------------------------------===//
29 #include "llvm/Transforms/Scalar.h"
30 #include "llvm/ADT/STLExtras.h"
31 #include "llvm/ADT/SmallPtrSet.h"
32 #include "llvm/ADT/Statistic.h"
33 #include "llvm/Analysis/AssumptionCache.h"
34 #include "llvm/Analysis/CodeMetrics.h"
35 #include "llvm/Analysis/InstructionSimplify.h"
36 #include "llvm/Analysis/LoopInfo.h"
37 #include "llvm/Analysis/LoopPass.h"
38 #include "llvm/Analysis/ScalarEvolution.h"
39 #include "llvm/Analysis/TargetTransformInfo.h"
40 #include "llvm/IR/Constants.h"
41 #include "llvm/IR/DerivedTypes.h"
42 #include "llvm/IR/Dominators.h"
43 #include "llvm/IR/Function.h"
44 #include "llvm/IR/Instructions.h"
45 #include "llvm/Support/CommandLine.h"
46 #include "llvm/Support/Debug.h"
47 #include "llvm/Support/raw_ostream.h"
48 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
49 #include "llvm/Transforms/Utils/Cloning.h"
50 #include "llvm/Transforms/Utils/Local.h"
56 #define DEBUG_TYPE "loop-unswitch"
58 STATISTIC(NumBranches, "Number of branches unswitched");
59 STATISTIC(NumSwitches, "Number of switches unswitched");
60 STATISTIC(NumSelects , "Number of selects unswitched");
61 STATISTIC(NumTrivial , "Number of unswitches that are trivial");
62 STATISTIC(NumSimplify, "Number of simplifications of unswitched code");
63 STATISTIC(TotalInsts, "Total number of instructions analyzed");
65 // The specific value of 100 here was chosen based only on intuition and a
66 // few specific examples.
67 static cl::opt<unsigned>
68 Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"),
69 cl::init(100), cl::Hidden);
73 class LUAnalysisCache {
75 typedef DenseMap<const SwitchInst*, SmallPtrSet<const Value *, 8> >
78 typedef UnswitchedValsMap::iterator UnswitchedValsIt;
80 struct LoopProperties {
81 unsigned CanBeUnswitchedCount;
82 unsigned SizeEstimation;
83 UnswitchedValsMap UnswitchedVals;
86 // Here we use std::map instead of DenseMap, since we need to keep valid
87 // LoopProperties pointer for current loop for better performance.
88 typedef std::map<const Loop*, LoopProperties> LoopPropsMap;
89 typedef LoopPropsMap::iterator LoopPropsMapIt;
91 LoopPropsMap LoopsProperties;
92 UnswitchedValsMap *CurLoopInstructions;
93 LoopProperties *CurrentLoopProperties;
95 // Max size of code we can produce on remained iterations.
101 CurLoopInstructions(nullptr), CurrentLoopProperties(nullptr),
105 // Analyze loop. Check its size, calculate is it possible to unswitch
106 // it. Returns true if we can unswitch this loop.
107 bool countLoop(const Loop *L, const TargetTransformInfo &TTI,
108 AssumptionCache *AC);
110 // Clean all data related to given loop.
111 void forgetLoop(const Loop *L);
113 // Mark case value as unswitched.
114 // Since SI instruction can be partly unswitched, in order to avoid
115 // extra unswitching in cloned loops keep track all unswitched values.
116 void setUnswitched(const SwitchInst *SI, const Value *V);
118 // Check was this case value unswitched before or not.
119 bool isUnswitched(const SwitchInst *SI, const Value *V);
121 // Clone all loop-unswitch related loop properties.
122 // Redistribute unswitching quotas.
123 // Note, that new loop data is stored inside the VMap.
124 void cloneData(const Loop *NewLoop, const Loop *OldLoop,
125 const ValueToValueMapTy &VMap);
128 class LoopUnswitch : public LoopPass {
129 LoopInfo *LI; // Loop information
133 // LoopProcessWorklist - Used to check if second loop needs processing
134 // after RewriteLoopBodyWithConditionConstant rewrites first loop.
135 std::vector<Loop*> LoopProcessWorklist;
137 LUAnalysisCache BranchesInfo;
139 bool OptimizeForSize;
144 BasicBlock *loopHeader;
145 BasicBlock *loopPreheader;
147 // LoopBlocks contains all of the basic blocks of the loop, including the
148 // preheader of the loop, the body of the loop, and the exit blocks of the
149 // loop, in that order.
150 std::vector<BasicBlock*> LoopBlocks;
151 // NewBlocks contained cloned copy of basic blocks from LoopBlocks.
152 std::vector<BasicBlock*> NewBlocks;
155 static char ID; // Pass ID, replacement for typeid
156 explicit LoopUnswitch(bool Os = false) :
157 LoopPass(ID), OptimizeForSize(Os), redoLoop(false),
158 currentLoop(nullptr), DT(nullptr), loopHeader(nullptr),
159 loopPreheader(nullptr) {
160 initializeLoopUnswitchPass(*PassRegistry::getPassRegistry());
163 bool runOnLoop(Loop *L, LPPassManager &LPM) override;
164 bool processCurrentLoop();
166 /// This transformation requires natural loop information & requires that
167 /// loop preheaders be inserted into the CFG.
169 void getAnalysisUsage(AnalysisUsage &AU) const override {
170 AU.addRequired<AssumptionCacheTracker>();
171 AU.addRequiredID(LoopSimplifyID);
172 AU.addPreservedID(LoopSimplifyID);
173 AU.addRequired<LoopInfoWrapperPass>();
174 AU.addPreserved<LoopInfoWrapperPass>();
175 AU.addRequiredID(LCSSAID);
176 AU.addPreservedID(LCSSAID);
177 AU.addPreserved<DominatorTreeWrapperPass>();
178 AU.addPreserved<ScalarEvolution>();
179 AU.addRequired<TargetTransformInfoWrapperPass>();
184 void releaseMemory() override {
185 BranchesInfo.forgetLoop(currentLoop);
188 void initLoopData() {
189 loopHeader = currentLoop->getHeader();
190 loopPreheader = currentLoop->getLoopPreheader();
193 /// Split all of the edges from inside the loop to their exit blocks.
194 /// Update the appropriate Phi nodes as we do so.
195 void SplitExitEdges(Loop *L, const SmallVectorImpl<BasicBlock *> &ExitBlocks);
197 bool UnswitchIfProfitable(Value *LoopCond, Constant *Val);
198 void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val,
199 BasicBlock *ExitBlock);
200 void UnswitchNontrivialCondition(Value *LIC, Constant *OnVal, Loop *L);
202 void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
203 Constant *Val, bool isEqual);
205 void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val,
206 BasicBlock *TrueDest,
207 BasicBlock *FalseDest,
208 Instruction *InsertPt);
210 void SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L);
211 bool IsTrivialUnswitchCondition(Value *Cond, Constant **Val = nullptr,
212 BasicBlock **LoopExit = nullptr);
217 // Analyze loop. Check its size, calculate is it possible to unswitch
218 // it. Returns true if we can unswitch this loop.
219 bool LUAnalysisCache::countLoop(const Loop *L, const TargetTransformInfo &TTI,
220 AssumptionCache *AC) {
222 LoopPropsMapIt PropsIt;
224 std::tie(PropsIt, Inserted) =
225 LoopsProperties.insert(std::make_pair(L, LoopProperties()));
227 LoopProperties &Props = PropsIt->second;
232 // Limit the number of instructions to avoid causing significant code
233 // expansion, and the number of basic blocks, to avoid loops with
234 // large numbers of branches which cause loop unswitching to go crazy.
235 // This is a very ad-hoc heuristic.
237 SmallPtrSet<const Value *, 32> EphValues;
238 CodeMetrics::collectEphemeralValues(L, AC, EphValues);
240 // FIXME: This is overly conservative because it does not take into
241 // consideration code simplification opportunities and code that can
242 // be shared by the resultant unswitched loops.
244 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
246 Metrics.analyzeBasicBlock(*I, TTI, EphValues);
248 Props.SizeEstimation = std::min(Metrics.NumInsts, Metrics.NumBlocks * 5);
249 Props.CanBeUnswitchedCount = MaxSize / (Props.SizeEstimation);
250 MaxSize -= Props.SizeEstimation * Props.CanBeUnswitchedCount;
252 if (Metrics.notDuplicatable) {
253 DEBUG(dbgs() << "NOT unswitching loop %"
254 << L->getHeader()->getName() << ", contents cannot be "
260 if (!Props.CanBeUnswitchedCount) {
261 DEBUG(dbgs() << "NOT unswitching loop %"
262 << L->getHeader()->getName() << ", cost too high: "
263 << L->getBlocks().size() << "\n");
267 // Be careful. This links are good only before new loop addition.
268 CurrentLoopProperties = &Props;
269 CurLoopInstructions = &Props.UnswitchedVals;
274 // Clean all data related to given loop.
275 void LUAnalysisCache::forgetLoop(const Loop *L) {
277 LoopPropsMapIt LIt = LoopsProperties.find(L);
279 if (LIt != LoopsProperties.end()) {
280 LoopProperties &Props = LIt->second;
281 MaxSize += Props.CanBeUnswitchedCount * Props.SizeEstimation;
282 LoopsProperties.erase(LIt);
285 CurrentLoopProperties = nullptr;
286 CurLoopInstructions = nullptr;
289 // Mark case value as unswitched.
290 // Since SI instruction can be partly unswitched, in order to avoid
291 // extra unswitching in cloned loops keep track all unswitched values.
292 void LUAnalysisCache::setUnswitched(const SwitchInst *SI, const Value *V) {
293 (*CurLoopInstructions)[SI].insert(V);
296 // Check was this case value unswitched before or not.
297 bool LUAnalysisCache::isUnswitched(const SwitchInst *SI, const Value *V) {
298 return (*CurLoopInstructions)[SI].count(V);
301 // Clone all loop-unswitch related loop properties.
302 // Redistribute unswitching quotas.
303 // Note, that new loop data is stored inside the VMap.
304 void LUAnalysisCache::cloneData(const Loop *NewLoop, const Loop *OldLoop,
305 const ValueToValueMapTy &VMap) {
307 LoopProperties &NewLoopProps = LoopsProperties[NewLoop];
308 LoopProperties &OldLoopProps = *CurrentLoopProperties;
309 UnswitchedValsMap &Insts = OldLoopProps.UnswitchedVals;
311 // Reallocate "can-be-unswitched quota"
313 --OldLoopProps.CanBeUnswitchedCount;
314 unsigned Quota = OldLoopProps.CanBeUnswitchedCount;
315 NewLoopProps.CanBeUnswitchedCount = Quota / 2;
316 OldLoopProps.CanBeUnswitchedCount = Quota - Quota / 2;
318 NewLoopProps.SizeEstimation = OldLoopProps.SizeEstimation;
320 // Clone unswitched values info:
321 // for new loop switches we clone info about values that was
322 // already unswitched and has redundant successors.
323 for (UnswitchedValsIt I = Insts.begin(); I != Insts.end(); ++I) {
324 const SwitchInst *OldInst = I->first;
325 Value *NewI = VMap.lookup(OldInst);
326 const SwitchInst *NewInst = cast_or_null<SwitchInst>(NewI);
327 assert(NewInst && "All instructions that are in SrcBB must be in VMap.");
329 NewLoopProps.UnswitchedVals[NewInst] = OldLoopProps.UnswitchedVals[OldInst];
333 char LoopUnswitch::ID = 0;
334 INITIALIZE_PASS_BEGIN(LoopUnswitch, "loop-unswitch", "Unswitch loops",
336 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
337 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
338 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
339 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
340 INITIALIZE_PASS_DEPENDENCY(LCSSA)
341 INITIALIZE_PASS_END(LoopUnswitch, "loop-unswitch", "Unswitch loops",
344 Pass *llvm::createLoopUnswitchPass(bool Os) {
345 return new LoopUnswitch(Os);
348 /// FindLIVLoopCondition - Cond is a condition that occurs in L. If it is
349 /// invariant in the loop, or has an invariant piece, return the invariant.
350 /// Otherwise, return null.
351 static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) {
353 // We started analyze new instruction, increment scanned instructions counter.
356 // We can never unswitch on vector conditions.
357 if (Cond->getType()->isVectorTy())
360 // Constants should be folded, not unswitched on!
361 if (isa<Constant>(Cond)) return nullptr;
363 // TODO: Handle: br (VARIANT|INVARIANT).
365 // Hoist simple values out.
366 if (L->makeLoopInvariant(Cond, Changed))
369 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond))
370 if (BO->getOpcode() == Instruction::And ||
371 BO->getOpcode() == Instruction::Or) {
372 // If either the left or right side is invariant, we can unswitch on this,
373 // which will cause the branch to go away in one loop and the condition to
374 // simplify in the other one.
375 if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed))
377 if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed))
384 bool LoopUnswitch::runOnLoop(Loop *L, LPPassManager &LPM_Ref) {
385 if (skipOptnoneFunction(L))
388 AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(
389 *L->getHeader()->getParent());
390 LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
392 DominatorTreeWrapperPass *DTWP =
393 getAnalysisIfAvailable<DominatorTreeWrapperPass>();
394 DT = DTWP ? &DTWP->getDomTree() : nullptr;
396 Function *F = currentLoop->getHeader()->getParent();
397 bool Changed = false;
399 assert(currentLoop->isLCSSAForm(*DT));
401 Changed |= processCurrentLoop();
405 // FIXME: Reconstruct dom info, because it is not preserved properly.
412 /// processCurrentLoop - Do actual work and unswitch loop if possible
414 bool LoopUnswitch::processCurrentLoop() {
415 bool Changed = false;
419 // If LoopSimplify was unable to form a preheader, don't do any unswitching.
423 // Loops with indirectbr cannot be cloned.
424 if (!currentLoop->isSafeToClone())
427 // Without dedicated exits, splitting the exit edge may fail.
428 if (!currentLoop->hasDedicatedExits())
431 LLVMContext &Context = loopHeader->getContext();
433 // Probably we reach the quota of branches for this loop. If so
435 if (!BranchesInfo.countLoop(
436 currentLoop, getAnalysis<TargetTransformInfoWrapperPass>().getTTI(
437 *currentLoop->getHeader()->getParent()),
441 // Loop over all of the basic blocks in the loop. If we find an interior
442 // block that is branching on a loop-invariant condition, we can unswitch this
444 for (Loop::block_iterator I = currentLoop->block_begin(),
445 E = currentLoop->block_end(); I != E; ++I) {
446 TerminatorInst *TI = (*I)->getTerminator();
447 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
448 // If this isn't branching on an invariant condition, we can't unswitch
450 if (BI->isConditional()) {
451 // See if this, or some part of it, is loop invariant. If so, we can
452 // unswitch on it if we desire.
453 Value *LoopCond = FindLIVLoopCondition(BI->getCondition(),
454 currentLoop, Changed);
455 if (LoopCond && UnswitchIfProfitable(LoopCond,
456 ConstantInt::getTrue(Context))) {
461 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
462 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(),
463 currentLoop, Changed);
464 unsigned NumCases = SI->getNumCases();
465 if (LoopCond && NumCases) {
466 // Find a value to unswitch on:
467 // FIXME: this should chose the most expensive case!
468 // FIXME: scan for a case with a non-critical edge?
469 Constant *UnswitchVal = nullptr;
471 // Do not process same value again and again.
472 // At this point we have some cases already unswitched and
473 // some not yet unswitched. Let's find the first not yet unswitched one.
474 for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end();
476 Constant *UnswitchValCandidate = i.getCaseValue();
477 if (!BranchesInfo.isUnswitched(SI, UnswitchValCandidate)) {
478 UnswitchVal = UnswitchValCandidate;
486 if (UnswitchIfProfitable(LoopCond, UnswitchVal)) {
493 // Scan the instructions to check for unswitchable values.
494 for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end();
496 if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) {
497 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(),
498 currentLoop, Changed);
499 if (LoopCond && UnswitchIfProfitable(LoopCond,
500 ConstantInt::getTrue(Context))) {
509 /// isTrivialLoopExitBlock - Check to see if all paths from BB exit the
510 /// loop with no side effects (including infinite loops).
512 /// If true, we return true and set ExitBB to the block we
515 static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB,
517 std::set<BasicBlock*> &Visited) {
518 if (!Visited.insert(BB).second) {
519 // Already visited. Without more analysis, this could indicate an infinite
523 if (!L->contains(BB)) {
524 // Otherwise, this is a loop exit, this is fine so long as this is the
526 if (ExitBB) return false;
531 // Otherwise, this is an unvisited intra-loop node. Check all successors.
532 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) {
533 // Check to see if the successor is a trivial loop exit.
534 if (!isTrivialLoopExitBlockHelper(L, *SI, ExitBB, Visited))
538 // Okay, everything after this looks good, check to make sure that this block
539 // doesn't include any side effects.
540 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
541 if (I->mayHaveSideEffects())
547 /// isTrivialLoopExitBlock - Return true if the specified block unconditionally
548 /// leads to an exit from the specified loop, and has no side-effects in the
549 /// process. If so, return the block that is exited to, otherwise return null.
550 static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) {
551 std::set<BasicBlock*> Visited;
552 Visited.insert(L->getHeader()); // Branches to header make infinite loops.
553 BasicBlock *ExitBB = nullptr;
554 if (isTrivialLoopExitBlockHelper(L, BB, ExitBB, Visited))
559 /// IsTrivialUnswitchCondition - Check to see if this unswitch condition is
560 /// trivial: that is, that the condition controls whether or not the loop does
561 /// anything at all. If this is a trivial condition, unswitching produces no
562 /// code duplications (equivalently, it produces a simpler loop and a new empty
563 /// loop, which gets deleted).
565 /// If this is a trivial condition, return true, otherwise return false. When
566 /// returning true, this sets Cond and Val to the condition that controls the
567 /// trivial condition: when Cond dynamically equals Val, the loop is known to
568 /// exit. Finally, this sets LoopExit to the BB that the loop exits to when
571 bool LoopUnswitch::IsTrivialUnswitchCondition(Value *Cond, Constant **Val,
572 BasicBlock **LoopExit) {
573 BasicBlock *Header = currentLoop->getHeader();
574 TerminatorInst *HeaderTerm = Header->getTerminator();
575 LLVMContext &Context = Header->getContext();
577 BasicBlock *LoopExitBB = nullptr;
578 if (BranchInst *BI = dyn_cast<BranchInst>(HeaderTerm)) {
579 // If the header block doesn't end with a conditional branch on Cond, we
581 if (!BI->isConditional() || BI->getCondition() != Cond)
584 // Check to see if a successor of the branch is guaranteed to
585 // exit through a unique exit block without having any
586 // side-effects. If so, determine the value of Cond that causes it to do
588 if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
589 BI->getSuccessor(0)))) {
590 if (Val) *Val = ConstantInt::getTrue(Context);
591 } else if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
592 BI->getSuccessor(1)))) {
593 if (Val) *Val = ConstantInt::getFalse(Context);
595 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(HeaderTerm)) {
596 // If this isn't a switch on Cond, we can't handle it.
597 if (SI->getCondition() != Cond) return false;
599 // Check to see if a successor of the switch is guaranteed to go to the
600 // latch block or exit through a one exit block without having any
601 // side-effects. If so, determine the value of Cond that causes it to do
603 // Note that we can't trivially unswitch on the default case or
604 // on already unswitched cases.
605 for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end();
607 BasicBlock *LoopExitCandidate;
608 if ((LoopExitCandidate = isTrivialLoopExitBlock(currentLoop,
609 i.getCaseSuccessor()))) {
610 // Okay, we found a trivial case, remember the value that is trivial.
611 ConstantInt *CaseVal = i.getCaseValue();
613 // Check that it was not unswitched before, since already unswitched
614 // trivial vals are looks trivial too.
615 if (BranchesInfo.isUnswitched(SI, CaseVal))
617 LoopExitBB = LoopExitCandidate;
618 if (Val) *Val = CaseVal;
624 // If we didn't find a single unique LoopExit block, or if the loop exit block
625 // contains phi nodes, this isn't trivial.
626 if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin()))
627 return false; // Can't handle this.
629 if (LoopExit) *LoopExit = LoopExitBB;
631 // We already know that nothing uses any scalar values defined inside of this
632 // loop. As such, we just have to check to see if this loop will execute any
633 // side-effecting instructions (e.g. stores, calls, volatile loads) in the
634 // part of the loop that the code *would* execute. We already checked the
635 // tail, check the header now.
636 for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I)
637 if (I->mayHaveSideEffects())
642 /// UnswitchIfProfitable - We have found that we can unswitch currentLoop when
643 /// LoopCond == Val to simplify the loop. If we decide that this is profitable,
644 /// unswitch the loop, reprocess the pieces, then return true.
645 bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val) {
646 Function *F = loopHeader->getParent();
647 Constant *CondVal = nullptr;
648 BasicBlock *ExitBlock = nullptr;
650 if (IsTrivialUnswitchCondition(LoopCond, &CondVal, &ExitBlock)) {
651 // If the condition is trivial, always unswitch. There is no code growth
653 UnswitchTrivialCondition(currentLoop, LoopCond, CondVal, ExitBlock);
657 // Check to see if it would be profitable to unswitch current loop.
659 // Do not do non-trivial unswitch while optimizing for size.
660 if (OptimizeForSize || F->hasFnAttribute(Attribute::OptimizeForSize))
663 UnswitchNontrivialCondition(LoopCond, Val, currentLoop);
667 /// CloneLoop - Recursively clone the specified loop and all of its children,
668 /// mapping the blocks with the specified map.
669 static Loop *CloneLoop(Loop *L, Loop *PL, ValueToValueMapTy &VM,
670 LoopInfo *LI, LPPassManager *LPM) {
671 Loop *New = new Loop();
672 LPM->insertLoop(New, PL);
674 // Add all of the blocks in L to the new loop.
675 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
677 if (LI->getLoopFor(*I) == L)
678 New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), *LI);
680 // Add all of the subloops to the new loop.
681 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
682 CloneLoop(*I, New, VM, LI, LPM);
687 /// EmitPreheaderBranchOnCondition - Emit a conditional branch on two values
688 /// if LIC == Val, branch to TrueDst, otherwise branch to FalseDest. Insert the
689 /// code immediately before InsertPt.
690 void LoopUnswitch::EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val,
691 BasicBlock *TrueDest,
692 BasicBlock *FalseDest,
693 Instruction *InsertPt) {
694 // Insert a conditional branch on LIC to the two preheaders. The original
695 // code is the true version and the new code is the false version.
696 Value *BranchVal = LIC;
697 if (!isa<ConstantInt>(Val) ||
698 Val->getType() != Type::getInt1Ty(LIC->getContext()))
699 BranchVal = new ICmpInst(InsertPt, ICmpInst::ICMP_EQ, LIC, Val);
700 else if (Val != ConstantInt::getTrue(Val->getContext()))
701 // We want to enter the new loop when the condition is true.
702 std::swap(TrueDest, FalseDest);
704 // Insert the new branch.
705 BranchInst *BI = BranchInst::Create(TrueDest, FalseDest, BranchVal, InsertPt);
707 // If either edge is critical, split it. This helps preserve LoopSimplify
708 // form for enclosing loops.
709 auto Options = CriticalEdgeSplittingOptions(DT, LI).setPreserveLCSSA();
710 SplitCriticalEdge(BI, 0, Options);
711 SplitCriticalEdge(BI, 1, Options);
714 /// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable
715 /// condition in it (a cond branch from its header block to its latch block,
716 /// where the path through the loop that doesn't execute its body has no
717 /// side-effects), unswitch it. This doesn't involve any code duplication, just
718 /// moving the conditional branch outside of the loop and updating loop info.
719 void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond,
721 BasicBlock *ExitBlock) {
722 DEBUG(dbgs() << "loop-unswitch: Trivial-Unswitch loop %"
723 << loopHeader->getName() << " [" << L->getBlocks().size()
724 << " blocks] in Function " << L->getHeader()->getParent()->getName()
725 << " on cond: " << *Val << " == " << *Cond << "\n");
727 // First step, split the preheader, so that we know that there is a safe place
728 // to insert the conditional branch. We will change loopPreheader to have a
729 // conditional branch on Cond.
730 BasicBlock *NewPH = SplitEdge(loopPreheader, loopHeader, DT, LI);
732 // Now that we have a place to insert the conditional branch, create a place
733 // to branch to: this is the exit block out of the loop that we should
736 // Split this block now, so that the loop maintains its exit block, and so
737 // that the jump from the preheader can execute the contents of the exit block
738 // without actually branching to it (the exit block should be dominated by the
739 // loop header, not the preheader).
740 assert(!L->contains(ExitBlock) && "Exit block is in the loop?");
741 BasicBlock *NewExit = SplitBlock(ExitBlock, ExitBlock->begin(), DT, LI);
743 // Okay, now we have a position to branch from and a position to branch to,
744 // insert the new conditional branch.
745 EmitPreheaderBranchOnCondition(Cond, Val, NewExit, NewPH,
746 loopPreheader->getTerminator());
747 LPM->deleteSimpleAnalysisValue(loopPreheader->getTerminator(), L);
748 loopPreheader->getTerminator()->eraseFromParent();
750 // We need to reprocess this loop, it could be unswitched again.
753 // Now that we know that the loop is never entered when this condition is a
754 // particular value, rewrite the loop with this info. We know that this will
755 // at least eliminate the old branch.
756 RewriteLoopBodyWithConditionConstant(L, Cond, Val, false);
760 /// SplitExitEdges - Split all of the edges from inside the loop to their exit
761 /// blocks. Update the appropriate Phi nodes as we do so.
762 void LoopUnswitch::SplitExitEdges(Loop *L,
763 const SmallVectorImpl<BasicBlock *> &ExitBlocks){
765 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
766 BasicBlock *ExitBlock = ExitBlocks[i];
767 SmallVector<BasicBlock *, 4> Preds(pred_begin(ExitBlock),
768 pred_end(ExitBlock));
770 // Although SplitBlockPredecessors doesn't preserve loop-simplify in
771 // general, if we call it on all predecessors of all exits then it does.
772 SplitBlockPredecessors(ExitBlock, Preds, ".us-lcssa",
773 /*AliasAnalysis*/ nullptr, DT, LI,
774 /*PreserveLCSSA*/ true);
778 /// UnswitchNontrivialCondition - We determined that the loop is profitable
779 /// to unswitch when LIC equal Val. Split it into loop versions and test the
780 /// condition outside of either loop. Return the loops created as Out1/Out2.
781 void LoopUnswitch::UnswitchNontrivialCondition(Value *LIC, Constant *Val,
783 Function *F = loopHeader->getParent();
784 DEBUG(dbgs() << "loop-unswitch: Unswitching loop %"
785 << loopHeader->getName() << " [" << L->getBlocks().size()
786 << " blocks] in Function " << F->getName()
787 << " when '" << *Val << "' == " << *LIC << "\n");
789 if (ScalarEvolution *SE = getAnalysisIfAvailable<ScalarEvolution>())
795 // First step, split the preheader and exit blocks, and add these blocks to
796 // the LoopBlocks list.
797 BasicBlock *NewPreheader = SplitEdge(loopPreheader, loopHeader, DT, LI);
798 LoopBlocks.push_back(NewPreheader);
800 // We want the loop to come after the preheader, but before the exit blocks.
801 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
803 SmallVector<BasicBlock*, 8> ExitBlocks;
804 L->getUniqueExitBlocks(ExitBlocks);
806 // Split all of the edges from inside the loop to their exit blocks. Update
807 // the appropriate Phi nodes as we do so.
808 SplitExitEdges(L, ExitBlocks);
810 // The exit blocks may have been changed due to edge splitting, recompute.
812 L->getUniqueExitBlocks(ExitBlocks);
814 // Add exit blocks to the loop blocks.
815 LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end());
817 // Next step, clone all of the basic blocks that make up the loop (including
818 // the loop preheader and exit blocks), keeping track of the mapping between
819 // the instructions and blocks.
820 NewBlocks.reserve(LoopBlocks.size());
821 ValueToValueMapTy VMap;
822 for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
823 BasicBlock *NewBB = CloneBasicBlock(LoopBlocks[i], VMap, ".us", F);
825 NewBlocks.push_back(NewBB);
826 VMap[LoopBlocks[i]] = NewBB; // Keep the BB mapping.
827 LPM->cloneBasicBlockSimpleAnalysis(LoopBlocks[i], NewBB, L);
830 // Splice the newly inserted blocks into the function right before the
831 // original preheader.
832 F->getBasicBlockList().splice(NewPreheader, F->getBasicBlockList(),
833 NewBlocks[0], F->end());
835 // FIXME: We could register any cloned assumptions instead of clearing the
836 // whole function's cache.
839 // Now we create the new Loop object for the versioned loop.
840 Loop *NewLoop = CloneLoop(L, L->getParentLoop(), VMap, LI, LPM);
842 // Recalculate unswitching quota, inherit simplified switches info for NewBB,
843 // Probably clone more loop-unswitch related loop properties.
844 BranchesInfo.cloneData(NewLoop, L, VMap);
846 Loop *ParentLoop = L->getParentLoop();
848 // Make sure to add the cloned preheader and exit blocks to the parent loop
850 ParentLoop->addBasicBlockToLoop(NewBlocks[0], *LI);
853 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
854 BasicBlock *NewExit = cast<BasicBlock>(VMap[ExitBlocks[i]]);
855 // The new exit block should be in the same loop as the old one.
856 if (Loop *ExitBBLoop = LI->getLoopFor(ExitBlocks[i]))
857 ExitBBLoop->addBasicBlockToLoop(NewExit, *LI);
859 assert(NewExit->getTerminator()->getNumSuccessors() == 1 &&
860 "Exit block should have been split to have one successor!");
861 BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0);
863 // If the successor of the exit block had PHI nodes, add an entry for
865 for (BasicBlock::iterator I = ExitSucc->begin();
866 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
867 Value *V = PN->getIncomingValueForBlock(ExitBlocks[i]);
868 ValueToValueMapTy::iterator It = VMap.find(V);
869 if (It != VMap.end()) V = It->second;
870 PN->addIncoming(V, NewExit);
873 if (LandingPadInst *LPad = NewExit->getLandingPadInst()) {
874 PHINode *PN = PHINode::Create(LPad->getType(), 0, "",
875 ExitSucc->getFirstInsertionPt());
877 for (pred_iterator I = pred_begin(ExitSucc), E = pred_end(ExitSucc);
880 LandingPadInst *LPI = BB->getLandingPadInst();
881 LPI->replaceAllUsesWith(PN);
882 PN->addIncoming(LPI, BB);
887 // Rewrite the code to refer to itself.
888 for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
889 for (BasicBlock::iterator I = NewBlocks[i]->begin(),
890 E = NewBlocks[i]->end(); I != E; ++I)
891 RemapInstruction(I, VMap,RF_NoModuleLevelChanges|RF_IgnoreMissingEntries);
893 // Rewrite the original preheader to select between versions of the loop.
894 BranchInst *OldBR = cast<BranchInst>(loopPreheader->getTerminator());
895 assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] &&
896 "Preheader splitting did not work correctly!");
898 // Emit the new branch that selects between the two versions of this loop.
899 EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR);
900 LPM->deleteSimpleAnalysisValue(OldBR, L);
901 OldBR->eraseFromParent();
903 LoopProcessWorklist.push_back(NewLoop);
906 // Keep a WeakVH holding onto LIC. If the first call to RewriteLoopBody
907 // deletes the instruction (for example by simplifying a PHI that feeds into
908 // the condition that we're unswitching on), we don't rewrite the second
910 WeakVH LICHandle(LIC);
912 // Now we rewrite the original code to know that the condition is true and the
913 // new code to know that the condition is false.
914 RewriteLoopBodyWithConditionConstant(L, LIC, Val, false);
916 // It's possible that simplifying one loop could cause the other to be
917 // changed to another value or a constant. If its a constant, don't simplify
919 if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop &&
920 LICHandle && !isa<Constant>(LICHandle))
921 RewriteLoopBodyWithConditionConstant(NewLoop, LICHandle, Val, true);
924 /// RemoveFromWorklist - Remove all instances of I from the worklist vector
926 static void RemoveFromWorklist(Instruction *I,
927 std::vector<Instruction*> &Worklist) {
929 Worklist.erase(std::remove(Worklist.begin(), Worklist.end(), I),
933 /// ReplaceUsesOfWith - When we find that I really equals V, remove I from the
934 /// program, replacing all uses with V and update the worklist.
935 static void ReplaceUsesOfWith(Instruction *I, Value *V,
936 std::vector<Instruction*> &Worklist,
937 Loop *L, LPPassManager *LPM) {
938 DEBUG(dbgs() << "Replace with '" << *V << "': " << *I);
940 // Add uses to the worklist, which may be dead now.
941 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
942 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
943 Worklist.push_back(Use);
945 // Add users to the worklist which may be simplified now.
946 for (User *U : I->users())
947 Worklist.push_back(cast<Instruction>(U));
948 LPM->deleteSimpleAnalysisValue(I, L);
949 RemoveFromWorklist(I, Worklist);
950 I->replaceAllUsesWith(V);
951 I->eraseFromParent();
955 // RewriteLoopBodyWithConditionConstant - We know either that the value LIC has
956 // the value specified by Val in the specified loop, or we know it does NOT have
957 // that value. Rewrite any uses of LIC or of properties correlated to it.
958 void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
961 assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?");
963 // FIXME: Support correlated properties, like:
970 // FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches,
971 // selects, switches.
972 std::vector<Instruction*> Worklist;
973 LLVMContext &Context = Val->getContext();
975 // If we know that LIC == Val, or that LIC == NotVal, just replace uses of LIC
976 // in the loop with the appropriate one directly.
977 if (IsEqual || (isa<ConstantInt>(Val) &&
978 Val->getType()->isIntegerTy(1))) {
983 Replacement = ConstantInt::get(Type::getInt1Ty(Val->getContext()),
984 !cast<ConstantInt>(Val)->getZExtValue());
986 for (User *U : LIC->users()) {
987 Instruction *UI = dyn_cast<Instruction>(U);
988 if (!UI || !L->contains(UI))
990 Worklist.push_back(UI);
993 for (std::vector<Instruction*>::iterator UI = Worklist.begin(),
994 UE = Worklist.end(); UI != UE; ++UI)
995 (*UI)->replaceUsesOfWith(LIC, Replacement);
997 SimplifyCode(Worklist, L);
1001 // Otherwise, we don't know the precise value of LIC, but we do know that it
1002 // is certainly NOT "Val". As such, simplify any uses in the loop that we
1003 // can. This case occurs when we unswitch switch statements.
1004 for (User *U : LIC->users()) {
1005 Instruction *UI = dyn_cast<Instruction>(U);
1006 if (!UI || !L->contains(UI))
1009 Worklist.push_back(UI);
1011 // TODO: We could do other simplifications, for example, turning
1012 // 'icmp eq LIC, Val' -> false.
1014 // If we know that LIC is not Val, use this info to simplify code.
1015 SwitchInst *SI = dyn_cast<SwitchInst>(UI);
1016 if (!SI || !isa<ConstantInt>(Val)) continue;
1018 SwitchInst::CaseIt DeadCase = SI->findCaseValue(cast<ConstantInt>(Val));
1019 // Default case is live for multiple values.
1020 if (DeadCase == SI->case_default()) continue;
1022 // Found a dead case value. Don't remove PHI nodes in the
1023 // successor if they become single-entry, those PHI nodes may
1024 // be in the Users list.
1026 BasicBlock *Switch = SI->getParent();
1027 BasicBlock *SISucc = DeadCase.getCaseSuccessor();
1028 BasicBlock *Latch = L->getLoopLatch();
1030 BranchesInfo.setUnswitched(SI, Val);
1032 if (!SI->findCaseDest(SISucc)) continue; // Edge is critical.
1033 // If the DeadCase successor dominates the loop latch, then the
1034 // transformation isn't safe since it will delete the sole predecessor edge
1036 if (Latch && DT->dominates(SISucc, Latch))
1039 // FIXME: This is a hack. We need to keep the successor around
1040 // and hooked up so as to preserve the loop structure, because
1041 // trying to update it is complicated. So instead we preserve the
1042 // loop structure and put the block on a dead code path.
1043 SplitEdge(Switch, SISucc, DT, LI);
1044 // Compute the successors instead of relying on the return value
1045 // of SplitEdge, since it may have split the switch successor
1047 BasicBlock *NewSISucc = DeadCase.getCaseSuccessor();
1048 BasicBlock *OldSISucc = *succ_begin(NewSISucc);
1049 // Create an "unreachable" destination.
1050 BasicBlock *Abort = BasicBlock::Create(Context, "us-unreachable",
1051 Switch->getParent(),
1053 new UnreachableInst(Context, Abort);
1054 // Force the new case destination to branch to the "unreachable"
1055 // block while maintaining a (dead) CFG edge to the old block.
1056 NewSISucc->getTerminator()->eraseFromParent();
1057 BranchInst::Create(Abort, OldSISucc,
1058 ConstantInt::getTrue(Context), NewSISucc);
1059 // Release the PHI operands for this edge.
1060 for (BasicBlock::iterator II = NewSISucc->begin();
1061 PHINode *PN = dyn_cast<PHINode>(II); ++II)
1062 PN->setIncomingValue(PN->getBasicBlockIndex(Switch),
1063 UndefValue::get(PN->getType()));
1064 // Tell the domtree about the new block. We don't fully update the
1065 // domtree here -- instead we force it to do a full recomputation
1066 // after the pass is complete -- but we do need to inform it of
1069 DT->addNewBlock(Abort, NewSISucc);
1072 SimplifyCode(Worklist, L);
1075 /// SimplifyCode - Okay, now that we have simplified some instructions in the
1076 /// loop, walk over it and constant prop, dce, and fold control flow where
1077 /// possible. Note that this is effectively a very simple loop-structure-aware
1078 /// optimizer. During processing of this loop, L could very well be deleted, so
1079 /// it must not be used.
1081 /// FIXME: When the loop optimizer is more mature, separate this out to a new
1084 void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L) {
1085 while (!Worklist.empty()) {
1086 Instruction *I = Worklist.back();
1087 Worklist.pop_back();
1090 if (isInstructionTriviallyDead(I)) {
1091 DEBUG(dbgs() << "Remove dead instruction '" << *I);
1093 // Add uses to the worklist, which may be dead now.
1094 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
1095 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
1096 Worklist.push_back(Use);
1097 LPM->deleteSimpleAnalysisValue(I, L);
1098 RemoveFromWorklist(I, Worklist);
1099 I->eraseFromParent();
1104 // See if instruction simplification can hack this up. This is common for
1105 // things like "select false, X, Y" after unswitching made the condition be
1106 // 'false'. TODO: update the domtree properly so we can pass it here.
1107 if (Value *V = SimplifyInstruction(I))
1108 if (LI->replacementPreservesLCSSAForm(I, V)) {
1109 ReplaceUsesOfWith(I, V, Worklist, L, LPM);
1113 // Special case hacks that appear commonly in unswitched code.
1114 if (BranchInst *BI = dyn_cast<BranchInst>(I)) {
1115 if (BI->isUnconditional()) {
1116 // If BI's parent is the only pred of the successor, fold the two blocks
1118 BasicBlock *Pred = BI->getParent();
1119 BasicBlock *Succ = BI->getSuccessor(0);
1120 BasicBlock *SinglePred = Succ->getSinglePredecessor();
1121 if (!SinglePred) continue; // Nothing to do.
1122 assert(SinglePred == Pred && "CFG broken");
1124 DEBUG(dbgs() << "Merging blocks: " << Pred->getName() << " <- "
1125 << Succ->getName() << "\n");
1127 // Resolve any single entry PHI nodes in Succ.
1128 while (PHINode *PN = dyn_cast<PHINode>(Succ->begin()))
1129 ReplaceUsesOfWith(PN, PN->getIncomingValue(0), Worklist, L, LPM);
1131 // If Succ has any successors with PHI nodes, update them to have
1132 // entries coming from Pred instead of Succ.
1133 Succ->replaceAllUsesWith(Pred);
1135 // Move all of the successor contents from Succ to Pred.
1136 Pred->getInstList().splice(BI, Succ->getInstList(), Succ->begin(),
1138 LPM->deleteSimpleAnalysisValue(BI, L);
1139 BI->eraseFromParent();
1140 RemoveFromWorklist(BI, Worklist);
1142 // Remove Succ from the loop tree.
1143 LI->removeBlock(Succ);
1144 LPM->deleteSimpleAnalysisValue(Succ, L);
1145 Succ->eraseFromParent();