1 //===- TailRecursionElimination.cpp - Eliminate Tail Calls ----------------===//
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
7 //===----------------------------------------------------------------------===//
9 // This file transforms calls of the current function (self recursion) followed
10 // by a return instruction with a branch to the entry of the function, creating
11 // a loop. This pass also implements the following extensions to the basic
14 // 1. Trivial instructions between the call and return do not prevent the
15 // transformation from taking place, though currently the analysis cannot
16 // support moving any really useful instructions (only dead ones).
17 // 2. This pass transforms functions that are prevented from being tail
18 // recursive by an associative and commutative expression to use an
19 // accumulator variable, thus compiling the typical naive factorial or
20 // 'fib' implementation into efficient code.
21 // 3. TRE is performed if the function returns void, if the return
22 // returns the result returned by the call, or if the function returns a
23 // run-time constant on all exits from the function. It is possible, though
24 // unlikely, that the return returns something else (like constant 0), and
25 // can still be TRE'd. It can be TRE'd if ALL OTHER return instructions in
26 // the function return the exact same value.
27 // 4. If it can prove that callees do not access their caller stack frame,
28 // they are marked as eligible for tail call elimination (by the code
31 // There are several improvements that could be made:
33 // 1. If the function has any alloca instructions, these instructions will be
34 // moved out of the entry block of the function, causing them to be
35 // evaluated each time through the tail recursion. Safely keeping allocas
36 // in the entry block requires analysis to proves that the tail-called
37 // function does not read or write the stack object.
38 // 2. Tail recursion is only performed if the call immediately precedes the
39 // return instruction. It's possible that there could be a jump between
40 // the call and the return.
41 // 3. There can be intervening operations between the call and the return that
42 // prevent the TRE from occurring. For example, there could be GEP's and
43 // stores to memory that will not be read or written by the call. This
44 // requires some substantial analysis (such as with DSA) to prove safe to
45 // move ahead of the call, but doing so could allow many more TREs to be
46 // performed, for example in TreeAdd/TreeAlloc from the treeadd benchmark.
47 // 4. The algorithm we use to detect if callees access their caller stack
48 // frames is very primitive.
50 //===----------------------------------------------------------------------===//
52 #include "llvm/Transforms/Scalar/TailRecursionElimination.h"
53 #include "llvm/ADT/STLExtras.h"
54 #include "llvm/ADT/SmallPtrSet.h"
55 #include "llvm/ADT/Statistic.h"
56 #include "llvm/Analysis/CFG.h"
57 #include "llvm/Analysis/CaptureTracking.h"
58 #include "llvm/Analysis/DomTreeUpdater.h"
59 #include "llvm/Analysis/GlobalsModRef.h"
60 #include "llvm/Analysis/InlineCost.h"
61 #include "llvm/Analysis/InstructionSimplify.h"
62 #include "llvm/Analysis/Loads.h"
63 #include "llvm/Analysis/OptimizationRemarkEmitter.h"
64 #include "llvm/Analysis/PostDominators.h"
65 #include "llvm/Analysis/TargetTransformInfo.h"
66 #include "llvm/IR/CFG.h"
67 #include "llvm/IR/CallSite.h"
68 #include "llvm/IR/Constants.h"
69 #include "llvm/IR/DataLayout.h"
70 #include "llvm/IR/DerivedTypes.h"
71 #include "llvm/IR/DiagnosticInfo.h"
72 #include "llvm/IR/Dominators.h"
73 #include "llvm/IR/Function.h"
74 #include "llvm/IR/InstIterator.h"
75 #include "llvm/IR/Instructions.h"
76 #include "llvm/IR/IntrinsicInst.h"
77 #include "llvm/IR/Module.h"
78 #include "llvm/IR/ValueHandle.h"
79 #include "llvm/InitializePasses.h"
80 #include "llvm/Pass.h"
81 #include "llvm/Support/Debug.h"
82 #include "llvm/Support/raw_ostream.h"
83 #include "llvm/Transforms/Scalar.h"
84 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
87 #define DEBUG_TYPE "tailcallelim"
89 STATISTIC(NumEliminated, "Number of tail calls removed");
90 STATISTIC(NumRetDuped, "Number of return duplicated");
91 STATISTIC(NumAccumAdded, "Number of accumulators introduced");
93 /// Scan the specified function for alloca instructions.
94 /// If it contains any dynamic allocas, returns false.
95 static bool canTRE(Function &F) {
96 // Because of PR962, we don't TRE dynamic allocas.
97 return llvm::all_of(instructions(F), [](Instruction &I) {
98 auto *AI = dyn_cast<AllocaInst>(&I);
99 return !AI || AI->isStaticAlloca();
104 struct AllocaDerivedValueTracker {
105 // Start at a root value and walk its use-def chain to mark calls that use the
106 // value or a derived value in AllocaUsers, and places where it may escape in
108 void walk(Value *Root) {
109 SmallVector<Use *, 32> Worklist;
110 SmallPtrSet<Use *, 32> Visited;
112 auto AddUsesToWorklist = [&](Value *V) {
113 for (auto &U : V->uses()) {
114 if (!Visited.insert(&U).second)
116 Worklist.push_back(&U);
120 AddUsesToWorklist(Root);
122 while (!Worklist.empty()) {
123 Use *U = Worklist.pop_back_val();
124 Instruction *I = cast<Instruction>(U->getUser());
126 switch (I->getOpcode()) {
127 case Instruction::Call:
128 case Instruction::Invoke: {
130 // If the alloca-derived argument is passed byval it is not an escape
131 // point, or a use of an alloca. Calling with byval copies the contents
132 // of the alloca into argument registers or stack slots, which exist
133 // beyond the lifetime of the current frame.
134 if (CS.isArgOperand(U) && CS.isByValArgument(CS.getArgumentNo(U)))
137 CS.isDataOperand(U) && CS.doesNotCapture(CS.getDataOperandNo(U));
138 callUsesLocalStack(CS, IsNocapture);
140 // If the alloca-derived argument is passed in as nocapture, then it
141 // can't propagate to the call's return. That would be capturing.
146 case Instruction::Load: {
147 // The result of a load is not alloca-derived (unless an alloca has
148 // otherwise escaped, but this is a local analysis).
151 case Instruction::Store: {
152 if (U->getOperandNo() == 0)
153 EscapePoints.insert(I);
154 continue; // Stores have no users to analyze.
156 case Instruction::BitCast:
157 case Instruction::GetElementPtr:
158 case Instruction::PHI:
159 case Instruction::Select:
160 case Instruction::AddrSpaceCast:
163 EscapePoints.insert(I);
167 AddUsesToWorklist(I);
171 void callUsesLocalStack(CallSite CS, bool IsNocapture) {
172 // Add it to the list of alloca users.
173 AllocaUsers.insert(CS.getInstruction());
175 // If it's nocapture then it can't capture this alloca.
179 // If it can write to memory, it can leak the alloca value.
180 if (!CS.onlyReadsMemory())
181 EscapePoints.insert(CS.getInstruction());
184 SmallPtrSet<Instruction *, 32> AllocaUsers;
185 SmallPtrSet<Instruction *, 32> EscapePoints;
189 static bool markTails(Function &F, bool &AllCallsAreTailCalls,
190 OptimizationRemarkEmitter *ORE) {
191 if (F.callsFunctionThatReturnsTwice())
193 AllCallsAreTailCalls = true;
195 // The local stack holds all alloca instructions and all byval arguments.
196 AllocaDerivedValueTracker Tracker;
197 for (Argument &Arg : F.args()) {
198 if (Arg.hasByValAttr())
203 if (AllocaInst *AI = dyn_cast<AllocaInst>(&I))
207 bool Modified = false;
209 // Track whether a block is reachable after an alloca has escaped. Blocks that
210 // contain the escaping instruction will be marked as being visited without an
211 // escaped alloca, since that is how the block began.
217 DenseMap<BasicBlock *, VisitType> Visited;
219 // We propagate the fact that an alloca has escaped from block to successor.
220 // Visit the blocks that are propagating the escapedness first. To do this, we
221 // maintain two worklists.
222 SmallVector<BasicBlock *, 32> WorklistUnescaped, WorklistEscaped;
224 // We may enter a block and visit it thinking that no alloca has escaped yet,
225 // then see an escape point and go back around a loop edge and come back to
226 // the same block twice. Because of this, we defer setting tail on calls when
227 // we first encounter them in a block. Every entry in this list does not
228 // statically use an alloca via use-def chain analysis, but may find an alloca
229 // through other means if the block turns out to be reachable after an escape
231 SmallVector<CallInst *, 32> DeferredTails;
233 BasicBlock *BB = &F.getEntryBlock();
234 VisitType Escaped = UNESCAPED;
236 for (auto &I : *BB) {
237 if (Tracker.EscapePoints.count(&I))
240 CallInst *CI = dyn_cast<CallInst>(&I);
241 if (!CI || CI->isTailCall() || isa<DbgInfoIntrinsic>(&I))
244 bool IsNoTail = CI->isNoTailCall() || CI->hasOperandBundles();
246 if (!IsNoTail && CI->doesNotAccessMemory()) {
247 // A call to a readnone function whose arguments are all things computed
248 // outside this function can be marked tail. Even if you stored the
249 // alloca address into a global, a readnone function can't load the
252 // Note that this runs whether we know an alloca has escaped or not. If
253 // it has, then we can't trust Tracker.AllocaUsers to be accurate.
254 bool SafeToTail = true;
255 for (auto &Arg : CI->arg_operands()) {
256 if (isa<Constant>(Arg.getUser()))
258 if (Argument *A = dyn_cast<Argument>(Arg.getUser()))
259 if (!A->hasByValAttr())
267 return OptimizationRemark(DEBUG_TYPE, "tailcall-readnone", CI)
268 << "marked as tail call candidate (readnone)";
276 if (!IsNoTail && Escaped == UNESCAPED && !Tracker.AllocaUsers.count(CI)) {
277 DeferredTails.push_back(CI);
279 AllCallsAreTailCalls = false;
283 for (auto *SuccBB : make_range(succ_begin(BB), succ_end(BB))) {
284 auto &State = Visited[SuccBB];
285 if (State < Escaped) {
287 if (State == ESCAPED)
288 WorklistEscaped.push_back(SuccBB);
290 WorklistUnescaped.push_back(SuccBB);
294 if (!WorklistEscaped.empty()) {
295 BB = WorklistEscaped.pop_back_val();
299 while (!WorklistUnescaped.empty()) {
300 auto *NextBB = WorklistUnescaped.pop_back_val();
301 if (Visited[NextBB] == UNESCAPED) {
310 for (CallInst *CI : DeferredTails) {
311 if (Visited[CI->getParent()] != ESCAPED) {
312 // If the escape point was part way through the block, calls after the
313 // escape point wouldn't have been put into DeferredTails.
314 LLVM_DEBUG(dbgs() << "Marked as tail call candidate: " << *CI << "\n");
318 AllCallsAreTailCalls = false;
325 /// Return true if it is safe to move the specified
326 /// instruction from after the call to before the call, assuming that all
327 /// instructions between the call and this instruction are movable.
329 static bool canMoveAboveCall(Instruction *I, CallInst *CI, AliasAnalysis *AA) {
330 // FIXME: We can move load/store/call/free instructions above the call if the
331 // call does not mod/ref the memory location being processed.
332 if (I->mayHaveSideEffects()) // This also handles volatile loads.
335 if (LoadInst *L = dyn_cast<LoadInst>(I)) {
336 // Loads may always be moved above calls without side effects.
337 if (CI->mayHaveSideEffects()) {
338 // Non-volatile loads may be moved above a call with side effects if it
339 // does not write to memory and the load provably won't trap.
340 // Writes to memory only matter if they may alias the pointer
341 // being loaded from.
342 const DataLayout &DL = L->getModule()->getDataLayout();
343 if (isModSet(AA->getModRefInfo(CI, MemoryLocation::get(L))) ||
344 !isSafeToLoadUnconditionally(L->getPointerOperand(), L->getType(),
345 MaybeAlign(L->getAlignment()), DL, L))
350 // Otherwise, if this is a side-effect free instruction, check to make sure
351 // that it does not use the return value of the call. If it doesn't use the
352 // return value of the call, it must only use things that are defined before
353 // the call, or movable instructions between the call and the instruction
355 return !is_contained(I->operands(), CI);
358 /// Return true if the specified value is the same when the return would exit
359 /// as it was when the initial iteration of the recursive function was executed.
361 /// We currently handle static constants and arguments that are not modified as
362 /// part of the recursion.
363 static bool isDynamicConstant(Value *V, CallInst *CI, ReturnInst *RI) {
364 if (isa<Constant>(V)) return true; // Static constants are always dyn consts
366 // Check to see if this is an immutable argument, if so, the value
367 // will be available to initialize the accumulator.
368 if (Argument *Arg = dyn_cast<Argument>(V)) {
369 // Figure out which argument number this is...
371 Function *F = CI->getParent()->getParent();
372 for (Function::arg_iterator AI = F->arg_begin(); &*AI != Arg; ++AI)
375 // If we are passing this argument into call as the corresponding
376 // argument operand, then the argument is dynamically constant.
377 // Otherwise, we cannot transform this function safely.
378 if (CI->getArgOperand(ArgNo) == Arg)
382 // Switch cases are always constant integers. If the value is being switched
383 // on and the return is only reachable from one of its cases, it's
384 // effectively constant.
385 if (BasicBlock *UniquePred = RI->getParent()->getUniquePredecessor())
386 if (SwitchInst *SI = dyn_cast<SwitchInst>(UniquePred->getTerminator()))
387 if (SI->getCondition() == V)
388 return SI->getDefaultDest() != RI->getParent();
390 // Not a constant or immutable argument, we can't safely transform.
394 /// Check to see if the function containing the specified tail call consistently
395 /// returns the same runtime-constant value at all exit points except for
396 /// IgnoreRI. If so, return the returned value.
397 static Value *getCommonReturnValue(ReturnInst *IgnoreRI, CallInst *CI) {
398 Function *F = CI->getParent()->getParent();
399 Value *ReturnedValue = nullptr;
401 for (BasicBlock &BBI : *F) {
402 ReturnInst *RI = dyn_cast<ReturnInst>(BBI.getTerminator());
403 if (RI == nullptr || RI == IgnoreRI) continue;
405 // We can only perform this transformation if the value returned is
406 // evaluatable at the start of the initial invocation of the function,
407 // instead of at the end of the evaluation.
409 Value *RetOp = RI->getOperand(0);
410 if (!isDynamicConstant(RetOp, CI, RI))
413 if (ReturnedValue && RetOp != ReturnedValue)
414 return nullptr; // Cannot transform if differing values are returned.
415 ReturnedValue = RetOp;
417 return ReturnedValue;
420 /// If the specified instruction can be transformed using accumulator recursion
421 /// elimination, return the constant which is the start of the accumulator
422 /// value. Otherwise return null.
423 static Value *canTransformAccumulatorRecursion(Instruction *I, CallInst *CI) {
424 if (!I->isAssociative() || !I->isCommutative()) return nullptr;
425 assert(I->getNumOperands() == 2 &&
426 "Associative/commutative operations should have 2 args!");
428 // Exactly one operand should be the result of the call instruction.
429 if ((I->getOperand(0) == CI && I->getOperand(1) == CI) ||
430 (I->getOperand(0) != CI && I->getOperand(1) != CI))
433 // The only user of this instruction we allow is a single return instruction.
434 if (!I->hasOneUse() || !isa<ReturnInst>(I->user_back()))
437 // Ok, now we have to check all of the other return instructions in this
438 // function. If they return non-constants or differing values, then we cannot
439 // transform the function safely.
440 return getCommonReturnValue(cast<ReturnInst>(I->user_back()), CI);
443 static Instruction *firstNonDbg(BasicBlock::iterator I) {
444 while (isa<DbgInfoIntrinsic>(I))
449 static CallInst *findTRECandidate(Instruction *TI,
450 bool CannotTailCallElimCallsMarkedTail,
451 const TargetTransformInfo *TTI) {
452 BasicBlock *BB = TI->getParent();
453 Function *F = BB->getParent();
455 if (&BB->front() == TI) // Make sure there is something before the terminator.
458 // Scan backwards from the return, checking to see if there is a tail call in
459 // this block. If so, set CI to it.
460 CallInst *CI = nullptr;
461 BasicBlock::iterator BBI(TI);
463 CI = dyn_cast<CallInst>(BBI);
464 if (CI && CI->getCalledFunction() == F)
467 if (BBI == BB->begin())
468 return nullptr; // Didn't find a potential tail call.
472 // If this call is marked as a tail call, and if there are dynamic allocas in
473 // the function, we cannot perform this optimization.
474 if (CI->isTailCall() && CannotTailCallElimCallsMarkedTail)
477 // As a special case, detect code like this:
478 // double fabs(double f) { return __builtin_fabs(f); } // a 'fabs' call
479 // and disable this xform in this case, because the code generator will
480 // lower the call to fabs into inline code.
481 if (BB == &F->getEntryBlock() &&
482 firstNonDbg(BB->front().getIterator()) == CI &&
483 firstNonDbg(std::next(BB->begin())) == TI && CI->getCalledFunction() &&
484 !TTI->isLoweredToCall(CI->getCalledFunction())) {
485 // A single-block function with just a call and a return. Check that
486 // the arguments match.
487 CallSite::arg_iterator I = CallSite(CI).arg_begin(),
488 E = CallSite(CI).arg_end();
489 Function::arg_iterator FI = F->arg_begin(),
491 for (; I != E && FI != FE; ++I, ++FI)
492 if (*I != &*FI) break;
493 if (I == E && FI == FE)
500 static bool eliminateRecursiveTailCall(
501 CallInst *CI, ReturnInst *Ret, BasicBlock *&OldEntry,
502 bool &TailCallsAreMarkedTail, SmallVectorImpl<PHINode *> &ArgumentPHIs,
503 AliasAnalysis *AA, OptimizationRemarkEmitter *ORE, DomTreeUpdater &DTU) {
504 // If we are introducing accumulator recursion to eliminate operations after
505 // the call instruction that are both associative and commutative, the initial
506 // value for the accumulator is placed in this variable. If this value is set
507 // then we actually perform accumulator recursion elimination instead of
508 // simple tail recursion elimination. If the operation is an LLVM instruction
509 // (eg: "add") then it is recorded in AccumulatorRecursionInstr. If not, then
510 // we are handling the case when the return instruction returns a constant C
511 // which is different to the constant returned by other return instructions
512 // (which is recorded in AccumulatorRecursionEliminationInitVal). This is a
513 // special case of accumulator recursion, the operation being "return C".
514 Value *AccumulatorRecursionEliminationInitVal = nullptr;
515 Instruction *AccumulatorRecursionInstr = nullptr;
517 // Ok, we found a potential tail call. We can currently only transform the
518 // tail call if all of the instructions between the call and the return are
519 // movable to above the call itself, leaving the call next to the return.
520 // Check that this is the case now.
521 BasicBlock::iterator BBI(CI);
522 for (++BBI; &*BBI != Ret; ++BBI) {
523 if (canMoveAboveCall(&*BBI, CI, AA))
526 // If we can't move the instruction above the call, it might be because it
527 // is an associative and commutative operation that could be transformed
528 // using accumulator recursion elimination. Check to see if this is the
529 // case, and if so, remember the initial accumulator value for later.
530 if ((AccumulatorRecursionEliminationInitVal =
531 canTransformAccumulatorRecursion(&*BBI, CI))) {
532 // Yes, this is accumulator recursion. Remember which instruction
534 AccumulatorRecursionInstr = &*BBI;
536 return false; // Otherwise, we cannot eliminate the tail recursion!
540 // We can only transform call/return pairs that either ignore the return value
541 // of the call and return void, ignore the value of the call and return a
542 // constant, return the value returned by the tail call, or that are being
543 // accumulator recursion variable eliminated.
544 if (Ret->getNumOperands() == 1 && Ret->getReturnValue() != CI &&
545 !isa<UndefValue>(Ret->getReturnValue()) &&
546 AccumulatorRecursionEliminationInitVal == nullptr &&
547 !getCommonReturnValue(nullptr, CI)) {
548 // One case remains that we are able to handle: the current return
549 // instruction returns a constant, and all other return instructions
550 // return a different constant.
551 if (!isDynamicConstant(Ret->getReturnValue(), CI, Ret))
552 return false; // Current return instruction does not return a constant.
553 // Check that all other return instructions return a common constant. If
554 // so, record it in AccumulatorRecursionEliminationInitVal.
555 AccumulatorRecursionEliminationInitVal = getCommonReturnValue(Ret, CI);
556 if (!AccumulatorRecursionEliminationInitVal)
560 BasicBlock *BB = Ret->getParent();
561 Function *F = BB->getParent();
565 return OptimizationRemark(DEBUG_TYPE, "tailcall-recursion", CI)
566 << "transforming tail recursion into loop";
569 // OK! We can transform this tail call. If this is the first one found,
570 // create the new entry block, allowing us to branch back to the old entry.
572 OldEntry = &F->getEntryBlock();
573 BasicBlock *NewEntry = BasicBlock::Create(F->getContext(), "", F, OldEntry);
574 NewEntry->takeName(OldEntry);
575 OldEntry->setName("tailrecurse");
576 BranchInst *BI = BranchInst::Create(OldEntry, NewEntry);
577 BI->setDebugLoc(CI->getDebugLoc());
579 // If this tail call is marked 'tail' and if there are any allocas in the
580 // entry block, move them up to the new entry block.
581 TailCallsAreMarkedTail = CI->isTailCall();
582 if (TailCallsAreMarkedTail)
583 // Move all fixed sized allocas from OldEntry to NewEntry.
584 for (BasicBlock::iterator OEBI = OldEntry->begin(), E = OldEntry->end(),
585 NEBI = NewEntry->begin(); OEBI != E; )
586 if (AllocaInst *AI = dyn_cast<AllocaInst>(OEBI++))
587 if (isa<ConstantInt>(AI->getArraySize()))
588 AI->moveBefore(&*NEBI);
590 // Now that we have created a new block, which jumps to the entry
591 // block, insert a PHI node for each argument of the function.
592 // For now, we initialize each PHI to only have the real arguments
593 // which are passed in.
594 Instruction *InsertPos = &OldEntry->front();
595 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
597 PHINode *PN = PHINode::Create(I->getType(), 2,
598 I->getName() + ".tr", InsertPos);
599 I->replaceAllUsesWith(PN); // Everyone use the PHI node now!
600 PN->addIncoming(&*I, NewEntry);
601 ArgumentPHIs.push_back(PN);
603 // The entry block was changed from OldEntry to NewEntry.
604 // The forward DominatorTree needs to be recalculated when the EntryBB is
605 // changed. In this corner-case we recalculate the entire tree.
606 DTU.recalculate(*NewEntry->getParent());
609 // If this function has self recursive calls in the tail position where some
610 // are marked tail and some are not, only transform one flavor or another. We
611 // have to choose whether we move allocas in the entry block to the new entry
612 // block or not, so we can't make a good choice for both. NOTE: We could do
613 // slightly better here in the case that the function has no entry block
615 if (TailCallsAreMarkedTail && !CI->isTailCall())
618 // Ok, now that we know we have a pseudo-entry block WITH all of the
619 // required PHI nodes, add entries into the PHI node for the actual
620 // parameters passed into the tail-recursive call.
621 for (unsigned i = 0, e = CI->getNumArgOperands(); i != e; ++i)
622 ArgumentPHIs[i]->addIncoming(CI->getArgOperand(i), BB);
624 // If we are introducing an accumulator variable to eliminate the recursion,
625 // do so now. Note that we _know_ that no subsequent tail recursion
626 // eliminations will happen on this function because of the way the
627 // accumulator recursion predicate is set up.
629 if (AccumulatorRecursionEliminationInitVal) {
630 Instruction *AccRecInstr = AccumulatorRecursionInstr;
631 // Start by inserting a new PHI node for the accumulator.
632 pred_iterator PB = pred_begin(OldEntry), PE = pred_end(OldEntry);
633 PHINode *AccPN = PHINode::Create(
634 AccumulatorRecursionEliminationInitVal->getType(),
635 std::distance(PB, PE) + 1, "accumulator.tr", &OldEntry->front());
637 // Loop over all of the predecessors of the tail recursion block. For the
638 // real entry into the function we seed the PHI with the initial value,
639 // computed earlier. For any other existing branches to this block (due to
640 // other tail recursions eliminated) the accumulator is not modified.
641 // Because we haven't added the branch in the current block to OldEntry yet,
642 // it will not show up as a predecessor.
643 for (pred_iterator PI = PB; PI != PE; ++PI) {
645 if (P == &F->getEntryBlock())
646 AccPN->addIncoming(AccumulatorRecursionEliminationInitVal, P);
648 AccPN->addIncoming(AccPN, P);
652 // Add an incoming argument for the current block, which is computed by
653 // our associative and commutative accumulator instruction.
654 AccPN->addIncoming(AccRecInstr, BB);
656 // Next, rewrite the accumulator recursion instruction so that it does not
657 // use the result of the call anymore, instead, use the PHI node we just
659 AccRecInstr->setOperand(AccRecInstr->getOperand(0) != CI, AccPN);
661 // Add an incoming argument for the current block, which is just the
662 // constant returned by the current return instruction.
663 AccPN->addIncoming(Ret->getReturnValue(), BB);
666 // Finally, rewrite any return instructions in the program to return the PHI
667 // node instead of the "initval" that they do currently. This loop will
668 // actually rewrite the return value we are destroying, but that's ok.
669 for (BasicBlock &BBI : *F)
670 if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI.getTerminator()))
671 RI->setOperand(0, AccPN);
675 // Now that all of the PHI nodes are in place, remove the call and
676 // ret instructions, replacing them with an unconditional branch.
677 BranchInst *NewBI = BranchInst::Create(OldEntry, Ret);
678 NewBI->setDebugLoc(CI->getDebugLoc());
680 BB->getInstList().erase(Ret); // Remove return.
681 BB->getInstList().erase(CI); // Remove call.
682 DTU.applyUpdates({{DominatorTree::Insert, BB, OldEntry}});
687 static bool foldReturnAndProcessPred(
688 BasicBlock *BB, ReturnInst *Ret, BasicBlock *&OldEntry,
689 bool &TailCallsAreMarkedTail, SmallVectorImpl<PHINode *> &ArgumentPHIs,
690 bool CannotTailCallElimCallsMarkedTail, const TargetTransformInfo *TTI,
691 AliasAnalysis *AA, OptimizationRemarkEmitter *ORE, DomTreeUpdater &DTU) {
694 // Make sure this block is a trivial return block.
695 assert(BB->getFirstNonPHIOrDbg() == Ret &&
696 "Trying to fold non-trivial return block");
698 // If the return block contains nothing but the return and PHI's,
699 // there might be an opportunity to duplicate the return in its
700 // predecessors and perform TRE there. Look for predecessors that end
701 // in unconditional branch and recursive call(s).
702 SmallVector<BranchInst*, 8> UncondBranchPreds;
703 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
704 BasicBlock *Pred = *PI;
705 Instruction *PTI = Pred->getTerminator();
706 if (BranchInst *BI = dyn_cast<BranchInst>(PTI))
707 if (BI->isUnconditional())
708 UncondBranchPreds.push_back(BI);
711 while (!UncondBranchPreds.empty()) {
712 BranchInst *BI = UncondBranchPreds.pop_back_val();
713 BasicBlock *Pred = BI->getParent();
714 if (CallInst *CI = findTRECandidate(BI, CannotTailCallElimCallsMarkedTail, TTI)){
715 LLVM_DEBUG(dbgs() << "FOLDING: " << *BB
716 << "INTO UNCOND BRANCH PRED: " << *Pred);
717 ReturnInst *RI = FoldReturnIntoUncondBranch(Ret, BB, Pred, &DTU);
719 // Cleanup: if all predecessors of BB have been eliminated by
720 // FoldReturnIntoUncondBranch, delete it. It is important to empty it,
721 // because the ret instruction in there is still using a value which
722 // eliminateRecursiveTailCall will attempt to remove.
723 if (!BB->hasAddressTaken() && pred_begin(BB) == pred_end(BB))
726 eliminateRecursiveTailCall(CI, RI, OldEntry, TailCallsAreMarkedTail,
727 ArgumentPHIs, AA, ORE, DTU);
736 static bool processReturningBlock(
737 ReturnInst *Ret, BasicBlock *&OldEntry, bool &TailCallsAreMarkedTail,
738 SmallVectorImpl<PHINode *> &ArgumentPHIs,
739 bool CannotTailCallElimCallsMarkedTail, const TargetTransformInfo *TTI,
740 AliasAnalysis *AA, OptimizationRemarkEmitter *ORE, DomTreeUpdater &DTU) {
741 CallInst *CI = findTRECandidate(Ret, CannotTailCallElimCallsMarkedTail, TTI);
745 return eliminateRecursiveTailCall(CI, Ret, OldEntry, TailCallsAreMarkedTail,
746 ArgumentPHIs, AA, ORE, DTU);
749 static bool eliminateTailRecursion(Function &F, const TargetTransformInfo *TTI,
751 OptimizationRemarkEmitter *ORE,
752 DomTreeUpdater &DTU) {
753 if (F.getFnAttribute("disable-tail-calls").getValueAsString() == "true")
756 bool MadeChange = false;
757 bool AllCallsAreTailCalls = false;
758 MadeChange |= markTails(F, AllCallsAreTailCalls, ORE);
759 if (!AllCallsAreTailCalls)
762 // If this function is a varargs function, we won't be able to PHI the args
763 // right, so don't even try to convert it...
764 if (F.getFunctionType()->isVarArg())
767 BasicBlock *OldEntry = nullptr;
768 bool TailCallsAreMarkedTail = false;
769 SmallVector<PHINode*, 8> ArgumentPHIs;
771 // If false, we cannot perform TRE on tail calls marked with the 'tail'
772 // attribute, because doing so would cause the stack size to increase (real
773 // TRE would deallocate variable sized allocas, TRE doesn't).
774 bool CanTRETailMarkedCall = canTRE(F);
776 // Change any tail recursive calls to loops.
778 // FIXME: The code generator produces really bad code when an 'escaping
779 // alloca' is changed from being a static alloca to being a dynamic alloca.
780 // Until this is resolved, disable this transformation if that would ever
781 // happen. This bug is PR962.
782 for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; /*in loop*/) {
783 BasicBlock *BB = &*BBI++; // foldReturnAndProcessPred may delete BB.
784 if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator())) {
785 bool Change = processReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail,
786 ArgumentPHIs, !CanTRETailMarkedCall,
788 if (!Change && BB->getFirstNonPHIOrDbg() == Ret)
789 Change = foldReturnAndProcessPred(
790 BB, Ret, OldEntry, TailCallsAreMarkedTail, ArgumentPHIs,
791 !CanTRETailMarkedCall, TTI, AA, ORE, DTU);
792 MadeChange |= Change;
796 // If we eliminated any tail recursions, it's possible that we inserted some
797 // silly PHI nodes which just merge an initial value (the incoming operand)
798 // with themselves. Check to see if we did and clean up our mess if so. This
799 // occurs when a function passes an argument straight through to its tail
801 for (PHINode *PN : ArgumentPHIs) {
802 // If the PHI Node is a dynamic constant, replace it with the value it is.
803 if (Value *PNV = SimplifyInstruction(PN, F.getParent()->getDataLayout())) {
804 PN->replaceAllUsesWith(PNV);
805 PN->eraseFromParent();
813 struct TailCallElim : public FunctionPass {
814 static char ID; // Pass identification, replacement for typeid
815 TailCallElim() : FunctionPass(ID) {
816 initializeTailCallElimPass(*PassRegistry::getPassRegistry());
819 void getAnalysisUsage(AnalysisUsage &AU) const override {
820 AU.addRequired<TargetTransformInfoWrapperPass>();
821 AU.addRequired<AAResultsWrapperPass>();
822 AU.addRequired<OptimizationRemarkEmitterWrapperPass>();
823 AU.addPreserved<GlobalsAAWrapperPass>();
824 AU.addPreserved<DominatorTreeWrapperPass>();
825 AU.addPreserved<PostDominatorTreeWrapperPass>();
828 bool runOnFunction(Function &F) override {
832 auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>();
833 auto *DT = DTWP ? &DTWP->getDomTree() : nullptr;
834 auto *PDTWP = getAnalysisIfAvailable<PostDominatorTreeWrapperPass>();
835 auto *PDT = PDTWP ? &PDTWP->getPostDomTree() : nullptr;
836 // There is no noticable performance difference here between Lazy and Eager
837 // UpdateStrategy based on some test results. It is feasible to switch the
838 // UpdateStrategy to Lazy if we find it profitable later.
839 DomTreeUpdater DTU(DT, PDT, DomTreeUpdater::UpdateStrategy::Eager);
841 return eliminateTailRecursion(
842 F, &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F),
843 &getAnalysis<AAResultsWrapperPass>().getAAResults(),
844 &getAnalysis<OptimizationRemarkEmitterWrapperPass>().getORE(), DTU);
849 char TailCallElim::ID = 0;
850 INITIALIZE_PASS_BEGIN(TailCallElim, "tailcallelim", "Tail Call Elimination",
852 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
853 INITIALIZE_PASS_DEPENDENCY(OptimizationRemarkEmitterWrapperPass)
854 INITIALIZE_PASS_END(TailCallElim, "tailcallelim", "Tail Call Elimination",
857 // Public interface to the TailCallElimination pass
858 FunctionPass *llvm::createTailCallEliminationPass() {
859 return new TailCallElim();
862 PreservedAnalyses TailCallElimPass::run(Function &F,
863 FunctionAnalysisManager &AM) {
865 TargetTransformInfo &TTI = AM.getResult<TargetIRAnalysis>(F);
866 AliasAnalysis &AA = AM.getResult<AAManager>(F);
867 auto &ORE = AM.getResult<OptimizationRemarkEmitterAnalysis>(F);
868 auto *DT = AM.getCachedResult<DominatorTreeAnalysis>(F);
869 auto *PDT = AM.getCachedResult<PostDominatorTreeAnalysis>(F);
870 // There is no noticable performance difference here between Lazy and Eager
871 // UpdateStrategy based on some test results. It is feasible to switch the
872 // UpdateStrategy to Lazy if we find it profitable later.
873 DomTreeUpdater DTU(DT, PDT, DomTreeUpdater::UpdateStrategy::Eager);
874 bool Changed = eliminateTailRecursion(F, &TTI, &AA, &ORE, DTU);
877 return PreservedAnalyses::all();
878 PreservedAnalyses PA;
879 PA.preserve<GlobalsAA>();
880 PA.preserve<DominatorTreeAnalysis>();
881 PA.preserve<PostDominatorTreeAnalysis>();