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1 //===- TailRecursionElimination.cpp - Eliminate Tail Calls ----------------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
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
13 // algorithm:
14 //
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
30 //     generator).
31 //
32 // There are several improvements that could be made:
33 //
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.
50 //
51 //===----------------------------------------------------------------------===//
52 
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/Constants.h"
64 #include "llvm/IR/DerivedTypes.h"
65 #include "llvm/IR/Function.h"
66 #include "llvm/IR/Instructions.h"
67 #include "llvm/IR/IntrinsicInst.h"
68 #include "llvm/IR/Module.h"
69 #include "llvm/Pass.h"
70 #include "llvm/Support/CFG.h"
71 #include "llvm/Support/CallSite.h"
72 #include "llvm/Support/Debug.h"
73 #include "llvm/Support/ValueHandle.h"
74 #include "llvm/Support/raw_ostream.h"
75 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
76 #include "llvm/Transforms/Utils/Local.h"
77 using namespace llvm;
78 
79 STATISTIC(NumEliminated, "Number of tail calls removed");
80 STATISTIC(NumRetDuped,   "Number of return duplicated");
81 STATISTIC(NumAccumAdded, "Number of accumulators introduced");
82 
83 namespace {
84   struct TailCallElim : public FunctionPass {
85     const TargetTransformInfo *TTI;
86 
87     static char ID; // Pass identification, replacement for typeid
TailCallElim__anon513e489c0111::TailCallElim88     TailCallElim() : FunctionPass(ID) {
89       initializeTailCallElimPass(*PassRegistry::getPassRegistry());
90     }
91 
92     virtual void getAnalysisUsage(AnalysisUsage &AU) const;
93 
94     virtual bool runOnFunction(Function &F);
95 
96   private:
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);
115   };
116 }
117 
118 char TailCallElim::ID = 0;
119 INITIALIZE_PASS_BEGIN(TailCallElim, "tailcallelim",
120                       "Tail Call Elimination", false, false)
INITIALIZE_AG_DEPENDENCY(TargetTransformInfo)121 INITIALIZE_AG_DEPENDENCY(TargetTransformInfo)
122 INITIALIZE_PASS_END(TailCallElim, "tailcallelim",
123                     "Tail Call Elimination", false, false)
124 
125 // Public interface to the TailCallElimination pass
126 FunctionPass *llvm::createTailCallEliminationPass() {
127   return new TailCallElim();
128 }
129 
getAnalysisUsage(AnalysisUsage & AU) const130 void TailCallElim::getAnalysisUsage(AnalysisUsage &AU) const {
131   AU.addRequired<TargetTransformInfo>();
132 }
133 
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,
136 /// returns false.
CanTRE(AllocaInst * AI)137 static bool CanTRE(AllocaInst *AI) {
138   // Because of PR962, we don't TRE allocas outside the entry block.
139 
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());
146 }
147 
148 namespace {
149 struct AllocaCaptureTracker : public CaptureTracker {
AllocaCaptureTracker__anon513e489c0211::AllocaCaptureTracker150   AllocaCaptureTracker() : Captured(false) {}
151 
tooManyUses__anon513e489c0211::AllocaCaptureTracker152   void tooManyUses() LLVM_OVERRIDE { Captured = true; }
153 
shouldExplore__anon513e489c0211::AllocaCaptureTracker154   bool shouldExplore(Use *U) LLVM_OVERRIDE {
155     Value *V = U->getUser();
156     if (isa<CallInst>(V) || isa<InvokeInst>(V))
157       UsesAlloca.insert(V);
158     return true;
159   }
160 
captured__anon513e489c0211::AllocaCaptureTracker161   bool captured(Use *U) LLVM_OVERRIDE {
162     if (isa<ReturnInst>(U->getUser()))
163       return false;
164     Captured = true;
165     return true;
166   }
167 
168   bool Captured;
169   SmallPtrSet<const Value *, 16> UsesAlloca;
170 };
171 } // end anonymous namespace
172 
runOnFunction(Function & F)173 bool TailCallElim::runOnFunction(Function &F) {
174   // If this function is a varargs function, we won't be able to PHI the args
175   // right, so don't even try to convert it...
176   if (F.getFunctionType()->isVarArg()) return false;
177 
178   TTI = &getAnalysis<TargetTransformInfo>();
179   BasicBlock *OldEntry = 0;
180   bool TailCallsAreMarkedTail = false;
181   SmallVector<PHINode*, 8> ArgumentPHIs;
182   bool MadeChange = false;
183 
184   // CanTRETailMarkedCall - If false, we cannot perform TRE on tail calls
185   // marked with the 'tail' attribute, because doing so would cause the stack
186   // size to increase (real TRE would deallocate variable sized allocas, TRE
187   // doesn't).
188   bool CanTRETailMarkedCall = true;
189 
190   // Find calls that can be marked tail.
191   AllocaCaptureTracker ACT;
192   for (Function::iterator BB = F.begin(), EE = F.end(); BB != EE; ++BB) {
193     for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
194       if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) {
195         CanTRETailMarkedCall &= CanTRE(AI);
196         PointerMayBeCaptured(AI, &ACT);
197         // If any allocas are captured, exit.
198         if (ACT.Captured)
199           return false;
200       }
201     }
202   }
203 
204   // Second pass, change any tail recursive calls to loops.
205   //
206   // FIXME: The code generator produces really bad code when an 'escaping
207   // alloca' is changed from being a static alloca to being a dynamic alloca.
208   // Until this is resolved, disable this transformation if that would ever
209   // happen.  This bug is PR962.
210   if (ACT.UsesAlloca.empty()) {
211     for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
212       if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator())) {
213         bool Change = ProcessReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail,
214                                             ArgumentPHIs, !CanTRETailMarkedCall);
215         if (!Change && BB->getFirstNonPHIOrDbg() == Ret)
216           Change = FoldReturnAndProcessPred(BB, Ret, OldEntry,
217                                             TailCallsAreMarkedTail, ArgumentPHIs,
218                                             !CanTRETailMarkedCall);
219         MadeChange |= Change;
220       }
221     }
222   }
223 
224   // If we eliminated any tail recursions, it's possible that we inserted some
225   // silly PHI nodes which just merge an initial value (the incoming operand)
226   // with themselves.  Check to see if we did and clean up our mess if so.  This
227   // occurs when a function passes an argument straight through to its tail
228   // call.
229   if (!ArgumentPHIs.empty()) {
230     for (unsigned i = 0, e = ArgumentPHIs.size(); i != e; ++i) {
231       PHINode *PN = ArgumentPHIs[i];
232 
233       // If the PHI Node is a dynamic constant, replace it with the value it is.
234       if (Value *PNV = SimplifyInstruction(PN)) {
235         PN->replaceAllUsesWith(PNV);
236         PN->eraseFromParent();
237       }
238     }
239   }
240 
241   // At this point, we know that the function does not have any captured
242   // allocas. If additionally the function does not call setjmp, mark all calls
243   // in the function that do not access stack memory with the tail keyword. This
244   // implies ensuring that there does not exist any path from a call that takes
245   // in an alloca but does not capture it and the call which we wish to mark
246   // with "tail".
247   if (!F.callsFunctionThatReturnsTwice()) {
248     for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
249       for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
250         if (CallInst *CI = dyn_cast<CallInst>(I)) {
251           if (!ACT.UsesAlloca.count(CI)) {
252             CI->setTailCall();
253             MadeChange = true;
254           }
255         }
256       }
257     }
258   }
259 
260   return MadeChange;
261 }
262 
263 
264 /// CanMoveAboveCall - Return true if it is safe to move the specified
265 /// instruction from after the call to before the call, assuming that all
266 /// instructions between the call and this instruction are movable.
267 ///
CanMoveAboveCall(Instruction * I,CallInst * CI)268 bool TailCallElim::CanMoveAboveCall(Instruction *I, CallInst *CI) {
269   // FIXME: We can move load/store/call/free instructions above the call if the
270   // call does not mod/ref the memory location being processed.
271   if (I->mayHaveSideEffects())  // This also handles volatile loads.
272     return false;
273 
274   if (LoadInst *L = dyn_cast<LoadInst>(I)) {
275     // Loads may always be moved above calls without side effects.
276     if (CI->mayHaveSideEffects()) {
277       // Non-volatile loads may be moved above a call with side effects if it
278       // does not write to memory and the load provably won't trap.
279       // FIXME: Writes to memory only matter if they may alias the pointer
280       // being loaded from.
281       if (CI->mayWriteToMemory() ||
282           !isSafeToLoadUnconditionally(L->getPointerOperand(), L,
283                                        L->getAlignment()))
284         return false;
285     }
286   }
287 
288   // Otherwise, if this is a side-effect free instruction, check to make sure
289   // that it does not use the return value of the call.  If it doesn't use the
290   // return value of the call, it must only use things that are defined before
291   // the call, or movable instructions between the call and the instruction
292   // itself.
293   for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
294     if (I->getOperand(i) == CI)
295       return false;
296   return true;
297 }
298 
299 // isDynamicConstant - Return true if the specified value is the same when the
300 // return would exit as it was when the initial iteration of the recursive
301 // function was executed.
302 //
303 // We currently handle static constants and arguments that are not modified as
304 // part of the recursion.
305 //
isDynamicConstant(Value * V,CallInst * CI,ReturnInst * RI)306 static bool isDynamicConstant(Value *V, CallInst *CI, ReturnInst *RI) {
307   if (isa<Constant>(V)) return true; // Static constants are always dyn consts
308 
309   // Check to see if this is an immutable argument, if so, the value
310   // will be available to initialize the accumulator.
311   if (Argument *Arg = dyn_cast<Argument>(V)) {
312     // Figure out which argument number this is...
313     unsigned ArgNo = 0;
314     Function *F = CI->getParent()->getParent();
315     for (Function::arg_iterator AI = F->arg_begin(); &*AI != Arg; ++AI)
316       ++ArgNo;
317 
318     // If we are passing this argument into call as the corresponding
319     // argument operand, then the argument is dynamically constant.
320     // Otherwise, we cannot transform this function safely.
321     if (CI->getArgOperand(ArgNo) == Arg)
322       return true;
323   }
324 
325   // Switch cases are always constant integers. If the value is being switched
326   // on and the return is only reachable from one of its cases, it's
327   // effectively constant.
328   if (BasicBlock *UniquePred = RI->getParent()->getUniquePredecessor())
329     if (SwitchInst *SI = dyn_cast<SwitchInst>(UniquePred->getTerminator()))
330       if (SI->getCondition() == V)
331         return SI->getDefaultDest() != RI->getParent();
332 
333   // Not a constant or immutable argument, we can't safely transform.
334   return false;
335 }
336 
337 // getCommonReturnValue - Check to see if the function containing the specified
338 // tail call consistently returns the same runtime-constant value at all exit
339 // points except for IgnoreRI.  If so, return the returned value.
340 //
getCommonReturnValue(ReturnInst * IgnoreRI,CallInst * CI)341 static Value *getCommonReturnValue(ReturnInst *IgnoreRI, CallInst *CI) {
342   Function *F = CI->getParent()->getParent();
343   Value *ReturnedValue = 0;
344 
345   for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI) {
346     ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator());
347     if (RI == 0 || RI == IgnoreRI) continue;
348 
349     // We can only perform this transformation if the value returned is
350     // evaluatable at the start of the initial invocation of the function,
351     // instead of at the end of the evaluation.
352     //
353     Value *RetOp = RI->getOperand(0);
354     if (!isDynamicConstant(RetOp, CI, RI))
355       return 0;
356 
357     if (ReturnedValue && RetOp != ReturnedValue)
358       return 0;     // Cannot transform if differing values are returned.
359     ReturnedValue = RetOp;
360   }
361   return ReturnedValue;
362 }
363 
364 /// CanTransformAccumulatorRecursion - If the specified instruction can be
365 /// transformed using accumulator recursion elimination, return the constant
366 /// which is the start of the accumulator value.  Otherwise return null.
367 ///
CanTransformAccumulatorRecursion(Instruction * I,CallInst * CI)368 Value *TailCallElim::CanTransformAccumulatorRecursion(Instruction *I,
369                                                       CallInst *CI) {
370   if (!I->isAssociative() || !I->isCommutative()) return 0;
371   assert(I->getNumOperands() == 2 &&
372          "Associative/commutative operations should have 2 args!");
373 
374   // Exactly one operand should be the result of the call instruction.
375   if ((I->getOperand(0) == CI && I->getOperand(1) == CI) ||
376       (I->getOperand(0) != CI && I->getOperand(1) != CI))
377     return 0;
378 
379   // The only user of this instruction we allow is a single return instruction.
380   if (!I->hasOneUse() || !isa<ReturnInst>(I->use_back()))
381     return 0;
382 
383   // Ok, now we have to check all of the other return instructions in this
384   // function.  If they return non-constants or differing values, then we cannot
385   // transform the function safely.
386   return getCommonReturnValue(cast<ReturnInst>(I->use_back()), CI);
387 }
388 
FirstNonDbg(BasicBlock::iterator I)389 static Instruction *FirstNonDbg(BasicBlock::iterator I) {
390   while (isa<DbgInfoIntrinsic>(I))
391     ++I;
392   return &*I;
393 }
394 
395 CallInst*
FindTRECandidate(Instruction * TI,bool CannotTailCallElimCallsMarkedTail)396 TailCallElim::FindTRECandidate(Instruction *TI,
397                                bool CannotTailCallElimCallsMarkedTail) {
398   BasicBlock *BB = TI->getParent();
399   Function *F = BB->getParent();
400 
401   if (&BB->front() == TI) // Make sure there is something before the terminator.
402     return 0;
403 
404   // Scan backwards from the return, checking to see if there is a tail call in
405   // this block.  If so, set CI to it.
406   CallInst *CI = 0;
407   BasicBlock::iterator BBI = TI;
408   while (true) {
409     CI = dyn_cast<CallInst>(BBI);
410     if (CI && CI->getCalledFunction() == F)
411       break;
412 
413     if (BBI == BB->begin())
414       return 0;          // Didn't find a potential tail call.
415     --BBI;
416   }
417 
418   // If this call is marked as a tail call, and if there are dynamic allocas in
419   // the function, we cannot perform this optimization.
420   if (CI->isTailCall() && CannotTailCallElimCallsMarkedTail)
421     return 0;
422 
423   // As a special case, detect code like this:
424   //   double fabs(double f) { return __builtin_fabs(f); } // a 'fabs' call
425   // and disable this xform in this case, because the code generator will
426   // lower the call to fabs into inline code.
427   if (BB == &F->getEntryBlock() &&
428       FirstNonDbg(BB->front()) == CI &&
429       FirstNonDbg(llvm::next(BB->begin())) == TI &&
430       CI->getCalledFunction() &&
431       !TTI->isLoweredToCall(CI->getCalledFunction())) {
432     // A single-block function with just a call and a return. Check that
433     // the arguments match.
434     CallSite::arg_iterator I = CallSite(CI).arg_begin(),
435                            E = CallSite(CI).arg_end();
436     Function::arg_iterator FI = F->arg_begin(),
437                            FE = F->arg_end();
438     for (; I != E && FI != FE; ++I, ++FI)
439       if (*I != &*FI) break;
440     if (I == E && FI == FE)
441       return 0;
442   }
443 
444   return CI;
445 }
446 
EliminateRecursiveTailCall(CallInst * CI,ReturnInst * Ret,BasicBlock * & OldEntry,bool & TailCallsAreMarkedTail,SmallVectorImpl<PHINode * > & ArgumentPHIs,bool CannotTailCallElimCallsMarkedTail)447 bool TailCallElim::EliminateRecursiveTailCall(CallInst *CI, ReturnInst *Ret,
448                                        BasicBlock *&OldEntry,
449                                        bool &TailCallsAreMarkedTail,
450                                        SmallVectorImpl<PHINode *> &ArgumentPHIs,
451                                        bool CannotTailCallElimCallsMarkedTail) {
452   // If we are introducing accumulator recursion to eliminate operations after
453   // the call instruction that are both associative and commutative, the initial
454   // value for the accumulator is placed in this variable.  If this value is set
455   // then we actually perform accumulator recursion elimination instead of
456   // simple tail recursion elimination.  If the operation is an LLVM instruction
457   // (eg: "add") then it is recorded in AccumulatorRecursionInstr.  If not, then
458   // we are handling the case when the return instruction returns a constant C
459   // which is different to the constant returned by other return instructions
460   // (which is recorded in AccumulatorRecursionEliminationInitVal).  This is a
461   // special case of accumulator recursion, the operation being "return C".
462   Value *AccumulatorRecursionEliminationInitVal = 0;
463   Instruction *AccumulatorRecursionInstr = 0;
464 
465   // Ok, we found a potential tail call.  We can currently only transform the
466   // tail call if all of the instructions between the call and the return are
467   // movable to above the call itself, leaving the call next to the return.
468   // Check that this is the case now.
469   BasicBlock::iterator BBI = CI;
470   for (++BBI; &*BBI != Ret; ++BBI) {
471     if (CanMoveAboveCall(BBI, CI)) continue;
472 
473     // If we can't move the instruction above the call, it might be because it
474     // is an associative and commutative operation that could be transformed
475     // using accumulator recursion elimination.  Check to see if this is the
476     // case, and if so, remember the initial accumulator value for later.
477     if ((AccumulatorRecursionEliminationInitVal =
478                            CanTransformAccumulatorRecursion(BBI, CI))) {
479       // Yes, this is accumulator recursion.  Remember which instruction
480       // accumulates.
481       AccumulatorRecursionInstr = BBI;
482     } else {
483       return false;   // Otherwise, we cannot eliminate the tail recursion!
484     }
485   }
486 
487   // We can only transform call/return pairs that either ignore the return value
488   // of the call and return void, ignore the value of the call and return a
489   // constant, return the value returned by the tail call, or that are being
490   // accumulator recursion variable eliminated.
491   if (Ret->getNumOperands() == 1 && Ret->getReturnValue() != CI &&
492       !isa<UndefValue>(Ret->getReturnValue()) &&
493       AccumulatorRecursionEliminationInitVal == 0 &&
494       !getCommonReturnValue(0, CI)) {
495     // One case remains that we are able to handle: the current return
496     // instruction returns a constant, and all other return instructions
497     // return a different constant.
498     if (!isDynamicConstant(Ret->getReturnValue(), CI, Ret))
499       return false; // Current return instruction does not return a constant.
500     // Check that all other return instructions return a common constant.  If
501     // so, record it in AccumulatorRecursionEliminationInitVal.
502     AccumulatorRecursionEliminationInitVal = getCommonReturnValue(Ret, CI);
503     if (!AccumulatorRecursionEliminationInitVal)
504       return false;
505   }
506 
507   BasicBlock *BB = Ret->getParent();
508   Function *F = BB->getParent();
509 
510   // OK! We can transform this tail call.  If this is the first one found,
511   // create the new entry block, allowing us to branch back to the old entry.
512   if (OldEntry == 0) {
513     OldEntry = &F->getEntryBlock();
514     BasicBlock *NewEntry = BasicBlock::Create(F->getContext(), "", F, OldEntry);
515     NewEntry->takeName(OldEntry);
516     OldEntry->setName("tailrecurse");
517     BranchInst::Create(OldEntry, NewEntry);
518 
519     // If this tail call is marked 'tail' and if there are any allocas in the
520     // entry block, move them up to the new entry block.
521     TailCallsAreMarkedTail = CI->isTailCall();
522     if (TailCallsAreMarkedTail)
523       // Move all fixed sized allocas from OldEntry to NewEntry.
524       for (BasicBlock::iterator OEBI = OldEntry->begin(), E = OldEntry->end(),
525              NEBI = NewEntry->begin(); OEBI != E; )
526         if (AllocaInst *AI = dyn_cast<AllocaInst>(OEBI++))
527           if (isa<ConstantInt>(AI->getArraySize()))
528             AI->moveBefore(NEBI);
529 
530     // Now that we have created a new block, which jumps to the entry
531     // block, insert a PHI node for each argument of the function.
532     // For now, we initialize each PHI to only have the real arguments
533     // which are passed in.
534     Instruction *InsertPos = OldEntry->begin();
535     for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
536          I != E; ++I) {
537       PHINode *PN = PHINode::Create(I->getType(), 2,
538                                     I->getName() + ".tr", InsertPos);
539       I->replaceAllUsesWith(PN); // Everyone use the PHI node now!
540       PN->addIncoming(I, NewEntry);
541       ArgumentPHIs.push_back(PN);
542     }
543   }
544 
545   // If this function has self recursive calls in the tail position where some
546   // are marked tail and some are not, only transform one flavor or another.  We
547   // have to choose whether we move allocas in the entry block to the new entry
548   // block or not, so we can't make a good choice for both.  NOTE: We could do
549   // slightly better here in the case that the function has no entry block
550   // allocas.
551   if (TailCallsAreMarkedTail && !CI->isTailCall())
552     return false;
553 
554   // Ok, now that we know we have a pseudo-entry block WITH all of the
555   // required PHI nodes, add entries into the PHI node for the actual
556   // parameters passed into the tail-recursive call.
557   for (unsigned i = 0, e = CI->getNumArgOperands(); i != e; ++i)
558     ArgumentPHIs[i]->addIncoming(CI->getArgOperand(i), BB);
559 
560   // If we are introducing an accumulator variable to eliminate the recursion,
561   // do so now.  Note that we _know_ that no subsequent tail recursion
562   // eliminations will happen on this function because of the way the
563   // accumulator recursion predicate is set up.
564   //
565   if (AccumulatorRecursionEliminationInitVal) {
566     Instruction *AccRecInstr = AccumulatorRecursionInstr;
567     // Start by inserting a new PHI node for the accumulator.
568     pred_iterator PB = pred_begin(OldEntry), PE = pred_end(OldEntry);
569     PHINode *AccPN =
570       PHINode::Create(AccumulatorRecursionEliminationInitVal->getType(),
571                       std::distance(PB, PE) + 1,
572                       "accumulator.tr", OldEntry->begin());
573 
574     // Loop over all of the predecessors of the tail recursion block.  For the
575     // real entry into the function we seed the PHI with the initial value,
576     // computed earlier.  For any other existing branches to this block (due to
577     // other tail recursions eliminated) the accumulator is not modified.
578     // Because we haven't added the branch in the current block to OldEntry yet,
579     // it will not show up as a predecessor.
580     for (pred_iterator PI = PB; PI != PE; ++PI) {
581       BasicBlock *P = *PI;
582       if (P == &F->getEntryBlock())
583         AccPN->addIncoming(AccumulatorRecursionEliminationInitVal, P);
584       else
585         AccPN->addIncoming(AccPN, P);
586     }
587 
588     if (AccRecInstr) {
589       // Add an incoming argument for the current block, which is computed by
590       // our associative and commutative accumulator instruction.
591       AccPN->addIncoming(AccRecInstr, BB);
592 
593       // Next, rewrite the accumulator recursion instruction so that it does not
594       // use the result of the call anymore, instead, use the PHI node we just
595       // inserted.
596       AccRecInstr->setOperand(AccRecInstr->getOperand(0) != CI, AccPN);
597     } else {
598       // Add an incoming argument for the current block, which is just the
599       // constant returned by the current return instruction.
600       AccPN->addIncoming(Ret->getReturnValue(), BB);
601     }
602 
603     // Finally, rewrite any return instructions in the program to return the PHI
604     // node instead of the "initval" that they do currently.  This loop will
605     // actually rewrite the return value we are destroying, but that's ok.
606     for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI)
607       if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator()))
608         RI->setOperand(0, AccPN);
609     ++NumAccumAdded;
610   }
611 
612   // Now that all of the PHI nodes are in place, remove the call and
613   // ret instructions, replacing them with an unconditional branch.
614   BranchInst *NewBI = BranchInst::Create(OldEntry, Ret);
615   NewBI->setDebugLoc(CI->getDebugLoc());
616 
617   BB->getInstList().erase(Ret);  // Remove return.
618   BB->getInstList().erase(CI);   // Remove call.
619   ++NumEliminated;
620   return true;
621 }
622 
FoldReturnAndProcessPred(BasicBlock * BB,ReturnInst * Ret,BasicBlock * & OldEntry,bool & TailCallsAreMarkedTail,SmallVectorImpl<PHINode * > & ArgumentPHIs,bool CannotTailCallElimCallsMarkedTail)623 bool TailCallElim::FoldReturnAndProcessPred(BasicBlock *BB,
624                                        ReturnInst *Ret, BasicBlock *&OldEntry,
625                                        bool &TailCallsAreMarkedTail,
626                                        SmallVectorImpl<PHINode *> &ArgumentPHIs,
627                                        bool CannotTailCallElimCallsMarkedTail) {
628   bool Change = false;
629 
630   // If the return block contains nothing but the return and PHI's,
631   // there might be an opportunity to duplicate the return in its
632   // predecessors and perform TRC there. Look for predecessors that end
633   // in unconditional branch and recursive call(s).
634   SmallVector<BranchInst*, 8> UncondBranchPreds;
635   for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
636     BasicBlock *Pred = *PI;
637     TerminatorInst *PTI = Pred->getTerminator();
638     if (BranchInst *BI = dyn_cast<BranchInst>(PTI))
639       if (BI->isUnconditional())
640         UncondBranchPreds.push_back(BI);
641   }
642 
643   while (!UncondBranchPreds.empty()) {
644     BranchInst *BI = UncondBranchPreds.pop_back_val();
645     BasicBlock *Pred = BI->getParent();
646     if (CallInst *CI = FindTRECandidate(BI, CannotTailCallElimCallsMarkedTail)){
647       DEBUG(dbgs() << "FOLDING: " << *BB
648             << "INTO UNCOND BRANCH PRED: " << *Pred);
649       EliminateRecursiveTailCall(CI, FoldReturnIntoUncondBranch(Ret, BB, Pred),
650                                  OldEntry, TailCallsAreMarkedTail, ArgumentPHIs,
651                                  CannotTailCallElimCallsMarkedTail);
652       ++NumRetDuped;
653       Change = true;
654     }
655   }
656 
657   return Change;
658 }
659 
660 bool
ProcessReturningBlock(ReturnInst * Ret,BasicBlock * & OldEntry,bool & TailCallsAreMarkedTail,SmallVectorImpl<PHINode * > & ArgumentPHIs,bool CannotTailCallElimCallsMarkedTail)661 TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry,
662                                     bool &TailCallsAreMarkedTail,
663                                     SmallVectorImpl<PHINode *> &ArgumentPHIs,
664                                     bool CannotTailCallElimCallsMarkedTail) {
665   CallInst *CI = FindTRECandidate(Ret, CannotTailCallElimCallsMarkedTail);
666   if (!CI)
667     return false;
668 
669   return EliminateRecursiveTailCall(CI, Ret, OldEntry, TailCallsAreMarkedTail,
670                                     ArgumentPHIs,
671                                     CannotTailCallElimCallsMarkedTail);
672 }
673