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 #define DEBUG_TYPE "tailcallelim"
54 #include "llvm/Transforms/Scalar.h"
55 #include "llvm/ADT/STLExtras.h"
56 #include "llvm/ADT/SmallPtrSet.h"
57 #include "llvm/ADT/Statistic.h"
58 #include "llvm/Analysis/CaptureTracking.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/DerivedTypes.h"
67 #include "llvm/IR/Function.h"
68 #include "llvm/IR/Instructions.h"
69 #include "llvm/IR/IntrinsicInst.h"
70 #include "llvm/IR/Module.h"
71 #include "llvm/IR/ValueHandle.h"
72 #include "llvm/Pass.h"
73 #include "llvm/Support/Debug.h"
74 #include "llvm/Support/raw_ostream.h"
75 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
76 #include "llvm/Transforms/Utils/Local.h"
79 STATISTIC(NumEliminated, "Number of tail calls removed");
80 STATISTIC(NumRetDuped, "Number of return duplicated");
81 STATISTIC(NumAccumAdded, "Number of accumulators introduced");
84 struct TailCallElim : public FunctionPass {
85 const TargetTransformInfo *TTI;
87 static char ID; // Pass identification, replacement for typeid
88 TailCallElim() : FunctionPass(ID) {
89 initializeTailCallElimPass(*PassRegistry::getPassRegistry());
92 void getAnalysisUsage(AnalysisUsage &AU) const override;
94 bool runOnFunction(Function &F) override;
97 CallInst *FindTRECandidate(Instruction *I,
98 bool CannotTailCallElimCallsMarkedTail);
99 bool EliminateRecursiveTailCall(CallInst *CI, ReturnInst *Ret,
100 BasicBlock *&OldEntry,
101 bool &TailCallsAreMarkedTail,
102 SmallVectorImpl<PHINode *> &ArgumentPHIs,
103 bool CannotTailCallElimCallsMarkedTail);
104 bool FoldReturnAndProcessPred(BasicBlock *BB,
105 ReturnInst *Ret, BasicBlock *&OldEntry,
106 bool &TailCallsAreMarkedTail,
107 SmallVectorImpl<PHINode *> &ArgumentPHIs,
108 bool CannotTailCallElimCallsMarkedTail);
109 bool ProcessReturningBlock(ReturnInst *RI, BasicBlock *&OldEntry,
110 bool &TailCallsAreMarkedTail,
111 SmallVectorImpl<PHINode *> &ArgumentPHIs,
112 bool CannotTailCallElimCallsMarkedTail);
113 bool CanMoveAboveCall(Instruction *I, CallInst *CI);
114 Value *CanTransformAccumulatorRecursion(Instruction *I, CallInst *CI);
118 char TailCallElim::ID = 0;
119 INITIALIZE_PASS_BEGIN(TailCallElim, "tailcallelim",
120 "Tail Call Elimination", false, false)
121 INITIALIZE_AG_DEPENDENCY(TargetTransformInfo)
122 INITIALIZE_PASS_END(TailCallElim, "tailcallelim",
123 "Tail Call Elimination", false, false)
125 // Public interface to the TailCallElimination pass
126 FunctionPass *llvm::createTailCallEliminationPass() {
127 return new TailCallElim();
130 void TailCallElim::getAnalysisUsage(AnalysisUsage &AU) const {
131 AU.addRequired<TargetTransformInfo>();
134 /// CanTRE - Scan the specified basic block for alloca instructions.
135 /// If it contains any that are variable-sized or not in the entry block,
137 static bool CanTRE(AllocaInst *AI) {
138 // Because of PR962, we don't TRE allocas outside the entry block.
140 // If this alloca is in the body of the function, or if it is a variable
141 // sized allocation, we cannot tail call eliminate calls marked 'tail'
142 // with this mechanism.
143 BasicBlock *BB = AI->getParent();
144 return BB == &BB->getParent()->getEntryBlock() &&
145 isa<ConstantInt>(AI->getArraySize());
149 struct AllocaCaptureTracker : public CaptureTracker {
150 AllocaCaptureTracker() : Captured(false) {}
152 void tooManyUses() override { Captured = true; }
154 bool shouldExplore(const Use *U) override {
155 Value *V = U->getUser();
156 if (isa<CallInst>(V) || isa<InvokeInst>(V))
157 UsesAlloca.insert(V);
161 bool captured(const Use *U) override {
162 if (isa<ReturnInst>(U->getUser()))
169 SmallPtrSet<const Value *, 16> UsesAlloca;
171 } // end anonymous namespace
173 bool TailCallElim::runOnFunction(Function &F) {
174 if (skipOptnoneFunction(F))
177 // If this function is a varargs function, we won't be able to PHI the args
178 // right, so don't even try to convert it...
179 if (F.getFunctionType()->isVarArg()) return false;
181 TTI = &getAnalysis<TargetTransformInfo>();
182 BasicBlock *OldEntry = 0;
183 bool TailCallsAreMarkedTail = false;
184 SmallVector<PHINode*, 8> ArgumentPHIs;
185 bool MadeChange = false;
187 // CanTRETailMarkedCall - If false, we cannot perform TRE on tail calls
188 // marked with the 'tail' attribute, because doing so would cause the stack
189 // size to increase (real TRE would deallocate variable sized allocas, TRE
191 bool CanTRETailMarkedCall = true;
193 // Find calls that can be marked tail.
194 AllocaCaptureTracker ACT;
195 for (Function::iterator BB = F.begin(), EE = F.end(); BB != EE; ++BB) {
196 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
197 if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) {
198 CanTRETailMarkedCall &= CanTRE(AI);
199 PointerMayBeCaptured(AI, &ACT);
200 // If any allocas are captured, exit.
207 // If any byval or inalloca args are captured, exit. They are also allocated
208 // in our stack frame.
209 for (Argument &Arg : F.args()) {
210 if (Arg.hasByValOrInAllocaAttr())
211 PointerMayBeCaptured(&Arg, &ACT);
216 // Second pass, change any tail recursive calls to loops.
218 // FIXME: The code generator produces really bad code when an 'escaping
219 // alloca' is changed from being a static alloca to being a dynamic alloca.
220 // Until this is resolved, disable this transformation if that would ever
221 // happen. This bug is PR962.
222 if (ACT.UsesAlloca.empty()) {
223 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
224 if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator())) {
225 bool Change = ProcessReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail,
226 ArgumentPHIs, !CanTRETailMarkedCall);
227 if (!Change && BB->getFirstNonPHIOrDbg() == Ret)
228 Change = FoldReturnAndProcessPred(BB, Ret, OldEntry,
229 TailCallsAreMarkedTail, ArgumentPHIs,
230 !CanTRETailMarkedCall);
231 MadeChange |= Change;
236 // If we eliminated any tail recursions, it's possible that we inserted some
237 // silly PHI nodes which just merge an initial value (the incoming operand)
238 // with themselves. Check to see if we did and clean up our mess if so. This
239 // occurs when a function passes an argument straight through to its tail
241 if (!ArgumentPHIs.empty()) {
242 for (unsigned i = 0, e = ArgumentPHIs.size(); i != e; ++i) {
243 PHINode *PN = ArgumentPHIs[i];
245 // If the PHI Node is a dynamic constant, replace it with the value it is.
246 if (Value *PNV = SimplifyInstruction(PN)) {
247 PN->replaceAllUsesWith(PNV);
248 PN->eraseFromParent();
253 // At this point, we know that the function does not have any captured
254 // allocas. If additionally the function does not call setjmp, mark all calls
255 // in the function that do not access stack memory with the tail keyword. This
256 // implies ensuring that there does not exist any path from a call that takes
257 // in an alloca but does not capture it and the call which we wish to mark
259 if (!F.callsFunctionThatReturnsTwice()) {
260 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
261 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
262 if (CallInst *CI = dyn_cast<CallInst>(I)) {
263 if (!ACT.UsesAlloca.count(CI)) {
276 /// CanMoveAboveCall - Return true if it is safe to move the specified
277 /// instruction from after the call to before the call, assuming that all
278 /// instructions between the call and this instruction are movable.
280 bool TailCallElim::CanMoveAboveCall(Instruction *I, CallInst *CI) {
281 // FIXME: We can move load/store/call/free instructions above the call if the
282 // call does not mod/ref the memory location being processed.
283 if (I->mayHaveSideEffects()) // This also handles volatile loads.
286 if (LoadInst *L = dyn_cast<LoadInst>(I)) {
287 // Loads may always be moved above calls without side effects.
288 if (CI->mayHaveSideEffects()) {
289 // Non-volatile loads may be moved above a call with side effects if it
290 // does not write to memory and the load provably won't trap.
291 // FIXME: Writes to memory only matter if they may alias the pointer
292 // being loaded from.
293 if (CI->mayWriteToMemory() ||
294 !isSafeToLoadUnconditionally(L->getPointerOperand(), L,
300 // Otherwise, if this is a side-effect free instruction, check to make sure
301 // that it does not use the return value of the call. If it doesn't use the
302 // return value of the call, it must only use things that are defined before
303 // the call, or movable instructions between the call and the instruction
305 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
306 if (I->getOperand(i) == CI)
311 // isDynamicConstant - Return true if the specified value is the same when the
312 // return would exit as it was when the initial iteration of the recursive
313 // function was executed.
315 // We currently handle static constants and arguments that are not modified as
316 // part of the recursion.
318 static bool isDynamicConstant(Value *V, CallInst *CI, ReturnInst *RI) {
319 if (isa<Constant>(V)) return true; // Static constants are always dyn consts
321 // Check to see if this is an immutable argument, if so, the value
322 // will be available to initialize the accumulator.
323 if (Argument *Arg = dyn_cast<Argument>(V)) {
324 // Figure out which argument number this is...
326 Function *F = CI->getParent()->getParent();
327 for (Function::arg_iterator AI = F->arg_begin(); &*AI != Arg; ++AI)
330 // If we are passing this argument into call as the corresponding
331 // argument operand, then the argument is dynamically constant.
332 // Otherwise, we cannot transform this function safely.
333 if (CI->getArgOperand(ArgNo) == Arg)
337 // Switch cases are always constant integers. If the value is being switched
338 // on and the return is only reachable from one of its cases, it's
339 // effectively constant.
340 if (BasicBlock *UniquePred = RI->getParent()->getUniquePredecessor())
341 if (SwitchInst *SI = dyn_cast<SwitchInst>(UniquePred->getTerminator()))
342 if (SI->getCondition() == V)
343 return SI->getDefaultDest() != RI->getParent();
345 // Not a constant or immutable argument, we can't safely transform.
349 // getCommonReturnValue - Check to see if the function containing the specified
350 // tail call consistently returns the same runtime-constant value at all exit
351 // points except for IgnoreRI. If so, return the returned value.
353 static Value *getCommonReturnValue(ReturnInst *IgnoreRI, CallInst *CI) {
354 Function *F = CI->getParent()->getParent();
355 Value *ReturnedValue = 0;
357 for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI) {
358 ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator());
359 if (RI == 0 || RI == IgnoreRI) continue;
361 // We can only perform this transformation if the value returned is
362 // evaluatable at the start of the initial invocation of the function,
363 // instead of at the end of the evaluation.
365 Value *RetOp = RI->getOperand(0);
366 if (!isDynamicConstant(RetOp, CI, RI))
369 if (ReturnedValue && RetOp != ReturnedValue)
370 return 0; // Cannot transform if differing values are returned.
371 ReturnedValue = RetOp;
373 return ReturnedValue;
376 /// CanTransformAccumulatorRecursion - If the specified instruction can be
377 /// transformed using accumulator recursion elimination, return the constant
378 /// which is the start of the accumulator value. Otherwise return null.
380 Value *TailCallElim::CanTransformAccumulatorRecursion(Instruction *I,
382 if (!I->isAssociative() || !I->isCommutative()) return 0;
383 assert(I->getNumOperands() == 2 &&
384 "Associative/commutative operations should have 2 args!");
386 // Exactly one operand should be the result of the call instruction.
387 if ((I->getOperand(0) == CI && I->getOperand(1) == CI) ||
388 (I->getOperand(0) != CI && I->getOperand(1) != CI))
391 // The only user of this instruction we allow is a single return instruction.
392 if (!I->hasOneUse() || !isa<ReturnInst>(I->user_back()))
395 // Ok, now we have to check all of the other return instructions in this
396 // function. If they return non-constants or differing values, then we cannot
397 // transform the function safely.
398 return getCommonReturnValue(cast<ReturnInst>(I->user_back()), CI);
401 static Instruction *FirstNonDbg(BasicBlock::iterator I) {
402 while (isa<DbgInfoIntrinsic>(I))
408 TailCallElim::FindTRECandidate(Instruction *TI,
409 bool CannotTailCallElimCallsMarkedTail) {
410 BasicBlock *BB = TI->getParent();
411 Function *F = BB->getParent();
413 if (&BB->front() == TI) // Make sure there is something before the terminator.
416 // Scan backwards from the return, checking to see if there is a tail call in
417 // this block. If so, set CI to it.
419 BasicBlock::iterator BBI = TI;
421 CI = dyn_cast<CallInst>(BBI);
422 if (CI && CI->getCalledFunction() == F)
425 if (BBI == BB->begin())
426 return 0; // Didn't find a potential tail call.
430 // If this call is marked as a tail call, and if there are dynamic allocas in
431 // the function, we cannot perform this optimization.
432 if (CI->isTailCall() && CannotTailCallElimCallsMarkedTail)
435 // As a special case, detect code like this:
436 // double fabs(double f) { return __builtin_fabs(f); } // a 'fabs' call
437 // and disable this xform in this case, because the code generator will
438 // lower the call to fabs into inline code.
439 if (BB == &F->getEntryBlock() &&
440 FirstNonDbg(BB->front()) == CI &&
441 FirstNonDbg(std::next(BB->begin())) == TI &&
442 CI->getCalledFunction() &&
443 !TTI->isLoweredToCall(CI->getCalledFunction())) {
444 // A single-block function with just a call and a return. Check that
445 // the arguments match.
446 CallSite::arg_iterator I = CallSite(CI).arg_begin(),
447 E = CallSite(CI).arg_end();
448 Function::arg_iterator FI = F->arg_begin(),
450 for (; I != E && FI != FE; ++I, ++FI)
451 if (*I != &*FI) break;
452 if (I == E && FI == FE)
459 bool TailCallElim::EliminateRecursiveTailCall(CallInst *CI, ReturnInst *Ret,
460 BasicBlock *&OldEntry,
461 bool &TailCallsAreMarkedTail,
462 SmallVectorImpl<PHINode *> &ArgumentPHIs,
463 bool CannotTailCallElimCallsMarkedTail) {
464 // If we are introducing accumulator recursion to eliminate operations after
465 // the call instruction that are both associative and commutative, the initial
466 // value for the accumulator is placed in this variable. If this value is set
467 // then we actually perform accumulator recursion elimination instead of
468 // simple tail recursion elimination. If the operation is an LLVM instruction
469 // (eg: "add") then it is recorded in AccumulatorRecursionInstr. If not, then
470 // we are handling the case when the return instruction returns a constant C
471 // which is different to the constant returned by other return instructions
472 // (which is recorded in AccumulatorRecursionEliminationInitVal). This is a
473 // special case of accumulator recursion, the operation being "return C".
474 Value *AccumulatorRecursionEliminationInitVal = 0;
475 Instruction *AccumulatorRecursionInstr = 0;
477 // Ok, we found a potential tail call. We can currently only transform the
478 // tail call if all of the instructions between the call and the return are
479 // movable to above the call itself, leaving the call next to the return.
480 // Check that this is the case now.
481 BasicBlock::iterator BBI = CI;
482 for (++BBI; &*BBI != Ret; ++BBI) {
483 if (CanMoveAboveCall(BBI, CI)) continue;
485 // If we can't move the instruction above the call, it might be because it
486 // is an associative and commutative operation that could be transformed
487 // using accumulator recursion elimination. Check to see if this is the
488 // case, and if so, remember the initial accumulator value for later.
489 if ((AccumulatorRecursionEliminationInitVal =
490 CanTransformAccumulatorRecursion(BBI, CI))) {
491 // Yes, this is accumulator recursion. Remember which instruction
493 AccumulatorRecursionInstr = BBI;
495 return false; // Otherwise, we cannot eliminate the tail recursion!
499 // We can only transform call/return pairs that either ignore the return value
500 // of the call and return void, ignore the value of the call and return a
501 // constant, return the value returned by the tail call, or that are being
502 // accumulator recursion variable eliminated.
503 if (Ret->getNumOperands() == 1 && Ret->getReturnValue() != CI &&
504 !isa<UndefValue>(Ret->getReturnValue()) &&
505 AccumulatorRecursionEliminationInitVal == 0 &&
506 !getCommonReturnValue(0, CI)) {
507 // One case remains that we are able to handle: the current return
508 // instruction returns a constant, and all other return instructions
509 // return a different constant.
510 if (!isDynamicConstant(Ret->getReturnValue(), CI, Ret))
511 return false; // Current return instruction does not return a constant.
512 // Check that all other return instructions return a common constant. If
513 // so, record it in AccumulatorRecursionEliminationInitVal.
514 AccumulatorRecursionEliminationInitVal = getCommonReturnValue(Ret, CI);
515 if (!AccumulatorRecursionEliminationInitVal)
519 BasicBlock *BB = Ret->getParent();
520 Function *F = BB->getParent();
522 // OK! We can transform this tail call. If this is the first one found,
523 // create the new entry block, allowing us to branch back to the old entry.
525 OldEntry = &F->getEntryBlock();
526 BasicBlock *NewEntry = BasicBlock::Create(F->getContext(), "", F, OldEntry);
527 NewEntry->takeName(OldEntry);
528 OldEntry->setName("tailrecurse");
529 BranchInst::Create(OldEntry, NewEntry);
531 // If this tail call is marked 'tail' and if there are any allocas in the
532 // entry block, move them up to the new entry block.
533 TailCallsAreMarkedTail = CI->isTailCall();
534 if (TailCallsAreMarkedTail)
535 // Move all fixed sized allocas from OldEntry to NewEntry.
536 for (BasicBlock::iterator OEBI = OldEntry->begin(), E = OldEntry->end(),
537 NEBI = NewEntry->begin(); OEBI != E; )
538 if (AllocaInst *AI = dyn_cast<AllocaInst>(OEBI++))
539 if (isa<ConstantInt>(AI->getArraySize()))
540 AI->moveBefore(NEBI);
542 // Now that we have created a new block, which jumps to the entry
543 // block, insert a PHI node for each argument of the function.
544 // For now, we initialize each PHI to only have the real arguments
545 // which are passed in.
546 Instruction *InsertPos = OldEntry->begin();
547 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
549 PHINode *PN = PHINode::Create(I->getType(), 2,
550 I->getName() + ".tr", InsertPos);
551 I->replaceAllUsesWith(PN); // Everyone use the PHI node now!
552 PN->addIncoming(I, NewEntry);
553 ArgumentPHIs.push_back(PN);
557 // If this function has self recursive calls in the tail position where some
558 // are marked tail and some are not, only transform one flavor or another. We
559 // have to choose whether we move allocas in the entry block to the new entry
560 // block or not, so we can't make a good choice for both. NOTE: We could do
561 // slightly better here in the case that the function has no entry block
563 if (TailCallsAreMarkedTail && !CI->isTailCall())
566 // Ok, now that we know we have a pseudo-entry block WITH all of the
567 // required PHI nodes, add entries into the PHI node for the actual
568 // parameters passed into the tail-recursive call.
569 for (unsigned i = 0, e = CI->getNumArgOperands(); i != e; ++i)
570 ArgumentPHIs[i]->addIncoming(CI->getArgOperand(i), BB);
572 // If we are introducing an accumulator variable to eliminate the recursion,
573 // do so now. Note that we _know_ that no subsequent tail recursion
574 // eliminations will happen on this function because of the way the
575 // accumulator recursion predicate is set up.
577 if (AccumulatorRecursionEliminationInitVal) {
578 Instruction *AccRecInstr = AccumulatorRecursionInstr;
579 // Start by inserting a new PHI node for the accumulator.
580 pred_iterator PB = pred_begin(OldEntry), PE = pred_end(OldEntry);
582 PHINode::Create(AccumulatorRecursionEliminationInitVal->getType(),
583 std::distance(PB, PE) + 1,
584 "accumulator.tr", OldEntry->begin());
586 // Loop over all of the predecessors of the tail recursion block. For the
587 // real entry into the function we seed the PHI with the initial value,
588 // computed earlier. For any other existing branches to this block (due to
589 // other tail recursions eliminated) the accumulator is not modified.
590 // Because we haven't added the branch in the current block to OldEntry yet,
591 // it will not show up as a predecessor.
592 for (pred_iterator PI = PB; PI != PE; ++PI) {
594 if (P == &F->getEntryBlock())
595 AccPN->addIncoming(AccumulatorRecursionEliminationInitVal, P);
597 AccPN->addIncoming(AccPN, P);
601 // Add an incoming argument for the current block, which is computed by
602 // our associative and commutative accumulator instruction.
603 AccPN->addIncoming(AccRecInstr, BB);
605 // Next, rewrite the accumulator recursion instruction so that it does not
606 // use the result of the call anymore, instead, use the PHI node we just
608 AccRecInstr->setOperand(AccRecInstr->getOperand(0) != CI, AccPN);
610 // Add an incoming argument for the current block, which is just the
611 // constant returned by the current return instruction.
612 AccPN->addIncoming(Ret->getReturnValue(), BB);
615 // Finally, rewrite any return instructions in the program to return the PHI
616 // node instead of the "initval" that they do currently. This loop will
617 // actually rewrite the return value we are destroying, but that's ok.
618 for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI)
619 if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator()))
620 RI->setOperand(0, AccPN);
624 // Now that all of the PHI nodes are in place, remove the call and
625 // ret instructions, replacing them with an unconditional branch.
626 BranchInst *NewBI = BranchInst::Create(OldEntry, Ret);
627 NewBI->setDebugLoc(CI->getDebugLoc());
629 BB->getInstList().erase(Ret); // Remove return.
630 BB->getInstList().erase(CI); // Remove call.
635 bool TailCallElim::FoldReturnAndProcessPred(BasicBlock *BB,
636 ReturnInst *Ret, BasicBlock *&OldEntry,
637 bool &TailCallsAreMarkedTail,
638 SmallVectorImpl<PHINode *> &ArgumentPHIs,
639 bool CannotTailCallElimCallsMarkedTail) {
642 // If the return block contains nothing but the return and PHI's,
643 // there might be an opportunity to duplicate the return in its
644 // predecessors and perform TRC there. Look for predecessors that end
645 // in unconditional branch and recursive call(s).
646 SmallVector<BranchInst*, 8> UncondBranchPreds;
647 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
648 BasicBlock *Pred = *PI;
649 TerminatorInst *PTI = Pred->getTerminator();
650 if (BranchInst *BI = dyn_cast<BranchInst>(PTI))
651 if (BI->isUnconditional())
652 UncondBranchPreds.push_back(BI);
655 while (!UncondBranchPreds.empty()) {
656 BranchInst *BI = UncondBranchPreds.pop_back_val();
657 BasicBlock *Pred = BI->getParent();
658 if (CallInst *CI = FindTRECandidate(BI, CannotTailCallElimCallsMarkedTail)){
659 DEBUG(dbgs() << "FOLDING: " << *BB
660 << "INTO UNCOND BRANCH PRED: " << *Pred);
661 EliminateRecursiveTailCall(CI, FoldReturnIntoUncondBranch(Ret, BB, Pred),
662 OldEntry, TailCallsAreMarkedTail, ArgumentPHIs,
663 CannotTailCallElimCallsMarkedTail);
673 TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry,
674 bool &TailCallsAreMarkedTail,
675 SmallVectorImpl<PHINode *> &ArgumentPHIs,
676 bool CannotTailCallElimCallsMarkedTail) {
677 CallInst *CI = FindTRECandidate(Ret, CannotTailCallElimCallsMarkedTail);
681 return EliminateRecursiveTailCall(CI, Ret, OldEntry, TailCallsAreMarkedTail,
683 CannotTailCallElimCallsMarkedTail);