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/IR/Module.h"
46 #include "llvm/IR/MDBuilder.h"
47 #include "llvm/Support/CommandLine.h"
48 #include "llvm/Support/Debug.h"
49 #include "llvm/Support/raw_ostream.h"
50 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
51 #include "llvm/Transforms/Utils/Cloning.h"
52 #include "llvm/Transforms/Utils/Local.h"
58 #define DEBUG_TYPE "loop-unswitch"
60 STATISTIC(NumBranches, "Number of branches unswitched");
61 STATISTIC(NumSwitches, "Number of switches unswitched");
62 STATISTIC(NumSelects , "Number of selects unswitched");
63 STATISTIC(NumTrivial , "Number of unswitches that are trivial");
64 STATISTIC(NumSimplify, "Number of simplifications of unswitched code");
65 STATISTIC(TotalInsts, "Total number of instructions analyzed");
67 // The specific value of 100 here was chosen based only on intuition and a
68 // few specific examples.
69 static cl::opt<unsigned>
70 Threshold("loop-unswitch-threshold", cl::desc("Max loop size to unswitch"),
71 cl::init(100), cl::Hidden);
75 class LUAnalysisCache {
77 typedef DenseMap<const SwitchInst*, SmallPtrSet<const Value *, 8> >
80 typedef UnswitchedValsMap::iterator UnswitchedValsIt;
82 struct LoopProperties {
83 unsigned CanBeUnswitchedCount;
84 unsigned WasUnswitchedCount;
85 unsigned SizeEstimation;
86 UnswitchedValsMap UnswitchedVals;
89 // Here we use std::map instead of DenseMap, since we need to keep valid
90 // LoopProperties pointer for current loop for better performance.
91 typedef std::map<const Loop*, LoopProperties> LoopPropsMap;
92 typedef LoopPropsMap::iterator LoopPropsMapIt;
94 LoopPropsMap LoopsProperties;
95 UnswitchedValsMap *CurLoopInstructions;
96 LoopProperties *CurrentLoopProperties;
98 // A loop unswitching with an estimated cost above this threshold
99 // is not performed. MaxSize is turned into unswitching quota for
100 // the current loop, and reduced correspondingly, though note that
101 // the quota is returned by releaseMemory() when the loop has been
102 // processed, so that MaxSize will return to its previous
103 // value. So in most cases MaxSize will equal the Threshold flag
104 // when a new loop is processed. An exception to that is that
105 // MaxSize will have a smaller value while processing nested loops
106 // that were introduced due to loop unswitching of an outer loop.
108 // FIXME: The way that MaxSize works is subtle and depends on the
109 // pass manager processing loops and calling releaseMemory() in a
110 // specific order. It would be good to find a more straightforward
111 // way of doing what MaxSize does.
116 : CurLoopInstructions(nullptr), CurrentLoopProperties(nullptr),
117 MaxSize(Threshold) {}
119 // Analyze loop. Check its size, calculate is it possible to unswitch
120 // it. Returns true if we can unswitch this loop.
121 bool countLoop(const Loop *L, const TargetTransformInfo &TTI,
122 AssumptionCache *AC);
124 // Clean all data related to given loop.
125 void forgetLoop(const Loop *L);
127 // Mark case value as unswitched.
128 // Since SI instruction can be partly unswitched, in order to avoid
129 // extra unswitching in cloned loops keep track all unswitched values.
130 void setUnswitched(const SwitchInst *SI, const Value *V);
132 // Check was this case value unswitched before or not.
133 bool isUnswitched(const SwitchInst *SI, const Value *V);
135 // Returns true if another unswitching could be done within the cost
137 bool CostAllowsUnswitching();
139 // Clone all loop-unswitch related loop properties.
140 // Redistribute unswitching quotas.
141 // Note, that new loop data is stored inside the VMap.
142 void cloneData(const Loop *NewLoop, const Loop *OldLoop,
143 const ValueToValueMapTy &VMap);
146 class LoopUnswitch : public LoopPass {
147 LoopInfo *LI; // Loop information
151 // LoopProcessWorklist - Used to check if second loop needs processing
152 // after RewriteLoopBodyWithConditionConstant rewrites first loop.
153 std::vector<Loop*> LoopProcessWorklist;
155 LUAnalysisCache BranchesInfo;
157 bool OptimizeForSize;
162 BasicBlock *loopHeader;
163 BasicBlock *loopPreheader;
165 // LoopBlocks contains all of the basic blocks of the loop, including the
166 // preheader of the loop, the body of the loop, and the exit blocks of the
167 // loop, in that order.
168 std::vector<BasicBlock*> LoopBlocks;
169 // NewBlocks contained cloned copy of basic blocks from LoopBlocks.
170 std::vector<BasicBlock*> NewBlocks;
173 static char ID; // Pass ID, replacement for typeid
174 explicit LoopUnswitch(bool Os = false) :
175 LoopPass(ID), OptimizeForSize(Os), redoLoop(false),
176 currentLoop(nullptr), DT(nullptr), loopHeader(nullptr),
177 loopPreheader(nullptr) {
178 initializeLoopUnswitchPass(*PassRegistry::getPassRegistry());
181 bool runOnLoop(Loop *L, LPPassManager &LPM) override;
182 bool processCurrentLoop();
184 /// This transformation requires natural loop information & requires that
185 /// loop preheaders be inserted into the CFG.
187 void getAnalysisUsage(AnalysisUsage &AU) const override {
188 AU.addRequired<AssumptionCacheTracker>();
189 AU.addRequiredID(LoopSimplifyID);
190 AU.addPreservedID(LoopSimplifyID);
191 AU.addRequired<LoopInfoWrapperPass>();
192 AU.addPreserved<LoopInfoWrapperPass>();
193 AU.addRequiredID(LCSSAID);
194 AU.addPreservedID(LCSSAID);
195 AU.addPreserved<DominatorTreeWrapperPass>();
196 AU.addPreserved<ScalarEvolution>();
197 AU.addRequired<TargetTransformInfoWrapperPass>();
202 void releaseMemory() override {
203 BranchesInfo.forgetLoop(currentLoop);
206 void initLoopData() {
207 loopHeader = currentLoop->getHeader();
208 loopPreheader = currentLoop->getLoopPreheader();
211 /// Split all of the edges from inside the loop to their exit blocks.
212 /// Update the appropriate Phi nodes as we do so.
213 void SplitExitEdges(Loop *L, const SmallVectorImpl<BasicBlock *> &ExitBlocks);
215 bool UnswitchIfProfitable(Value *LoopCond, Constant *Val,
216 TerminatorInst *TI = nullptr);
217 void UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val,
218 BasicBlock *ExitBlock, TerminatorInst *TI);
219 void UnswitchNontrivialCondition(Value *LIC, Constant *OnVal, Loop *L,
222 void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
223 Constant *Val, bool isEqual);
225 void EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val,
226 BasicBlock *TrueDest,
227 BasicBlock *FalseDest,
228 Instruction *InsertPt,
231 void SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L);
232 bool IsTrivialUnswitchCondition(Value *Cond, Constant **Val = nullptr,
233 BasicBlock **LoopExit = nullptr);
238 // Analyze loop. Check its size, calculate is it possible to unswitch
239 // it. Returns true if we can unswitch this loop.
240 bool LUAnalysisCache::countLoop(const Loop *L, const TargetTransformInfo &TTI,
241 AssumptionCache *AC) {
243 LoopPropsMapIt PropsIt;
245 std::tie(PropsIt, Inserted) =
246 LoopsProperties.insert(std::make_pair(L, LoopProperties()));
248 LoopProperties &Props = PropsIt->second;
253 // Limit the number of instructions to avoid causing significant code
254 // expansion, and the number of basic blocks, to avoid loops with
255 // large numbers of branches which cause loop unswitching to go crazy.
256 // This is a very ad-hoc heuristic.
258 SmallPtrSet<const Value *, 32> EphValues;
259 CodeMetrics::collectEphemeralValues(L, AC, EphValues);
261 // FIXME: This is overly conservative because it does not take into
262 // consideration code simplification opportunities and code that can
263 // be shared by the resultant unswitched loops.
265 for (Loop::block_iterator I = L->block_begin(), E = L->block_end(); I != E;
267 Metrics.analyzeBasicBlock(*I, TTI, EphValues);
269 Props.SizeEstimation = Metrics.NumInsts;
270 Props.CanBeUnswitchedCount = MaxSize / (Props.SizeEstimation);
271 Props.WasUnswitchedCount = 0;
272 MaxSize -= Props.SizeEstimation * Props.CanBeUnswitchedCount;
274 if (Metrics.notDuplicatable) {
275 DEBUG(dbgs() << "NOT unswitching loop %"
276 << L->getHeader()->getName() << ", contents cannot be "
282 // Be careful. This links are good only before new loop addition.
283 CurrentLoopProperties = &Props;
284 CurLoopInstructions = &Props.UnswitchedVals;
289 // Clean all data related to given loop.
290 void LUAnalysisCache::forgetLoop(const Loop *L) {
292 LoopPropsMapIt LIt = LoopsProperties.find(L);
294 if (LIt != LoopsProperties.end()) {
295 LoopProperties &Props = LIt->second;
296 MaxSize += (Props.CanBeUnswitchedCount + Props.WasUnswitchedCount) *
297 Props.SizeEstimation;
298 LoopsProperties.erase(LIt);
301 CurrentLoopProperties = nullptr;
302 CurLoopInstructions = nullptr;
305 // Mark case value as unswitched.
306 // Since SI instruction can be partly unswitched, in order to avoid
307 // extra unswitching in cloned loops keep track all unswitched values.
308 void LUAnalysisCache::setUnswitched(const SwitchInst *SI, const Value *V) {
309 (*CurLoopInstructions)[SI].insert(V);
312 // Check was this case value unswitched before or not.
313 bool LUAnalysisCache::isUnswitched(const SwitchInst *SI, const Value *V) {
314 return (*CurLoopInstructions)[SI].count(V);
317 bool LUAnalysisCache::CostAllowsUnswitching() {
318 return CurrentLoopProperties->CanBeUnswitchedCount > 0;
321 // Clone all loop-unswitch related loop properties.
322 // Redistribute unswitching quotas.
323 // Note, that new loop data is stored inside the VMap.
324 void LUAnalysisCache::cloneData(const Loop *NewLoop, const Loop *OldLoop,
325 const ValueToValueMapTy &VMap) {
327 LoopProperties &NewLoopProps = LoopsProperties[NewLoop];
328 LoopProperties &OldLoopProps = *CurrentLoopProperties;
329 UnswitchedValsMap &Insts = OldLoopProps.UnswitchedVals;
331 // Reallocate "can-be-unswitched quota"
333 --OldLoopProps.CanBeUnswitchedCount;
334 ++OldLoopProps.WasUnswitchedCount;
335 NewLoopProps.WasUnswitchedCount = 0;
336 unsigned Quota = OldLoopProps.CanBeUnswitchedCount;
337 NewLoopProps.CanBeUnswitchedCount = Quota / 2;
338 OldLoopProps.CanBeUnswitchedCount = Quota - Quota / 2;
340 NewLoopProps.SizeEstimation = OldLoopProps.SizeEstimation;
342 // Clone unswitched values info:
343 // for new loop switches we clone info about values that was
344 // already unswitched and has redundant successors.
345 for (UnswitchedValsIt I = Insts.begin(); I != Insts.end(); ++I) {
346 const SwitchInst *OldInst = I->first;
347 Value *NewI = VMap.lookup(OldInst);
348 const SwitchInst *NewInst = cast_or_null<SwitchInst>(NewI);
349 assert(NewInst && "All instructions that are in SrcBB must be in VMap.");
351 NewLoopProps.UnswitchedVals[NewInst] = OldLoopProps.UnswitchedVals[OldInst];
355 char LoopUnswitch::ID = 0;
356 INITIALIZE_PASS_BEGIN(LoopUnswitch, "loop-unswitch", "Unswitch loops",
358 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
359 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
360 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
361 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
362 INITIALIZE_PASS_DEPENDENCY(LCSSA)
363 INITIALIZE_PASS_END(LoopUnswitch, "loop-unswitch", "Unswitch loops",
366 Pass *llvm::createLoopUnswitchPass(bool Os) {
367 return new LoopUnswitch(Os);
370 /// FindLIVLoopCondition - Cond is a condition that occurs in L. If it is
371 /// invariant in the loop, or has an invariant piece, return the invariant.
372 /// Otherwise, return null.
373 static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) {
375 // We started analyze new instruction, increment scanned instructions counter.
378 // We can never unswitch on vector conditions.
379 if (Cond->getType()->isVectorTy())
382 // Constants should be folded, not unswitched on!
383 if (isa<Constant>(Cond)) return nullptr;
385 // TODO: Handle: br (VARIANT|INVARIANT).
387 // Hoist simple values out.
388 if (L->makeLoopInvariant(Cond, Changed))
391 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond))
392 if (BO->getOpcode() == Instruction::And ||
393 BO->getOpcode() == Instruction::Or) {
394 // If either the left or right side is invariant, we can unswitch on this,
395 // which will cause the branch to go away in one loop and the condition to
396 // simplify in the other one.
397 if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed))
399 if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed))
406 bool LoopUnswitch::runOnLoop(Loop *L, LPPassManager &LPM_Ref) {
407 if (skipOptnoneFunction(L))
410 AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(
411 *L->getHeader()->getParent());
412 LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
414 DominatorTreeWrapperPass *DTWP =
415 getAnalysisIfAvailable<DominatorTreeWrapperPass>();
416 DT = DTWP ? &DTWP->getDomTree() : nullptr;
418 Function *F = currentLoop->getHeader()->getParent();
419 bool Changed = false;
421 assert(currentLoop->isLCSSAForm(*DT));
423 Changed |= processCurrentLoop();
427 // FIXME: Reconstruct dom info, because it is not preserved properly.
434 /// processCurrentLoop - Do actual work and unswitch loop if possible
436 bool LoopUnswitch::processCurrentLoop() {
437 bool Changed = false;
441 // If LoopSimplify was unable to form a preheader, don't do any unswitching.
445 // Loops with indirectbr cannot be cloned.
446 if (!currentLoop->isSafeToClone())
449 // Without dedicated exits, splitting the exit edge may fail.
450 if (!currentLoop->hasDedicatedExits())
453 LLVMContext &Context = loopHeader->getContext();
455 // Probably we reach the quota of branches for this loop. If so
457 if (!BranchesInfo.countLoop(
458 currentLoop, getAnalysis<TargetTransformInfoWrapperPass>().getTTI(
459 *currentLoop->getHeader()->getParent()),
463 // Loop over all of the basic blocks in the loop. If we find an interior
464 // block that is branching on a loop-invariant condition, we can unswitch this
466 for (Loop::block_iterator I = currentLoop->block_begin(),
467 E = currentLoop->block_end(); I != E; ++I) {
468 TerminatorInst *TI = (*I)->getTerminator();
469 if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
470 // If this isn't branching on an invariant condition, we can't unswitch
472 if (BI->isConditional()) {
473 // See if this, or some part of it, is loop invariant. If so, we can
474 // unswitch on it if we desire.
475 Value *LoopCond = FindLIVLoopCondition(BI->getCondition(),
476 currentLoop, Changed);
478 UnswitchIfProfitable(LoopCond, ConstantInt::getTrue(Context), TI)) {
483 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
484 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(),
485 currentLoop, Changed);
486 unsigned NumCases = SI->getNumCases();
487 if (LoopCond && NumCases) {
488 // Find a value to unswitch on:
489 // FIXME: this should chose the most expensive case!
490 // FIXME: scan for a case with a non-critical edge?
491 Constant *UnswitchVal = nullptr;
493 // Do not process same value again and again.
494 // At this point we have some cases already unswitched and
495 // some not yet unswitched. Let's find the first not yet unswitched one.
496 for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end();
498 Constant *UnswitchValCandidate = i.getCaseValue();
499 if (!BranchesInfo.isUnswitched(SI, UnswitchValCandidate)) {
500 UnswitchVal = UnswitchValCandidate;
508 if (UnswitchIfProfitable(LoopCond, UnswitchVal)) {
515 // Scan the instructions to check for unswitchable values.
516 for (BasicBlock::iterator BBI = (*I)->begin(), E = (*I)->end();
518 if (SelectInst *SI = dyn_cast<SelectInst>(BBI)) {
519 Value *LoopCond = FindLIVLoopCondition(SI->getCondition(),
520 currentLoop, Changed);
521 if (LoopCond && UnswitchIfProfitable(LoopCond,
522 ConstantInt::getTrue(Context))) {
531 /// isTrivialLoopExitBlock - Check to see if all paths from BB exit the
532 /// loop with no side effects (including infinite loops).
534 /// If true, we return true and set ExitBB to the block we
537 static bool isTrivialLoopExitBlockHelper(Loop *L, BasicBlock *BB,
539 std::set<BasicBlock*> &Visited) {
540 if (!Visited.insert(BB).second) {
541 // Already visited. Without more analysis, this could indicate an infinite
545 if (!L->contains(BB)) {
546 // Otherwise, this is a loop exit, this is fine so long as this is the
548 if (ExitBB) return false;
553 // Otherwise, this is an unvisited intra-loop node. Check all successors.
554 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) {
555 // Check to see if the successor is a trivial loop exit.
556 if (!isTrivialLoopExitBlockHelper(L, *SI, ExitBB, Visited))
560 // Okay, everything after this looks good, check to make sure that this block
561 // doesn't include any side effects.
562 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
563 if (I->mayHaveSideEffects())
569 /// isTrivialLoopExitBlock - Return true if the specified block unconditionally
570 /// leads to an exit from the specified loop, and has no side-effects in the
571 /// process. If so, return the block that is exited to, otherwise return null.
572 static BasicBlock *isTrivialLoopExitBlock(Loop *L, BasicBlock *BB) {
573 std::set<BasicBlock*> Visited;
574 Visited.insert(L->getHeader()); // Branches to header make infinite loops.
575 BasicBlock *ExitBB = nullptr;
576 if (isTrivialLoopExitBlockHelper(L, BB, ExitBB, Visited))
581 /// IsTrivialUnswitchCondition - Check to see if this unswitch condition is
582 /// trivial: that is, that the condition controls whether or not the loop does
583 /// anything at all. If this is a trivial condition, unswitching produces no
584 /// code duplications (equivalently, it produces a simpler loop and a new empty
585 /// loop, which gets deleted).
587 /// If this is a trivial condition, return true, otherwise return false. When
588 /// returning true, this sets Cond and Val to the condition that controls the
589 /// trivial condition: when Cond dynamically equals Val, the loop is known to
590 /// exit. Finally, this sets LoopExit to the BB that the loop exits to when
593 bool LoopUnswitch::IsTrivialUnswitchCondition(Value *Cond, Constant **Val,
594 BasicBlock **LoopExit) {
595 BasicBlock *Header = currentLoop->getHeader();
596 TerminatorInst *HeaderTerm = Header->getTerminator();
597 LLVMContext &Context = Header->getContext();
599 BasicBlock *LoopExitBB = nullptr;
600 if (BranchInst *BI = dyn_cast<BranchInst>(HeaderTerm)) {
601 // If the header block doesn't end with a conditional branch on Cond, we
603 if (!BI->isConditional() || BI->getCondition() != Cond)
606 // Check to see if a successor of the branch is guaranteed to
607 // exit through a unique exit block without having any
608 // side-effects. If so, determine the value of Cond that causes it to do
610 if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
611 BI->getSuccessor(0)))) {
612 if (Val) *Val = ConstantInt::getTrue(Context);
613 } else if ((LoopExitBB = isTrivialLoopExitBlock(currentLoop,
614 BI->getSuccessor(1)))) {
615 if (Val) *Val = ConstantInt::getFalse(Context);
617 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(HeaderTerm)) {
618 // If this isn't a switch on Cond, we can't handle it.
619 if (SI->getCondition() != Cond) return false;
621 // Check to see if a successor of the switch is guaranteed to go to the
622 // latch block or exit through a one exit block without having any
623 // side-effects. If so, determine the value of Cond that causes it to do
625 // Note that we can't trivially unswitch on the default case or
626 // on already unswitched cases.
627 for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end();
629 BasicBlock *LoopExitCandidate;
630 if ((LoopExitCandidate = isTrivialLoopExitBlock(currentLoop,
631 i.getCaseSuccessor()))) {
632 // Okay, we found a trivial case, remember the value that is trivial.
633 ConstantInt *CaseVal = i.getCaseValue();
635 // Check that it was not unswitched before, since already unswitched
636 // trivial vals are looks trivial too.
637 if (BranchesInfo.isUnswitched(SI, CaseVal))
639 LoopExitBB = LoopExitCandidate;
640 if (Val) *Val = CaseVal;
646 // If we didn't find a single unique LoopExit block, or if the loop exit block
647 // contains phi nodes, this isn't trivial.
648 if (!LoopExitBB || isa<PHINode>(LoopExitBB->begin()))
649 return false; // Can't handle this.
651 if (LoopExit) *LoopExit = LoopExitBB;
653 // We already know that nothing uses any scalar values defined inside of this
654 // loop. As such, we just have to check to see if this loop will execute any
655 // side-effecting instructions (e.g. stores, calls, volatile loads) in the
656 // part of the loop that the code *would* execute. We already checked the
657 // tail, check the header now.
658 for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E; ++I)
659 if (I->mayHaveSideEffects())
664 /// UnswitchIfProfitable - We have found that we can unswitch currentLoop when
665 /// LoopCond == Val to simplify the loop. If we decide that this is profitable,
666 /// unswitch the loop, reprocess the pieces, then return true.
667 bool LoopUnswitch::UnswitchIfProfitable(Value *LoopCond, Constant *Val,
668 TerminatorInst *TI) {
669 Function *F = loopHeader->getParent();
670 Constant *CondVal = nullptr;
671 BasicBlock *ExitBlock = nullptr;
673 if (IsTrivialUnswitchCondition(LoopCond, &CondVal, &ExitBlock)) {
674 // If the condition is trivial, always unswitch. There is no code growth
676 UnswitchTrivialCondition(currentLoop, LoopCond, CondVal, ExitBlock, TI);
680 // Check to see if it would be profitable to unswitch current loop.
681 if (!BranchesInfo.CostAllowsUnswitching()) {
682 DEBUG(dbgs() << "NOT unswitching loop %"
683 << currentLoop->getHeader()->getName()
684 << " at non-trivial condition '" << *Val
685 << "' == " << *LoopCond << "\n"
686 << ". Cost too high.\n");
690 // Do not do non-trivial unswitch while optimizing for size.
691 if (OptimizeForSize || F->hasFnAttribute(Attribute::OptimizeForSize))
694 UnswitchNontrivialCondition(LoopCond, Val, currentLoop, TI);
698 /// CloneLoop - Recursively clone the specified loop and all of its children,
699 /// mapping the blocks with the specified map.
700 static Loop *CloneLoop(Loop *L, Loop *PL, ValueToValueMapTy &VM,
701 LoopInfo *LI, LPPassManager *LPM) {
702 Loop *New = new Loop();
703 LPM->insertLoop(New, PL);
705 // Add all of the blocks in L to the new loop.
706 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
708 if (LI->getLoopFor(*I) == L)
709 New->addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), *LI);
711 // Add all of the subloops to the new loop.
712 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
713 CloneLoop(*I, New, VM, LI, LPM);
718 static void copyMetadata(Instruction *DstInst, const Instruction *SrcInst,
720 if (!SrcInst || !SrcInst->hasMetadata())
723 SmallVector<std::pair<unsigned, MDNode *>, 4> MDs;
724 SrcInst->getAllMetadata(MDs);
725 for (auto &MD : MDs) {
729 case LLVMContext::MD_prof:
730 if (Swapped && MD.second->getNumOperands() == 3 &&
731 isa<MDString>(MD.second->getOperand(0))) {
732 MDString *MDName = cast<MDString>(MD.second->getOperand(0));
733 if (MDName->getString() == "branch_weights") {
734 auto *ValT = cast_or_null<ConstantAsMetadata>(
735 MD.second->getOperand(1))->getValue();
736 auto *ValF = cast_or_null<ConstantAsMetadata>(
737 MD.second->getOperand(2))->getValue();
738 assert(ValT && ValF && "Invalid Operands of branch_weights");
740 MDBuilder(DstInst->getParent()->getContext())
741 .createBranchWeights(cast<ConstantInt>(ValF)->getZExtValue(),
742 cast<ConstantInt>(ValT)->getZExtValue());
747 case LLVMContext::MD_dbg:
748 DstInst->setMetadata(MD.first, MD.second);
753 /// EmitPreheaderBranchOnCondition - Emit a conditional branch on two values
754 /// if LIC == Val, branch to TrueDst, otherwise branch to FalseDest. Insert the
755 /// code immediately before InsertPt.
756 void LoopUnswitch::EmitPreheaderBranchOnCondition(Value *LIC, Constant *Val,
757 BasicBlock *TrueDest,
758 BasicBlock *FalseDest,
759 Instruction *InsertPt,
760 TerminatorInst *TI) {
761 // Insert a conditional branch on LIC to the two preheaders. The original
762 // code is the true version and the new code is the false version.
763 Value *BranchVal = LIC;
764 bool Swapped = false;
765 if (!isa<ConstantInt>(Val) ||
766 Val->getType() != Type::getInt1Ty(LIC->getContext()))
767 BranchVal = new ICmpInst(InsertPt, ICmpInst::ICMP_EQ, LIC, Val);
768 else if (Val != ConstantInt::getTrue(Val->getContext())) {
769 // We want to enter the new loop when the condition is true.
770 std::swap(TrueDest, FalseDest);
774 // Insert the new branch.
775 BranchInst *BI = BranchInst::Create(TrueDest, FalseDest, BranchVal, InsertPt);
776 copyMetadata(BI, TI, Swapped);
778 // If either edge is critical, split it. This helps preserve LoopSimplify
779 // form for enclosing loops.
780 auto Options = CriticalEdgeSplittingOptions(DT, LI).setPreserveLCSSA();
781 SplitCriticalEdge(BI, 0, Options);
782 SplitCriticalEdge(BI, 1, Options);
785 /// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable
786 /// condition in it (a cond branch from its header block to its latch block,
787 /// where the path through the loop that doesn't execute its body has no
788 /// side-effects), unswitch it. This doesn't involve any code duplication, just
789 /// moving the conditional branch outside of the loop and updating loop info.
790 void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond, Constant *Val,
791 BasicBlock *ExitBlock,
792 TerminatorInst *TI) {
793 DEBUG(dbgs() << "loop-unswitch: Trivial-Unswitch loop %"
794 << loopHeader->getName() << " [" << L->getBlocks().size()
795 << " blocks] in Function "
796 << L->getHeader()->getParent()->getName() << " on cond: " << *Val
797 << " == " << *Cond << "\n");
799 // First step, split the preheader, so that we know that there is a safe place
800 // to insert the conditional branch. We will change loopPreheader to have a
801 // conditional branch on Cond.
802 BasicBlock *NewPH = SplitEdge(loopPreheader, loopHeader, DT, LI);
804 // Now that we have a place to insert the conditional branch, create a place
805 // to branch to: this is the exit block out of the loop that we should
808 // Split this block now, so that the loop maintains its exit block, and so
809 // that the jump from the preheader can execute the contents of the exit block
810 // without actually branching to it (the exit block should be dominated by the
811 // loop header, not the preheader).
812 assert(!L->contains(ExitBlock) && "Exit block is in the loop?");
813 BasicBlock *NewExit = SplitBlock(ExitBlock, ExitBlock->begin(), DT, LI);
815 // Okay, now we have a position to branch from and a position to branch to,
816 // insert the new conditional branch.
817 EmitPreheaderBranchOnCondition(Cond, Val, NewExit, NewPH,
818 loopPreheader->getTerminator(), TI);
819 LPM->deleteSimpleAnalysisValue(loopPreheader->getTerminator(), L);
820 loopPreheader->getTerminator()->eraseFromParent();
822 // We need to reprocess this loop, it could be unswitched again.
825 // Now that we know that the loop is never entered when this condition is a
826 // particular value, rewrite the loop with this info. We know that this will
827 // at least eliminate the old branch.
828 RewriteLoopBodyWithConditionConstant(L, Cond, Val, false);
832 /// SplitExitEdges - Split all of the edges from inside the loop to their exit
833 /// blocks. Update the appropriate Phi nodes as we do so.
834 void LoopUnswitch::SplitExitEdges(Loop *L,
835 const SmallVectorImpl<BasicBlock *> &ExitBlocks){
837 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
838 BasicBlock *ExitBlock = ExitBlocks[i];
839 SmallVector<BasicBlock *, 4> Preds(pred_begin(ExitBlock),
840 pred_end(ExitBlock));
842 // Although SplitBlockPredecessors doesn't preserve loop-simplify in
843 // general, if we call it on all predecessors of all exits then it does.
844 SplitBlockPredecessors(ExitBlock, Preds, ".us-lcssa",
845 /*AliasAnalysis*/ nullptr, DT, LI,
846 /*PreserveLCSSA*/ true);
850 /// UnswitchNontrivialCondition - We determined that the loop is profitable
851 /// to unswitch when LIC equal Val. Split it into loop versions and test the
852 /// condition outside of either loop. Return the loops created as Out1/Out2.
853 void LoopUnswitch::UnswitchNontrivialCondition(Value *LIC, Constant *Val,
854 Loop *L, TerminatorInst *TI) {
855 Function *F = loopHeader->getParent();
856 DEBUG(dbgs() << "loop-unswitch: Unswitching loop %"
857 << loopHeader->getName() << " [" << L->getBlocks().size()
858 << " blocks] in Function " << F->getName()
859 << " when '" << *Val << "' == " << *LIC << "\n");
861 if (ScalarEvolution *SE = getAnalysisIfAvailable<ScalarEvolution>())
867 // First step, split the preheader and exit blocks, and add these blocks to
868 // the LoopBlocks list.
869 BasicBlock *NewPreheader = SplitEdge(loopPreheader, loopHeader, DT, LI);
870 LoopBlocks.push_back(NewPreheader);
872 // We want the loop to come after the preheader, but before the exit blocks.
873 LoopBlocks.insert(LoopBlocks.end(), L->block_begin(), L->block_end());
875 SmallVector<BasicBlock*, 8> ExitBlocks;
876 L->getUniqueExitBlocks(ExitBlocks);
878 // Split all of the edges from inside the loop to their exit blocks. Update
879 // the appropriate Phi nodes as we do so.
880 SplitExitEdges(L, ExitBlocks);
882 // The exit blocks may have been changed due to edge splitting, recompute.
884 L->getUniqueExitBlocks(ExitBlocks);
886 // Add exit blocks to the loop blocks.
887 LoopBlocks.insert(LoopBlocks.end(), ExitBlocks.begin(), ExitBlocks.end());
889 // Next step, clone all of the basic blocks that make up the loop (including
890 // the loop preheader and exit blocks), keeping track of the mapping between
891 // the instructions and blocks.
892 NewBlocks.reserve(LoopBlocks.size());
893 ValueToValueMapTy VMap;
894 for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
895 BasicBlock *NewBB = CloneBasicBlock(LoopBlocks[i], VMap, ".us", F);
897 NewBlocks.push_back(NewBB);
898 VMap[LoopBlocks[i]] = NewBB; // Keep the BB mapping.
899 LPM->cloneBasicBlockSimpleAnalysis(LoopBlocks[i], NewBB, L);
902 // Splice the newly inserted blocks into the function right before the
903 // original preheader.
904 F->getBasicBlockList().splice(NewPreheader, F->getBasicBlockList(),
905 NewBlocks[0], F->end());
907 // FIXME: We could register any cloned assumptions instead of clearing the
908 // whole function's cache.
911 // Now we create the new Loop object for the versioned loop.
912 Loop *NewLoop = CloneLoop(L, L->getParentLoop(), VMap, LI, LPM);
914 // Recalculate unswitching quota, inherit simplified switches info for NewBB,
915 // Probably clone more loop-unswitch related loop properties.
916 BranchesInfo.cloneData(NewLoop, L, VMap);
918 Loop *ParentLoop = L->getParentLoop();
920 // Make sure to add the cloned preheader and exit blocks to the parent loop
922 ParentLoop->addBasicBlockToLoop(NewBlocks[0], *LI);
925 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
926 BasicBlock *NewExit = cast<BasicBlock>(VMap[ExitBlocks[i]]);
927 // The new exit block should be in the same loop as the old one.
928 if (Loop *ExitBBLoop = LI->getLoopFor(ExitBlocks[i]))
929 ExitBBLoop->addBasicBlockToLoop(NewExit, *LI);
931 assert(NewExit->getTerminator()->getNumSuccessors() == 1 &&
932 "Exit block should have been split to have one successor!");
933 BasicBlock *ExitSucc = NewExit->getTerminator()->getSuccessor(0);
935 // If the successor of the exit block had PHI nodes, add an entry for
937 for (BasicBlock::iterator I = ExitSucc->begin();
938 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
939 Value *V = PN->getIncomingValueForBlock(ExitBlocks[i]);
940 ValueToValueMapTy::iterator It = VMap.find(V);
941 if (It != VMap.end()) V = It->second;
942 PN->addIncoming(V, NewExit);
945 if (LandingPadInst *LPad = NewExit->getLandingPadInst()) {
946 PHINode *PN = PHINode::Create(LPad->getType(), 0, "",
947 ExitSucc->getFirstInsertionPt());
949 for (pred_iterator I = pred_begin(ExitSucc), E = pred_end(ExitSucc);
952 LandingPadInst *LPI = BB->getLandingPadInst();
953 LPI->replaceAllUsesWith(PN);
954 PN->addIncoming(LPI, BB);
959 // Rewrite the code to refer to itself.
960 for (unsigned i = 0, e = NewBlocks.size(); i != e; ++i)
961 for (BasicBlock::iterator I = NewBlocks[i]->begin(),
962 E = NewBlocks[i]->end(); I != E; ++I)
963 RemapInstruction(I, VMap,RF_NoModuleLevelChanges|RF_IgnoreMissingEntries);
965 // Rewrite the original preheader to select between versions of the loop.
966 BranchInst *OldBR = cast<BranchInst>(loopPreheader->getTerminator());
967 assert(OldBR->isUnconditional() && OldBR->getSuccessor(0) == LoopBlocks[0] &&
968 "Preheader splitting did not work correctly!");
970 // Emit the new branch that selects between the two versions of this loop.
971 EmitPreheaderBranchOnCondition(LIC, Val, NewBlocks[0], LoopBlocks[0], OldBR,
973 LPM->deleteSimpleAnalysisValue(OldBR, L);
974 OldBR->eraseFromParent();
976 LoopProcessWorklist.push_back(NewLoop);
979 // Keep a WeakVH holding onto LIC. If the first call to RewriteLoopBody
980 // deletes the instruction (for example by simplifying a PHI that feeds into
981 // the condition that we're unswitching on), we don't rewrite the second
983 WeakVH LICHandle(LIC);
985 // Now we rewrite the original code to know that the condition is true and the
986 // new code to know that the condition is false.
987 RewriteLoopBodyWithConditionConstant(L, LIC, Val, false);
989 // It's possible that simplifying one loop could cause the other to be
990 // changed to another value or a constant. If its a constant, don't simplify
992 if (!LoopProcessWorklist.empty() && LoopProcessWorklist.back() == NewLoop &&
993 LICHandle && !isa<Constant>(LICHandle))
994 RewriteLoopBodyWithConditionConstant(NewLoop, LICHandle, Val, true);
997 /// RemoveFromWorklist - Remove all instances of I from the worklist vector
999 static void RemoveFromWorklist(Instruction *I,
1000 std::vector<Instruction*> &Worklist) {
1002 Worklist.erase(std::remove(Worklist.begin(), Worklist.end(), I),
1006 /// ReplaceUsesOfWith - When we find that I really equals V, remove I from the
1007 /// program, replacing all uses with V and update the worklist.
1008 static void ReplaceUsesOfWith(Instruction *I, Value *V,
1009 std::vector<Instruction*> &Worklist,
1010 Loop *L, LPPassManager *LPM) {
1011 DEBUG(dbgs() << "Replace with '" << *V << "': " << *I);
1013 // Add uses to the worklist, which may be dead now.
1014 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
1015 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
1016 Worklist.push_back(Use);
1018 // Add users to the worklist which may be simplified now.
1019 for (User *U : I->users())
1020 Worklist.push_back(cast<Instruction>(U));
1021 LPM->deleteSimpleAnalysisValue(I, L);
1022 RemoveFromWorklist(I, Worklist);
1023 I->replaceAllUsesWith(V);
1024 I->eraseFromParent();
1028 // RewriteLoopBodyWithConditionConstant - We know either that the value LIC has
1029 // the value specified by Val in the specified loop, or we know it does NOT have
1030 // that value. Rewrite any uses of LIC or of properties correlated to it.
1031 void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
1034 assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?");
1036 // FIXME: Support correlated properties, like:
1043 // FOLD boolean conditions (X|LIC), (X&LIC). Fold conditional branches,
1044 // selects, switches.
1045 std::vector<Instruction*> Worklist;
1046 LLVMContext &Context = Val->getContext();
1048 // If we know that LIC == Val, or that LIC == NotVal, just replace uses of LIC
1049 // in the loop with the appropriate one directly.
1050 if (IsEqual || (isa<ConstantInt>(Val) &&
1051 Val->getType()->isIntegerTy(1))) {
1056 Replacement = ConstantInt::get(Type::getInt1Ty(Val->getContext()),
1057 !cast<ConstantInt>(Val)->getZExtValue());
1059 for (User *U : LIC->users()) {
1060 Instruction *UI = dyn_cast<Instruction>(U);
1061 if (!UI || !L->contains(UI))
1063 Worklist.push_back(UI);
1066 for (std::vector<Instruction*>::iterator UI = Worklist.begin(),
1067 UE = Worklist.end(); UI != UE; ++UI)
1068 (*UI)->replaceUsesOfWith(LIC, Replacement);
1070 SimplifyCode(Worklist, L);
1074 // Otherwise, we don't know the precise value of LIC, but we do know that it
1075 // is certainly NOT "Val". As such, simplify any uses in the loop that we
1076 // can. This case occurs when we unswitch switch statements.
1077 for (User *U : LIC->users()) {
1078 Instruction *UI = dyn_cast<Instruction>(U);
1079 if (!UI || !L->contains(UI))
1082 Worklist.push_back(UI);
1084 // TODO: We could do other simplifications, for example, turning
1085 // 'icmp eq LIC, Val' -> false.
1087 // If we know that LIC is not Val, use this info to simplify code.
1088 SwitchInst *SI = dyn_cast<SwitchInst>(UI);
1089 if (!SI || !isa<ConstantInt>(Val)) continue;
1091 SwitchInst::CaseIt DeadCase = SI->findCaseValue(cast<ConstantInt>(Val));
1092 // Default case is live for multiple values.
1093 if (DeadCase == SI->case_default()) continue;
1095 // Found a dead case value. Don't remove PHI nodes in the
1096 // successor if they become single-entry, those PHI nodes may
1097 // be in the Users list.
1099 BasicBlock *Switch = SI->getParent();
1100 BasicBlock *SISucc = DeadCase.getCaseSuccessor();
1101 BasicBlock *Latch = L->getLoopLatch();
1103 BranchesInfo.setUnswitched(SI, Val);
1105 if (!SI->findCaseDest(SISucc)) continue; // Edge is critical.
1106 // If the DeadCase successor dominates the loop latch, then the
1107 // transformation isn't safe since it will delete the sole predecessor edge
1109 if (Latch && DT->dominates(SISucc, Latch))
1112 // FIXME: This is a hack. We need to keep the successor around
1113 // and hooked up so as to preserve the loop structure, because
1114 // trying to update it is complicated. So instead we preserve the
1115 // loop structure and put the block on a dead code path.
1116 SplitEdge(Switch, SISucc, DT, LI);
1117 // Compute the successors instead of relying on the return value
1118 // of SplitEdge, since it may have split the switch successor
1120 BasicBlock *NewSISucc = DeadCase.getCaseSuccessor();
1121 BasicBlock *OldSISucc = *succ_begin(NewSISucc);
1122 // Create an "unreachable" destination.
1123 BasicBlock *Abort = BasicBlock::Create(Context, "us-unreachable",
1124 Switch->getParent(),
1126 new UnreachableInst(Context, Abort);
1127 // Force the new case destination to branch to the "unreachable"
1128 // block while maintaining a (dead) CFG edge to the old block.
1129 NewSISucc->getTerminator()->eraseFromParent();
1130 BranchInst::Create(Abort, OldSISucc,
1131 ConstantInt::getTrue(Context), NewSISucc);
1132 // Release the PHI operands for this edge.
1133 for (BasicBlock::iterator II = NewSISucc->begin();
1134 PHINode *PN = dyn_cast<PHINode>(II); ++II)
1135 PN->setIncomingValue(PN->getBasicBlockIndex(Switch),
1136 UndefValue::get(PN->getType()));
1137 // Tell the domtree about the new block. We don't fully update the
1138 // domtree here -- instead we force it to do a full recomputation
1139 // after the pass is complete -- but we do need to inform it of
1142 DT->addNewBlock(Abort, NewSISucc);
1145 SimplifyCode(Worklist, L);
1148 /// SimplifyCode - Okay, now that we have simplified some instructions in the
1149 /// loop, walk over it and constant prop, dce, and fold control flow where
1150 /// possible. Note that this is effectively a very simple loop-structure-aware
1151 /// optimizer. During processing of this loop, L could very well be deleted, so
1152 /// it must not be used.
1154 /// FIXME: When the loop optimizer is more mature, separate this out to a new
1157 void LoopUnswitch::SimplifyCode(std::vector<Instruction*> &Worklist, Loop *L) {
1158 const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
1159 while (!Worklist.empty()) {
1160 Instruction *I = Worklist.back();
1161 Worklist.pop_back();
1164 if (isInstructionTriviallyDead(I)) {
1165 DEBUG(dbgs() << "Remove dead instruction '" << *I);
1167 // Add uses to the worklist, which may be dead now.
1168 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
1169 if (Instruction *Use = dyn_cast<Instruction>(I->getOperand(i)))
1170 Worklist.push_back(Use);
1171 LPM->deleteSimpleAnalysisValue(I, L);
1172 RemoveFromWorklist(I, Worklist);
1173 I->eraseFromParent();
1178 // See if instruction simplification can hack this up. This is common for
1179 // things like "select false, X, Y" after unswitching made the condition be
1180 // 'false'. TODO: update the domtree properly so we can pass it here.
1181 if (Value *V = SimplifyInstruction(I, DL))
1182 if (LI->replacementPreservesLCSSAForm(I, V)) {
1183 ReplaceUsesOfWith(I, V, Worklist, L, LPM);
1187 // Special case hacks that appear commonly in unswitched code.
1188 if (BranchInst *BI = dyn_cast<BranchInst>(I)) {
1189 if (BI->isUnconditional()) {
1190 // If BI's parent is the only pred of the successor, fold the two blocks
1192 BasicBlock *Pred = BI->getParent();
1193 BasicBlock *Succ = BI->getSuccessor(0);
1194 BasicBlock *SinglePred = Succ->getSinglePredecessor();
1195 if (!SinglePred) continue; // Nothing to do.
1196 assert(SinglePred == Pred && "CFG broken");
1198 DEBUG(dbgs() << "Merging blocks: " << Pred->getName() << " <- "
1199 << Succ->getName() << "\n");
1201 // Resolve any single entry PHI nodes in Succ.
1202 while (PHINode *PN = dyn_cast<PHINode>(Succ->begin()))
1203 ReplaceUsesOfWith(PN, PN->getIncomingValue(0), Worklist, L, LPM);
1205 // If Succ has any successors with PHI nodes, update them to have
1206 // entries coming from Pred instead of Succ.
1207 Succ->replaceAllUsesWith(Pred);
1209 // Move all of the successor contents from Succ to Pred.
1210 Pred->getInstList().splice(BI, Succ->getInstList(), Succ->begin(),
1212 LPM->deleteSimpleAnalysisValue(BI, L);
1213 BI->eraseFromParent();
1214 RemoveFromWorklist(BI, Worklist);
1216 // Remove Succ from the loop tree.
1217 LI->removeBlock(Succ);
1218 LPM->deleteSimpleAnalysisValue(Succ, L);
1219 Succ->eraseFromParent();