1 //===- TailRecursionElimination.cpp - Eliminate Tail Calls ----------------===//
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 file transforms calls of the current function (self recursion) followed
11 // by a return instruction with a branch to the entry of the function, creating
12 // a loop. This pass also implements the following extensions to the basic
15 // 1. Trivial instructions between the call and return do not prevent the
16 // transformation from taking place, though currently the analysis cannot
17 // support moving any really useful instructions (only dead ones).
18 // 2. This pass transforms functions that are prevented from being tail
19 // recursive by an associative and commutative expression to use an
20 // accumulator variable, thus compiling the typical naive factorial or
21 // 'fib' implementation into efficient code.
22 // 3. TRE is performed if the function returns void, if the return
23 // returns the result returned by the call, or if the function returns a
24 // run-time constant on all exits from the function. It is possible, though
25 // unlikely, that the return returns something else (like constant 0), and
26 // can still be TRE'd. It can be TRE'd if ALL OTHER return instructions in
27 // the function return the exact same value.
28 // 4. If it can prove that callees do not access their caller stack frame,
29 // they are marked as eligible for tail call elimination (by the code
32 // There are several improvements that could be made:
34 // 1. If the function has any alloca instructions, these instructions will be
35 // moved out of the entry block of the function, causing them to be
36 // evaluated each time through the tail recursion. Safely keeping allocas
37 // in the entry block requires analysis to proves that the tail-called
38 // function does not read or write the stack object.
39 // 2. Tail recursion is only performed if the call immediately precedes the
40 // return instruction. It's possible that there could be a jump between
41 // the call and the return.
42 // 3. There can be intervening operations between the call and the return that
43 // prevent the TRE from occurring. For example, there could be GEP's and
44 // stores to memory that will not be read or written by the call. This
45 // requires some substantial analysis (such as with DSA) to prove safe to
46 // move ahead of the call, but doing so could allow many more TREs to be
47 // performed, for example in TreeAdd/TreeAlloc from the treeadd benchmark.
48 // 4. The algorithm we use to detect if callees access their caller stack
49 // frames is very primitive.
51 //===----------------------------------------------------------------------===//
53 #include "llvm/Transforms/Scalar.h"
54 #include "llvm/ADT/STLExtras.h"
55 #include "llvm/ADT/SmallPtrSet.h"
56 #include "llvm/ADT/Statistic.h"
57 #include "llvm/Analysis/CaptureTracking.h"
58 #include "llvm/Analysis/CFG.h"
59 #include "llvm/Analysis/InlineCost.h"
60 #include "llvm/Analysis/InstructionSimplify.h"
61 #include "llvm/Analysis/Loads.h"
62 #include "llvm/Analysis/TargetTransformInfo.h"
63 #include "llvm/IR/CFG.h"
64 #include "llvm/IR/CallSite.h"
65 #include "llvm/IR/Constants.h"
66 #include "llvm/IR/DataLayout.h"
67 #include "llvm/IR/DerivedTypes.h"
68 #include "llvm/IR/DiagnosticInfo.h"
69 #include "llvm/IR/Function.h"
70 #include "llvm/IR/Instructions.h"
71 #include "llvm/IR/IntrinsicInst.h"
72 #include "llvm/IR/Module.h"
73 #include "llvm/IR/ValueHandle.h"
74 #include "llvm/Pass.h"
75 #include "llvm/Support/Debug.h"
76 #include "llvm/Support/raw_ostream.h"
77 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
78 #include "llvm/Transforms/Utils/Local.h"
81 #define DEBUG_TYPE "tailcallelim"
83 STATISTIC(NumEliminated, "Number of tail calls removed");
84 STATISTIC(NumRetDuped, "Number of return duplicated");
85 STATISTIC(NumAccumAdded, "Number of accumulators introduced");
88 struct TailCallElim : public FunctionPass {
89 const TargetTransformInfo *TTI;
92 static char ID; // Pass identification, replacement for typeid
93 TailCallElim() : FunctionPass(ID) {
94 initializeTailCallElimPass(*PassRegistry::getPassRegistry());
97 void getAnalysisUsage(AnalysisUsage &AU) const override;
99 bool runOnFunction(Function &F) override;
102 bool runTRE(Function &F);
103 bool markTails(Function &F, bool &AllCallsAreTailCalls);
105 CallInst *FindTRECandidate(Instruction *I,
106 bool CannotTailCallElimCallsMarkedTail);
107 bool EliminateRecursiveTailCall(CallInst *CI, ReturnInst *Ret,
108 BasicBlock *&OldEntry,
109 bool &TailCallsAreMarkedTail,
110 SmallVectorImpl<PHINode *> &ArgumentPHIs,
111 bool CannotTailCallElimCallsMarkedTail);
112 bool FoldReturnAndProcessPred(BasicBlock *BB,
113 ReturnInst *Ret, BasicBlock *&OldEntry,
114 bool &TailCallsAreMarkedTail,
115 SmallVectorImpl<PHINode *> &ArgumentPHIs,
116 bool CannotTailCallElimCallsMarkedTail);
117 bool ProcessReturningBlock(ReturnInst *RI, BasicBlock *&OldEntry,
118 bool &TailCallsAreMarkedTail,
119 SmallVectorImpl<PHINode *> &ArgumentPHIs,
120 bool CannotTailCallElimCallsMarkedTail);
121 bool CanMoveAboveCall(Instruction *I, CallInst *CI);
122 Value *CanTransformAccumulatorRecursion(Instruction *I, CallInst *CI);
126 char TailCallElim::ID = 0;
127 INITIALIZE_PASS_BEGIN(TailCallElim, "tailcallelim",
128 "Tail Call Elimination", false, false)
129 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
130 INITIALIZE_PASS_END(TailCallElim, "tailcallelim",
131 "Tail Call Elimination", false, false)
133 // Public interface to the TailCallElimination pass
134 FunctionPass *llvm::createTailCallEliminationPass() {
135 return new TailCallElim();
138 void TailCallElim::getAnalysisUsage(AnalysisUsage &AU) const {
139 AU.addRequired<TargetTransformInfoWrapperPass>();
142 /// \brief Scan the specified function for alloca instructions.
143 /// If it contains any dynamic allocas, returns false.
144 static bool CanTRE(Function &F) {
145 // Because of PR962, we don't TRE dynamic allocas.
148 if (AllocaInst *AI = dyn_cast<AllocaInst>(&I)) {
149 if (!AI->isStaticAlloca())
158 bool TailCallElim::runOnFunction(Function &F) {
159 if (skipOptnoneFunction(F))
162 DL = F.getParent()->getDataLayout();
164 bool AllCallsAreTailCalls = false;
165 bool Modified = markTails(F, AllCallsAreTailCalls);
166 if (AllCallsAreTailCalls)
167 Modified |= runTRE(F);
172 struct AllocaDerivedValueTracker {
173 // Start at a root value and walk its use-def chain to mark calls that use the
174 // value or a derived value in AllocaUsers, and places where it may escape in
176 void walk(Value *Root) {
177 SmallVector<Use *, 32> Worklist;
178 SmallPtrSet<Use *, 32> Visited;
180 auto AddUsesToWorklist = [&](Value *V) {
181 for (auto &U : V->uses()) {
182 if (!Visited.insert(&U).second)
184 Worklist.push_back(&U);
188 AddUsesToWorklist(Root);
190 while (!Worklist.empty()) {
191 Use *U = Worklist.pop_back_val();
192 Instruction *I = cast<Instruction>(U->getUser());
194 switch (I->getOpcode()) {
195 case Instruction::Call:
196 case Instruction::Invoke: {
198 bool IsNocapture = !CS.isCallee(U) &&
199 CS.doesNotCapture(CS.getArgumentNo(U));
200 callUsesLocalStack(CS, IsNocapture);
202 // If the alloca-derived argument is passed in as nocapture, then it
203 // can't propagate to the call's return. That would be capturing.
208 case Instruction::Load: {
209 // The result of a load is not alloca-derived (unless an alloca has
210 // otherwise escaped, but this is a local analysis).
213 case Instruction::Store: {
214 if (U->getOperandNo() == 0)
215 EscapePoints.insert(I);
216 continue; // Stores have no users to analyze.
218 case Instruction::BitCast:
219 case Instruction::GetElementPtr:
220 case Instruction::PHI:
221 case Instruction::Select:
222 case Instruction::AddrSpaceCast:
225 EscapePoints.insert(I);
229 AddUsesToWorklist(I);
233 void callUsesLocalStack(CallSite CS, bool IsNocapture) {
234 // Add it to the list of alloca users.
235 AllocaUsers.insert(CS.getInstruction());
237 // If it's nocapture then it can't capture this alloca.
241 // If it can write to memory, it can leak the alloca value.
242 if (!CS.onlyReadsMemory())
243 EscapePoints.insert(CS.getInstruction());
246 SmallPtrSet<Instruction *, 32> AllocaUsers;
247 SmallPtrSet<Instruction *, 32> EscapePoints;
251 bool TailCallElim::markTails(Function &F, bool &AllCallsAreTailCalls) {
252 if (F.callsFunctionThatReturnsTwice())
254 AllCallsAreTailCalls = true;
256 // The local stack holds all alloca instructions and all byval arguments.
257 AllocaDerivedValueTracker Tracker;
258 for (Argument &Arg : F.args()) {
259 if (Arg.hasByValAttr())
264 if (AllocaInst *AI = dyn_cast<AllocaInst>(&I))
268 bool Modified = false;
270 // Track whether a block is reachable after an alloca has escaped. Blocks that
271 // contain the escaping instruction will be marked as being visited without an
272 // escaped alloca, since that is how the block began.
278 DenseMap<BasicBlock *, VisitType> Visited;
280 // We propagate the fact that an alloca has escaped from block to successor.
281 // Visit the blocks that are propagating the escapedness first. To do this, we
282 // maintain two worklists.
283 SmallVector<BasicBlock *, 32> WorklistUnescaped, WorklistEscaped;
285 // We may enter a block and visit it thinking that no alloca has escaped yet,
286 // then see an escape point and go back around a loop edge and come back to
287 // the same block twice. Because of this, we defer setting tail on calls when
288 // we first encounter them in a block. Every entry in this list does not
289 // statically use an alloca via use-def chain analysis, but may find an alloca
290 // through other means if the block turns out to be reachable after an escape
292 SmallVector<CallInst *, 32> DeferredTails;
294 BasicBlock *BB = &F.getEntryBlock();
295 VisitType Escaped = UNESCAPED;
297 for (auto &I : *BB) {
298 if (Tracker.EscapePoints.count(&I))
301 CallInst *CI = dyn_cast<CallInst>(&I);
302 if (!CI || CI->isTailCall())
305 if (CI->doesNotAccessMemory()) {
306 // A call to a readnone function whose arguments are all things computed
307 // outside this function can be marked tail. Even if you stored the
308 // alloca address into a global, a readnone function can't load the
311 // Note that this runs whether we know an alloca has escaped or not. If
312 // it has, then we can't trust Tracker.AllocaUsers to be accurate.
313 bool SafeToTail = true;
314 for (auto &Arg : CI->arg_operands()) {
315 if (isa<Constant>(Arg.getUser()))
317 if (Argument *A = dyn_cast<Argument>(Arg.getUser()))
318 if (!A->hasByValAttr())
324 emitOptimizationRemark(
325 F.getContext(), "tailcallelim", F, CI->getDebugLoc(),
326 "marked this readnone call a tail call candidate");
333 if (Escaped == UNESCAPED && !Tracker.AllocaUsers.count(CI)) {
334 DeferredTails.push_back(CI);
336 AllCallsAreTailCalls = false;
340 for (auto *SuccBB : make_range(succ_begin(BB), succ_end(BB))) {
341 auto &State = Visited[SuccBB];
342 if (State < Escaped) {
344 if (State == ESCAPED)
345 WorklistEscaped.push_back(SuccBB);
347 WorklistUnescaped.push_back(SuccBB);
351 if (!WorklistEscaped.empty()) {
352 BB = WorklistEscaped.pop_back_val();
356 while (!WorklistUnescaped.empty()) {
357 auto *NextBB = WorklistUnescaped.pop_back_val();
358 if (Visited[NextBB] == UNESCAPED) {
367 for (CallInst *CI : DeferredTails) {
368 if (Visited[CI->getParent()] != ESCAPED) {
369 // If the escape point was part way through the block, calls after the
370 // escape point wouldn't have been put into DeferredTails.
371 emitOptimizationRemark(F.getContext(), "tailcallelim", F,
373 "marked this call a tail call candidate");
377 AllCallsAreTailCalls = false;
384 bool TailCallElim::runTRE(Function &F) {
385 // If this function is a varargs function, we won't be able to PHI the args
386 // right, so don't even try to convert it...
387 if (F.getFunctionType()->isVarArg()) return false;
389 TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
390 BasicBlock *OldEntry = nullptr;
391 bool TailCallsAreMarkedTail = false;
392 SmallVector<PHINode*, 8> ArgumentPHIs;
393 bool MadeChange = false;
395 // CanTRETailMarkedCall - If false, we cannot perform TRE on tail calls
396 // marked with the 'tail' attribute, because doing so would cause the stack
397 // size to increase (real TRE would deallocate variable sized allocas, TRE
399 bool CanTRETailMarkedCall = CanTRE(F);
401 // Change any tail recursive calls to loops.
403 // FIXME: The code generator produces really bad code when an 'escaping
404 // alloca' is changed from being a static alloca to being a dynamic alloca.
405 // Until this is resolved, disable this transformation if that would ever
406 // happen. This bug is PR962.
407 SmallVector<BasicBlock*, 8> BBToErase;
408 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
409 if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator())) {
410 bool Change = ProcessReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail,
411 ArgumentPHIs, !CanTRETailMarkedCall);
412 if (!Change && BB->getFirstNonPHIOrDbg() == Ret) {
413 Change = FoldReturnAndProcessPred(BB, Ret, OldEntry,
414 TailCallsAreMarkedTail, ArgumentPHIs,
415 !CanTRETailMarkedCall);
416 // FoldReturnAndProcessPred may have emptied some BB. Remember to
418 if (Change && BB->empty())
419 BBToErase.push_back(BB);
422 MadeChange |= Change;
426 for (auto BB: BBToErase)
427 BB->eraseFromParent();
429 // If we eliminated any tail recursions, it's possible that we inserted some
430 // silly PHI nodes which just merge an initial value (the incoming operand)
431 // with themselves. Check to see if we did and clean up our mess if so. This
432 // occurs when a function passes an argument straight through to its tail
434 for (unsigned i = 0, e = ArgumentPHIs.size(); i != e; ++i) {
435 PHINode *PN = ArgumentPHIs[i];
437 // If the PHI Node is a dynamic constant, replace it with the value it is.
438 if (Value *PNV = SimplifyInstruction(PN)) {
439 PN->replaceAllUsesWith(PNV);
440 PN->eraseFromParent();
448 /// CanMoveAboveCall - Return true if it is safe to move the specified
449 /// instruction from after the call to before the call, assuming that all
450 /// instructions between the call and this instruction are movable.
452 bool TailCallElim::CanMoveAboveCall(Instruction *I, CallInst *CI) {
453 // FIXME: We can move load/store/call/free instructions above the call if the
454 // call does not mod/ref the memory location being processed.
455 if (I->mayHaveSideEffects()) // This also handles volatile loads.
458 if (LoadInst *L = dyn_cast<LoadInst>(I)) {
459 // Loads may always be moved above calls without side effects.
460 if (CI->mayHaveSideEffects()) {
461 // Non-volatile loads may be moved above a call with side effects if it
462 // does not write to memory and the load provably won't trap.
463 // FIXME: Writes to memory only matter if they may alias the pointer
464 // being loaded from.
465 if (CI->mayWriteToMemory() ||
466 !isSafeToLoadUnconditionally(L->getPointerOperand(), L,
467 L->getAlignment(), DL))
472 // Otherwise, if this is a side-effect free instruction, check to make sure
473 // that it does not use the return value of the call. If it doesn't use the
474 // return value of the call, it must only use things that are defined before
475 // the call, or movable instructions between the call and the instruction
477 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
478 if (I->getOperand(i) == CI)
483 // isDynamicConstant - Return true if the specified value is the same when the
484 // return would exit as it was when the initial iteration of the recursive
485 // function was executed.
487 // We currently handle static constants and arguments that are not modified as
488 // part of the recursion.
490 static bool isDynamicConstant(Value *V, CallInst *CI, ReturnInst *RI) {
491 if (isa<Constant>(V)) return true; // Static constants are always dyn consts
493 // Check to see if this is an immutable argument, if so, the value
494 // will be available to initialize the accumulator.
495 if (Argument *Arg = dyn_cast<Argument>(V)) {
496 // Figure out which argument number this is...
498 Function *F = CI->getParent()->getParent();
499 for (Function::arg_iterator AI = F->arg_begin(); &*AI != Arg; ++AI)
502 // If we are passing this argument into call as the corresponding
503 // argument operand, then the argument is dynamically constant.
504 // Otherwise, we cannot transform this function safely.
505 if (CI->getArgOperand(ArgNo) == Arg)
509 // Switch cases are always constant integers. If the value is being switched
510 // on and the return is only reachable from one of its cases, it's
511 // effectively constant.
512 if (BasicBlock *UniquePred = RI->getParent()->getUniquePredecessor())
513 if (SwitchInst *SI = dyn_cast<SwitchInst>(UniquePred->getTerminator()))
514 if (SI->getCondition() == V)
515 return SI->getDefaultDest() != RI->getParent();
517 // Not a constant or immutable argument, we can't safely transform.
521 // getCommonReturnValue - Check to see if the function containing the specified
522 // tail call consistently returns the same runtime-constant value at all exit
523 // points except for IgnoreRI. If so, return the returned value.
525 static Value *getCommonReturnValue(ReturnInst *IgnoreRI, CallInst *CI) {
526 Function *F = CI->getParent()->getParent();
527 Value *ReturnedValue = nullptr;
529 for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI) {
530 ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator());
531 if (RI == nullptr || RI == IgnoreRI) continue;
533 // We can only perform this transformation if the value returned is
534 // evaluatable at the start of the initial invocation of the function,
535 // instead of at the end of the evaluation.
537 Value *RetOp = RI->getOperand(0);
538 if (!isDynamicConstant(RetOp, CI, RI))
541 if (ReturnedValue && RetOp != ReturnedValue)
542 return nullptr; // Cannot transform if differing values are returned.
543 ReturnedValue = RetOp;
545 return ReturnedValue;
548 /// CanTransformAccumulatorRecursion - If the specified instruction can be
549 /// transformed using accumulator recursion elimination, return the constant
550 /// which is the start of the accumulator value. Otherwise return null.
552 Value *TailCallElim::CanTransformAccumulatorRecursion(Instruction *I,
554 if (!I->isAssociative() || !I->isCommutative()) return nullptr;
555 assert(I->getNumOperands() == 2 &&
556 "Associative/commutative operations should have 2 args!");
558 // Exactly one operand should be the result of the call instruction.
559 if ((I->getOperand(0) == CI && I->getOperand(1) == CI) ||
560 (I->getOperand(0) != CI && I->getOperand(1) != CI))
563 // The only user of this instruction we allow is a single return instruction.
564 if (!I->hasOneUse() || !isa<ReturnInst>(I->user_back()))
567 // Ok, now we have to check all of the other return instructions in this
568 // function. If they return non-constants or differing values, then we cannot
569 // transform the function safely.
570 return getCommonReturnValue(cast<ReturnInst>(I->user_back()), CI);
573 static Instruction *FirstNonDbg(BasicBlock::iterator I) {
574 while (isa<DbgInfoIntrinsic>(I))
580 TailCallElim::FindTRECandidate(Instruction *TI,
581 bool CannotTailCallElimCallsMarkedTail) {
582 BasicBlock *BB = TI->getParent();
583 Function *F = BB->getParent();
585 if (&BB->front() == TI) // Make sure there is something before the terminator.
588 // Scan backwards from the return, checking to see if there is a tail call in
589 // this block. If so, set CI to it.
590 CallInst *CI = nullptr;
591 BasicBlock::iterator BBI = TI;
593 CI = dyn_cast<CallInst>(BBI);
594 if (CI && CI->getCalledFunction() == F)
597 if (BBI == BB->begin())
598 return nullptr; // Didn't find a potential tail call.
602 // If this call is marked as a tail call, and if there are dynamic allocas in
603 // the function, we cannot perform this optimization.
604 if (CI->isTailCall() && CannotTailCallElimCallsMarkedTail)
607 // As a special case, detect code like this:
608 // double fabs(double f) { return __builtin_fabs(f); } // a 'fabs' call
609 // and disable this xform in this case, because the code generator will
610 // lower the call to fabs into inline code.
611 if (BB == &F->getEntryBlock() &&
612 FirstNonDbg(BB->front()) == CI &&
613 FirstNonDbg(std::next(BB->begin())) == TI &&
614 CI->getCalledFunction() &&
615 !TTI->isLoweredToCall(CI->getCalledFunction())) {
616 // A single-block function with just a call and a return. Check that
617 // the arguments match.
618 CallSite::arg_iterator I = CallSite(CI).arg_begin(),
619 E = CallSite(CI).arg_end();
620 Function::arg_iterator FI = F->arg_begin(),
622 for (; I != E && FI != FE; ++I, ++FI)
623 if (*I != &*FI) break;
624 if (I == E && FI == FE)
631 bool TailCallElim::EliminateRecursiveTailCall(CallInst *CI, ReturnInst *Ret,
632 BasicBlock *&OldEntry,
633 bool &TailCallsAreMarkedTail,
634 SmallVectorImpl<PHINode *> &ArgumentPHIs,
635 bool CannotTailCallElimCallsMarkedTail) {
636 // If we are introducing accumulator recursion to eliminate operations after
637 // the call instruction that are both associative and commutative, the initial
638 // value for the accumulator is placed in this variable. If this value is set
639 // then we actually perform accumulator recursion elimination instead of
640 // simple tail recursion elimination. If the operation is an LLVM instruction
641 // (eg: "add") then it is recorded in AccumulatorRecursionInstr. If not, then
642 // we are handling the case when the return instruction returns a constant C
643 // which is different to the constant returned by other return instructions
644 // (which is recorded in AccumulatorRecursionEliminationInitVal). This is a
645 // special case of accumulator recursion, the operation being "return C".
646 Value *AccumulatorRecursionEliminationInitVal = nullptr;
647 Instruction *AccumulatorRecursionInstr = nullptr;
649 // Ok, we found a potential tail call. We can currently only transform the
650 // tail call if all of the instructions between the call and the return are
651 // movable to above the call itself, leaving the call next to the return.
652 // Check that this is the case now.
653 BasicBlock::iterator BBI = CI;
654 for (++BBI; &*BBI != Ret; ++BBI) {
655 if (CanMoveAboveCall(BBI, CI)) continue;
657 // If we can't move the instruction above the call, it might be because it
658 // is an associative and commutative operation that could be transformed
659 // using accumulator recursion elimination. Check to see if this is the
660 // case, and if so, remember the initial accumulator value for later.
661 if ((AccumulatorRecursionEliminationInitVal =
662 CanTransformAccumulatorRecursion(BBI, CI))) {
663 // Yes, this is accumulator recursion. Remember which instruction
665 AccumulatorRecursionInstr = BBI;
667 return false; // Otherwise, we cannot eliminate the tail recursion!
671 // We can only transform call/return pairs that either ignore the return value
672 // of the call and return void, ignore the value of the call and return a
673 // constant, return the value returned by the tail call, or that are being
674 // accumulator recursion variable eliminated.
675 if (Ret->getNumOperands() == 1 && Ret->getReturnValue() != CI &&
676 !isa<UndefValue>(Ret->getReturnValue()) &&
677 AccumulatorRecursionEliminationInitVal == nullptr &&
678 !getCommonReturnValue(nullptr, CI)) {
679 // One case remains that we are able to handle: the current return
680 // instruction returns a constant, and all other return instructions
681 // return a different constant.
682 if (!isDynamicConstant(Ret->getReturnValue(), CI, Ret))
683 return false; // Current return instruction does not return a constant.
684 // Check that all other return instructions return a common constant. If
685 // so, record it in AccumulatorRecursionEliminationInitVal.
686 AccumulatorRecursionEliminationInitVal = getCommonReturnValue(Ret, CI);
687 if (!AccumulatorRecursionEliminationInitVal)
691 BasicBlock *BB = Ret->getParent();
692 Function *F = BB->getParent();
694 emitOptimizationRemark(F->getContext(), "tailcallelim", *F, CI->getDebugLoc(),
695 "transforming tail recursion to loop");
697 // OK! We can transform this tail call. If this is the first one found,
698 // create the new entry block, allowing us to branch back to the old entry.
700 OldEntry = &F->getEntryBlock();
701 BasicBlock *NewEntry = BasicBlock::Create(F->getContext(), "", F, OldEntry);
702 NewEntry->takeName(OldEntry);
703 OldEntry->setName("tailrecurse");
704 BranchInst::Create(OldEntry, NewEntry);
706 // If this tail call is marked 'tail' and if there are any allocas in the
707 // entry block, move them up to the new entry block.
708 TailCallsAreMarkedTail = CI->isTailCall();
709 if (TailCallsAreMarkedTail)
710 // Move all fixed sized allocas from OldEntry to NewEntry.
711 for (BasicBlock::iterator OEBI = OldEntry->begin(), E = OldEntry->end(),
712 NEBI = NewEntry->begin(); OEBI != E; )
713 if (AllocaInst *AI = dyn_cast<AllocaInst>(OEBI++))
714 if (isa<ConstantInt>(AI->getArraySize()))
715 AI->moveBefore(NEBI);
717 // Now that we have created a new block, which jumps to the entry
718 // block, insert a PHI node for each argument of the function.
719 // For now, we initialize each PHI to only have the real arguments
720 // which are passed in.
721 Instruction *InsertPos = OldEntry->begin();
722 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
724 PHINode *PN = PHINode::Create(I->getType(), 2,
725 I->getName() + ".tr", InsertPos);
726 I->replaceAllUsesWith(PN); // Everyone use the PHI node now!
727 PN->addIncoming(I, NewEntry);
728 ArgumentPHIs.push_back(PN);
732 // If this function has self recursive calls in the tail position where some
733 // are marked tail and some are not, only transform one flavor or another. We
734 // have to choose whether we move allocas in the entry block to the new entry
735 // block or not, so we can't make a good choice for both. NOTE: We could do
736 // slightly better here in the case that the function has no entry block
738 if (TailCallsAreMarkedTail && !CI->isTailCall())
741 // Ok, now that we know we have a pseudo-entry block WITH all of the
742 // required PHI nodes, add entries into the PHI node for the actual
743 // parameters passed into the tail-recursive call.
744 for (unsigned i = 0, e = CI->getNumArgOperands(); i != e; ++i)
745 ArgumentPHIs[i]->addIncoming(CI->getArgOperand(i), BB);
747 // If we are introducing an accumulator variable to eliminate the recursion,
748 // do so now. Note that we _know_ that no subsequent tail recursion
749 // eliminations will happen on this function because of the way the
750 // accumulator recursion predicate is set up.
752 if (AccumulatorRecursionEliminationInitVal) {
753 Instruction *AccRecInstr = AccumulatorRecursionInstr;
754 // Start by inserting a new PHI node for the accumulator.
755 pred_iterator PB = pred_begin(OldEntry), PE = pred_end(OldEntry);
757 PHINode::Create(AccumulatorRecursionEliminationInitVal->getType(),
758 std::distance(PB, PE) + 1,
759 "accumulator.tr", OldEntry->begin());
761 // Loop over all of the predecessors of the tail recursion block. For the
762 // real entry into the function we seed the PHI with the initial value,
763 // computed earlier. For any other existing branches to this block (due to
764 // other tail recursions eliminated) the accumulator is not modified.
765 // Because we haven't added the branch in the current block to OldEntry yet,
766 // it will not show up as a predecessor.
767 for (pred_iterator PI = PB; PI != PE; ++PI) {
769 if (P == &F->getEntryBlock())
770 AccPN->addIncoming(AccumulatorRecursionEliminationInitVal, P);
772 AccPN->addIncoming(AccPN, P);
776 // Add an incoming argument for the current block, which is computed by
777 // our associative and commutative accumulator instruction.
778 AccPN->addIncoming(AccRecInstr, BB);
780 // Next, rewrite the accumulator recursion instruction so that it does not
781 // use the result of the call anymore, instead, use the PHI node we just
783 AccRecInstr->setOperand(AccRecInstr->getOperand(0) != CI, AccPN);
785 // Add an incoming argument for the current block, which is just the
786 // constant returned by the current return instruction.
787 AccPN->addIncoming(Ret->getReturnValue(), BB);
790 // Finally, rewrite any return instructions in the program to return the PHI
791 // node instead of the "initval" that they do currently. This loop will
792 // actually rewrite the return value we are destroying, but that's ok.
793 for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI)
794 if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator()))
795 RI->setOperand(0, AccPN);
799 // Now that all of the PHI nodes are in place, remove the call and
800 // ret instructions, replacing them with an unconditional branch.
801 BranchInst *NewBI = BranchInst::Create(OldEntry, Ret);
802 NewBI->setDebugLoc(CI->getDebugLoc());
804 BB->getInstList().erase(Ret); // Remove return.
805 BB->getInstList().erase(CI); // Remove call.
810 bool TailCallElim::FoldReturnAndProcessPred(BasicBlock *BB,
811 ReturnInst *Ret, BasicBlock *&OldEntry,
812 bool &TailCallsAreMarkedTail,
813 SmallVectorImpl<PHINode *> &ArgumentPHIs,
814 bool CannotTailCallElimCallsMarkedTail) {
817 // If the return block contains nothing but the return and PHI's,
818 // there might be an opportunity to duplicate the return in its
819 // predecessors and perform TRC there. Look for predecessors that end
820 // in unconditional branch and recursive call(s).
821 SmallVector<BranchInst*, 8> UncondBranchPreds;
822 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
823 BasicBlock *Pred = *PI;
824 TerminatorInst *PTI = Pred->getTerminator();
825 if (BranchInst *BI = dyn_cast<BranchInst>(PTI))
826 if (BI->isUnconditional())
827 UncondBranchPreds.push_back(BI);
830 while (!UncondBranchPreds.empty()) {
831 BranchInst *BI = UncondBranchPreds.pop_back_val();
832 BasicBlock *Pred = BI->getParent();
833 if (CallInst *CI = FindTRECandidate(BI, CannotTailCallElimCallsMarkedTail)){
834 DEBUG(dbgs() << "FOLDING: " << *BB
835 << "INTO UNCOND BRANCH PRED: " << *Pred);
836 ReturnInst *RI = FoldReturnIntoUncondBranch(Ret, BB, Pred);
838 // Cleanup: if all predecessors of BB have been eliminated by
839 // FoldReturnIntoUncondBranch, we would like to delete it, but we
840 // can not just nuke it as it is being used as an iterator by our caller.
841 // Just empty it, and the caller will erase it when it is safe to do so.
842 // It is important to empty it, because the ret instruction in there is
843 // still using a value which EliminateRecursiveTailCall will attempt
845 if (!BB->hasAddressTaken() && pred_begin(BB) == pred_end(BB))
846 BB->getInstList().clear();
848 EliminateRecursiveTailCall(CI, RI, OldEntry, TailCallsAreMarkedTail,
850 CannotTailCallElimCallsMarkedTail);
860 TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry,
861 bool &TailCallsAreMarkedTail,
862 SmallVectorImpl<PHINode *> &ArgumentPHIs,
863 bool CannotTailCallElimCallsMarkedTail) {
864 CallInst *CI = FindTRECandidate(Ret, CannotTailCallElimCallsMarkedTail);
868 return EliminateRecursiveTailCall(CI, Ret, OldEntry, TailCallsAreMarkedTail,
870 CannotTailCallElimCallsMarkedTail);