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 preceeds 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/Transforms/Utils/Local.h"
56 #include "llvm/Constants.h"
57 #include "llvm/DerivedTypes.h"
58 #include "llvm/Function.h"
59 #include "llvm/Instructions.h"
60 #include "llvm/Pass.h"
61 #include "llvm/Analysis/CaptureTracking.h"
62 #include "llvm/Analysis/InlineCost.h"
63 #include "llvm/Analysis/Loads.h"
64 #include "llvm/Support/CallSite.h"
65 #include "llvm/Support/CFG.h"
66 #include "llvm/ADT/Statistic.h"
69 STATISTIC(NumEliminated, "Number of tail calls removed");
70 STATISTIC(NumAccumAdded, "Number of accumulators introduced");
73 struct TailCallElim : public FunctionPass {
74 static char ID; // Pass identification, replacement for typeid
75 TailCallElim() : FunctionPass(ID) {}
77 virtual bool runOnFunction(Function &F);
80 bool ProcessReturningBlock(ReturnInst *RI, BasicBlock *&OldEntry,
81 bool &TailCallsAreMarkedTail,
82 SmallVector<PHINode*, 8> &ArgumentPHIs,
83 bool CannotTailCallElimCallsMarkedTail);
84 bool CanMoveAboveCall(Instruction *I, CallInst *CI);
85 Value *CanTransformAccumulatorRecursion(Instruction *I, CallInst *CI);
89 char TailCallElim::ID = 0;
90 INITIALIZE_PASS(TailCallElim, "tailcallelim",
91 "Tail Call Elimination", false, false);
93 // Public interface to the TailCallElimination pass
94 FunctionPass *llvm::createTailCallEliminationPass() {
95 return new TailCallElim();
98 /// AllocaMightEscapeToCalls - Return true if this alloca may be accessed by
99 /// callees of this function. We only do very simple analysis right now, this
100 /// could be expanded in the future to use mod/ref information for particular
101 /// call sites if desired.
102 static bool AllocaMightEscapeToCalls(AllocaInst *AI) {
103 // FIXME: do simple 'address taken' analysis.
107 /// CheckForEscapingAllocas - Scan the specified basic block for alloca
108 /// instructions. If it contains any that might be accessed by calls, return
110 static bool CheckForEscapingAllocas(BasicBlock *BB,
111 bool &CannotTCETailMarkedCall) {
113 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
114 if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) {
115 RetVal |= AllocaMightEscapeToCalls(AI);
117 // If this alloca is in the body of the function, or if it is a variable
118 // sized allocation, we cannot tail call eliminate calls marked 'tail'
119 // with this mechanism.
120 if (BB != &BB->getParent()->getEntryBlock() ||
121 !isa<ConstantInt>(AI->getArraySize()))
122 CannotTCETailMarkedCall = true;
127 bool TailCallElim::runOnFunction(Function &F) {
128 // If this function is a varargs function, we won't be able to PHI the args
129 // right, so don't even try to convert it...
130 if (F.getFunctionType()->isVarArg()) return false;
132 BasicBlock *OldEntry = 0;
133 bool TailCallsAreMarkedTail = false;
134 SmallVector<PHINode*, 8> ArgumentPHIs;
135 bool MadeChange = false;
137 bool FunctionContainsEscapingAllocas = false;
139 // CannotTCETailMarkedCall - If true, we cannot perform TCE on tail calls
140 // marked with the 'tail' attribute, because doing so would cause the stack
141 // size to increase (real TCE would deallocate variable sized allocas, TCE
143 bool CannotTCETailMarkedCall = false;
145 // Loop over the function, looking for any returning blocks, and keeping track
146 // of whether this function has any non-trivially used allocas.
147 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
148 if (FunctionContainsEscapingAllocas && CannotTCETailMarkedCall)
151 FunctionContainsEscapingAllocas |=
152 CheckForEscapingAllocas(BB, CannotTCETailMarkedCall);
155 /// FIXME: The code generator produces really bad code when an 'escaping
156 /// alloca' is changed from being a static alloca to being a dynamic alloca.
157 /// Until this is resolved, disable this transformation if that would ever
158 /// happen. This bug is PR962.
159 if (FunctionContainsEscapingAllocas)
162 // Second pass, change any tail calls to loops.
163 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
164 if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator()))
165 MadeChange |= ProcessReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail,
166 ArgumentPHIs,CannotTCETailMarkedCall);
168 // If we eliminated any tail recursions, it's possible that we inserted some
169 // silly PHI nodes which just merge an initial value (the incoming operand)
170 // with themselves. Check to see if we did and clean up our mess if so. This
171 // occurs when a function passes an argument straight through to its tail
173 if (!ArgumentPHIs.empty()) {
174 for (unsigned i = 0, e = ArgumentPHIs.size(); i != e; ++i) {
175 PHINode *PN = ArgumentPHIs[i];
177 // If the PHI Node is a dynamic constant, replace it with the value it is.
178 if (Value *PNV = PN->hasConstantValue()) {
179 PN->replaceAllUsesWith(PNV);
180 PN->eraseFromParent();
185 // Finally, if this function contains no non-escaping allocas, mark all calls
186 // in the function as eligible for tail calls (there is no stack memory for
188 if (!FunctionContainsEscapingAllocas)
189 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
190 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
191 if (CallInst *CI = dyn_cast<CallInst>(I)) {
200 /// CanMoveAboveCall - Return true if it is safe to move the specified
201 /// instruction from after the call to before the call, assuming that all
202 /// instructions between the call and this instruction are movable.
204 bool TailCallElim::CanMoveAboveCall(Instruction *I, CallInst *CI) {
205 // FIXME: We can move load/store/call/free instructions above the call if the
206 // call does not mod/ref the memory location being processed.
207 if (I->mayHaveSideEffects()) // This also handles volatile loads.
210 if (LoadInst *L = dyn_cast<LoadInst>(I)) {
211 // Loads may always be moved above calls without side effects.
212 if (CI->mayHaveSideEffects()) {
213 // Non-volatile loads may be moved above a call with side effects if it
214 // does not write to memory and the load provably won't trap.
215 // FIXME: Writes to memory only matter if they may alias the pointer
216 // being loaded from.
217 if (CI->mayWriteToMemory() ||
218 !isSafeToLoadUnconditionally(L->getPointerOperand(), L,
224 // Otherwise, if this is a side-effect free instruction, check to make sure
225 // that it does not use the return value of the call. If it doesn't use the
226 // return value of the call, it must only use things that are defined before
227 // the call, or movable instructions between the call and the instruction
229 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
230 if (I->getOperand(i) == CI)
235 // isDynamicConstant - Return true if the specified value is the same when the
236 // return would exit as it was when the initial iteration of the recursive
237 // function was executed.
239 // We currently handle static constants and arguments that are not modified as
240 // part of the recursion.
242 static bool isDynamicConstant(Value *V, CallInst *CI, ReturnInst *RI) {
243 if (isa<Constant>(V)) return true; // Static constants are always dyn consts
245 // Check to see if this is an immutable argument, if so, the value
246 // will be available to initialize the accumulator.
247 if (Argument *Arg = dyn_cast<Argument>(V)) {
248 // Figure out which argument number this is...
250 Function *F = CI->getParent()->getParent();
251 for (Function::arg_iterator AI = F->arg_begin(); &*AI != Arg; ++AI)
254 // If we are passing this argument into call as the corresponding
255 // argument operand, then the argument is dynamically constant.
256 // Otherwise, we cannot transform this function safely.
257 if (CI->getArgOperand(ArgNo) == Arg)
261 // Switch cases are always constant integers. If the value is being switched
262 // on and the return is only reachable from one of its cases, it's
263 // effectively constant.
264 if (BasicBlock *UniquePred = RI->getParent()->getUniquePredecessor())
265 if (SwitchInst *SI = dyn_cast<SwitchInst>(UniquePred->getTerminator()))
266 if (SI->getCondition() == V)
267 return SI->getDefaultDest() != RI->getParent();
269 // Not a constant or immutable argument, we can't safely transform.
273 // getCommonReturnValue - Check to see if the function containing the specified
274 // tail call consistently returns the same runtime-constant value at all exit
275 // points except for IgnoreRI. If so, return the returned value.
277 static Value *getCommonReturnValue(ReturnInst *IgnoreRI, CallInst *CI) {
278 Function *F = CI->getParent()->getParent();
279 Value *ReturnedValue = 0;
281 for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI) {
282 ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator());
283 if (RI == 0 || RI == IgnoreRI) continue;
285 // We can only perform this transformation if the value returned is
286 // evaluatable at the start of the initial invocation of the function,
287 // instead of at the end of the evaluation.
289 Value *RetOp = RI->getOperand(0);
290 if (!isDynamicConstant(RetOp, CI, RI))
293 if (ReturnedValue && RetOp != ReturnedValue)
294 return 0; // Cannot transform if differing values are returned.
295 ReturnedValue = RetOp;
297 return ReturnedValue;
300 /// CanTransformAccumulatorRecursion - If the specified instruction can be
301 /// transformed using accumulator recursion elimination, return the constant
302 /// which is the start of the accumulator value. Otherwise return null.
304 Value *TailCallElim::CanTransformAccumulatorRecursion(Instruction *I,
306 if (!I->isAssociative() || !I->isCommutative()) return 0;
307 assert(I->getNumOperands() == 2 &&
308 "Associative/commutative operations should have 2 args!");
310 // Exactly one operand should be the result of the call instruction.
311 if ((I->getOperand(0) == CI && I->getOperand(1) == CI) ||
312 (I->getOperand(0) != CI && I->getOperand(1) != CI))
315 // The only user of this instruction we allow is a single return instruction.
316 if (!I->hasOneUse() || !isa<ReturnInst>(I->use_back()))
319 // Ok, now we have to check all of the other return instructions in this
320 // function. If they return non-constants or differing values, then we cannot
321 // transform the function safely.
322 return getCommonReturnValue(cast<ReturnInst>(I->use_back()), CI);
325 bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry,
326 bool &TailCallsAreMarkedTail,
327 SmallVector<PHINode*, 8> &ArgumentPHIs,
328 bool CannotTailCallElimCallsMarkedTail) {
329 BasicBlock *BB = Ret->getParent();
330 Function *F = BB->getParent();
332 if (&BB->front() == Ret) // Make sure there is something before the ret...
335 // Scan backwards from the return, checking to see if there is a tail call in
336 // this block. If so, set CI to it.
338 BasicBlock::iterator BBI = Ret;
340 CI = dyn_cast<CallInst>(BBI);
341 if (CI && CI->getCalledFunction() == F)
344 if (BBI == BB->begin())
345 return false; // Didn't find a potential tail call.
349 // If this call is marked as a tail call, and if there are dynamic allocas in
350 // the function, we cannot perform this optimization.
351 if (CI->isTailCall() && CannotTailCallElimCallsMarkedTail)
354 // As a special case, detect code like this:
355 // double fabs(double f) { return __builtin_fabs(f); } // a 'fabs' call
356 // and disable this xform in this case, because the code generator will
357 // lower the call to fabs into inline code.
358 if (BB == &F->getEntryBlock() &&
359 &BB->front() == CI && &*++BB->begin() == Ret &&
361 // A single-block function with just a call and a return. Check that
362 // the arguments match.
363 CallSite::arg_iterator I = CallSite(CI).arg_begin(),
364 E = CallSite(CI).arg_end();
365 Function::arg_iterator FI = F->arg_begin(),
367 for (; I != E && FI != FE; ++I, ++FI)
368 if (*I != &*FI) break;
369 if (I == E && FI == FE)
373 // If we are introducing accumulator recursion to eliminate operations after
374 // the call instruction that are both associative and commutative, the initial
375 // value for the accumulator is placed in this variable. If this value is set
376 // then we actually perform accumulator recursion elimination instead of
377 // simple tail recursion elimination. If the operation is an LLVM instruction
378 // (eg: "add") then it is recorded in AccumulatorRecursionInstr. If not, then
379 // we are handling the case when the return instruction returns a constant C
380 // which is different to the constant returned by other return instructions
381 // (which is recorded in AccumulatorRecursionEliminationInitVal). This is a
382 // special case of accumulator recursion, the operation being "return C".
383 Value *AccumulatorRecursionEliminationInitVal = 0;
384 Instruction *AccumulatorRecursionInstr = 0;
386 // Ok, we found a potential tail call. We can currently only transform the
387 // tail call if all of the instructions between the call and the return are
388 // movable to above the call itself, leaving the call next to the return.
389 // Check that this is the case now.
390 for (BBI = CI, ++BBI; &*BBI != Ret; ++BBI) {
391 if (CanMoveAboveCall(BBI, CI)) continue;
393 // If we can't move the instruction above the call, it might be because it
394 // is an associative and commutative operation that could be tranformed
395 // using accumulator recursion elimination. Check to see if this is the
396 // case, and if so, remember the initial accumulator value for later.
397 if ((AccumulatorRecursionEliminationInitVal =
398 CanTransformAccumulatorRecursion(BBI, CI))) {
399 // Yes, this is accumulator recursion. Remember which instruction
401 AccumulatorRecursionInstr = BBI;
403 return false; // Otherwise, we cannot eliminate the tail recursion!
407 // We can only transform call/return pairs that either ignore the return value
408 // of the call and return void, ignore the value of the call and return a
409 // constant, return the value returned by the tail call, or that are being
410 // accumulator recursion variable eliminated.
411 if (Ret->getNumOperands() == 1 && Ret->getReturnValue() != CI &&
412 !isa<UndefValue>(Ret->getReturnValue()) &&
413 AccumulatorRecursionEliminationInitVal == 0 &&
414 !getCommonReturnValue(0, CI)) {
415 // One case remains that we are able to handle: the current return
416 // instruction returns a constant, and all other return instructions
417 // return a different constant.
418 if (!isDynamicConstant(Ret->getReturnValue(), CI, Ret))
419 return false; // Current return instruction does not return a constant.
420 // Check that all other return instructions return a common constant. If
421 // so, record it in AccumulatorRecursionEliminationInitVal.
422 AccumulatorRecursionEliminationInitVal = getCommonReturnValue(Ret, CI);
423 if (!AccumulatorRecursionEliminationInitVal)
427 // OK! We can transform this tail call. If this is the first one found,
428 // create the new entry block, allowing us to branch back to the old entry.
430 OldEntry = &F->getEntryBlock();
431 BasicBlock *NewEntry = BasicBlock::Create(F->getContext(), "", F, OldEntry);
432 NewEntry->takeName(OldEntry);
433 OldEntry->setName("tailrecurse");
434 BranchInst::Create(OldEntry, NewEntry);
436 // If this tail call is marked 'tail' and if there are any allocas in the
437 // entry block, move them up to the new entry block.
438 TailCallsAreMarkedTail = CI->isTailCall();
439 if (TailCallsAreMarkedTail)
440 // Move all fixed sized allocas from OldEntry to NewEntry.
441 for (BasicBlock::iterator OEBI = OldEntry->begin(), E = OldEntry->end(),
442 NEBI = NewEntry->begin(); OEBI != E; )
443 if (AllocaInst *AI = dyn_cast<AllocaInst>(OEBI++))
444 if (isa<ConstantInt>(AI->getArraySize()))
445 AI->moveBefore(NEBI);
447 // Now that we have created a new block, which jumps to the entry
448 // block, insert a PHI node for each argument of the function.
449 // For now, we initialize each PHI to only have the real arguments
450 // which are passed in.
451 Instruction *InsertPos = OldEntry->begin();
452 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
454 PHINode *PN = PHINode::Create(I->getType(),
455 I->getName() + ".tr", InsertPos);
456 I->replaceAllUsesWith(PN); // Everyone use the PHI node now!
457 PN->addIncoming(I, NewEntry);
458 ArgumentPHIs.push_back(PN);
462 // If this function has self recursive calls in the tail position where some
463 // are marked tail and some are not, only transform one flavor or another. We
464 // have to choose whether we move allocas in the entry block to the new entry
465 // block or not, so we can't make a good choice for both. NOTE: We could do
466 // slightly better here in the case that the function has no entry block
468 if (TailCallsAreMarkedTail && !CI->isTailCall())
471 // Ok, now that we know we have a pseudo-entry block WITH all of the
472 // required PHI nodes, add entries into the PHI node for the actual
473 // parameters passed into the tail-recursive call.
474 for (unsigned i = 0, e = CI->getNumArgOperands(); i != e; ++i)
475 ArgumentPHIs[i]->addIncoming(CI->getArgOperand(i), BB);
477 // If we are introducing an accumulator variable to eliminate the recursion,
478 // do so now. Note that we _know_ that no subsequent tail recursion
479 // eliminations will happen on this function because of the way the
480 // accumulator recursion predicate is set up.
482 if (AccumulatorRecursionEliminationInitVal) {
483 Instruction *AccRecInstr = AccumulatorRecursionInstr;
484 // Start by inserting a new PHI node for the accumulator.
486 PHINode::Create(AccumulatorRecursionEliminationInitVal->getType(),
487 "accumulator.tr", OldEntry->begin());
489 // Loop over all of the predecessors of the tail recursion block. For the
490 // real entry into the function we seed the PHI with the initial value,
491 // computed earlier. For any other existing branches to this block (due to
492 // other tail recursions eliminated) the accumulator is not modified.
493 // Because we haven't added the branch in the current block to OldEntry yet,
494 // it will not show up as a predecessor.
495 for (pred_iterator PI = pred_begin(OldEntry), PE = pred_end(OldEntry);
498 if (P == &F->getEntryBlock())
499 AccPN->addIncoming(AccumulatorRecursionEliminationInitVal, P);
501 AccPN->addIncoming(AccPN, P);
505 // Add an incoming argument for the current block, which is computed by
506 // our associative and commutative accumulator instruction.
507 AccPN->addIncoming(AccRecInstr, BB);
509 // Next, rewrite the accumulator recursion instruction so that it does not
510 // use the result of the call anymore, instead, use the PHI node we just
512 AccRecInstr->setOperand(AccRecInstr->getOperand(0) != CI, AccPN);
514 // Add an incoming argument for the current block, which is just the
515 // constant returned by the current return instruction.
516 AccPN->addIncoming(Ret->getReturnValue(), BB);
519 // Finally, rewrite any return instructions in the program to return the PHI
520 // node instead of the "initval" that they do currently. This loop will
521 // actually rewrite the return value we are destroying, but that's ok.
522 for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI)
523 if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator()))
524 RI->setOperand(0, AccPN);
528 // Now that all of the PHI nodes are in place, remove the call and
529 // ret instructions, replacing them with an unconditional branch.
530 BranchInst::Create(OldEntry, Ret);
531 BB->getInstList().erase(Ret); // Remove return.
532 BB->getInstList().erase(CI); // Remove call.