1 //===- TailRecursionElimination.cpp - Eliminate Tail Calls ----------------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source 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 expression to use an accumulator variable,
20 // thus compiling the typical naive factorial or 'fib' implementation into
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 theier 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/Constants.h"
56 #include "llvm/DerivedTypes.h"
57 #include "llvm/Function.h"
58 #include "llvm/Instructions.h"
59 #include "llvm/Pass.h"
60 #include "llvm/Support/CFG.h"
61 #include "llvm/ADT/Statistic.h"
62 #include "llvm/Support/Compiler.h"
65 STATISTIC(NumEliminated, "Number of tail calls removed");
66 STATISTIC(NumAccumAdded, "Number of accumulators introduced");
69 struct VISIBILITY_HIDDEN TailCallElim : public FunctionPass {
70 static char ID; // Pass identification, replacement for typeid
71 TailCallElim() : FunctionPass((intptr_t)&ID) {}
73 virtual bool runOnFunction(Function &F);
76 bool ProcessReturningBlock(ReturnInst *RI, BasicBlock *&OldEntry,
77 bool &TailCallsAreMarkedTail,
78 std::vector<PHINode*> &ArgumentPHIs,
79 bool CannotTailCallElimCallsMarkedTail);
80 bool CanMoveAboveCall(Instruction *I, CallInst *CI);
81 Value *CanTransformAccumulatorRecursion(Instruction *I, CallInst *CI);
83 char TailCallElim::ID = 0;
84 RegisterPass<TailCallElim> X("tailcallelim", "Tail Call Elimination");
87 // Public interface to the TailCallElimination pass
88 FunctionPass *llvm::createTailCallEliminationPass() {
89 return new TailCallElim();
93 /// AllocaMightEscapeToCalls - Return true if this alloca may be accessed by
94 /// callees of this function. We only do very simple analysis right now, this
95 /// could be expanded in the future to use mod/ref information for particular
96 /// call sites if desired.
97 static bool AllocaMightEscapeToCalls(AllocaInst *AI) {
98 // FIXME: do simple 'address taken' analysis.
102 /// FunctionContainsAllocas - Scan the specified basic block for alloca
103 /// instructions. If it contains any that might be accessed by calls, return
105 static bool CheckForEscapingAllocas(BasicBlock *BB,
106 bool &CannotTCETailMarkedCall) {
108 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
109 if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) {
110 RetVal |= AllocaMightEscapeToCalls(AI);
112 // If this alloca is in the body of the function, or if it is a variable
113 // sized allocation, we cannot tail call eliminate calls marked 'tail'
114 // with this mechanism.
115 if (BB != &BB->getParent()->getEntryBlock() ||
116 !isa<ConstantInt>(AI->getArraySize()))
117 CannotTCETailMarkedCall = true;
122 bool TailCallElim::runOnFunction(Function &F) {
123 // If this function is a varargs function, we won't be able to PHI the args
124 // right, so don't even try to convert it...
125 if (F.getFunctionType()->isVarArg()) return false;
127 BasicBlock *OldEntry = 0;
128 bool TailCallsAreMarkedTail = false;
129 std::vector<PHINode*> ArgumentPHIs;
130 bool MadeChange = false;
132 bool FunctionContainsEscapingAllocas = false;
134 // CannotTCETailMarkedCall - If true, we cannot perform TCE on tail calls
135 // marked with the 'tail' attribute, because doing so would cause the stack
136 // size to increase (real TCE would deallocate variable sized allocas, TCE
138 bool CannotTCETailMarkedCall = false;
140 // Loop over the function, looking for any returning blocks, and keeping track
141 // of whether this function has any non-trivially used allocas.
142 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
143 if (FunctionContainsEscapingAllocas && CannotTCETailMarkedCall)
146 FunctionContainsEscapingAllocas |=
147 CheckForEscapingAllocas(BB, CannotTCETailMarkedCall);
150 /// FIXME: The code generator produces really bad code when an 'escaping
151 /// alloca' is changed from being a static alloca to being a dynamic alloca.
152 /// Until this is resolved, disable this transformation if that would ever
153 /// happen. This bug is PR962.
154 if (FunctionContainsEscapingAllocas)
158 // Second pass, change any tail calls to loops.
159 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
160 if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator()))
161 MadeChange |= ProcessReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail,
162 ArgumentPHIs,CannotTCETailMarkedCall);
164 // If we eliminated any tail recursions, it's possible that we inserted some
165 // silly PHI nodes which just merge an initial value (the incoming operand)
166 // with themselves. Check to see if we did and clean up our mess if so. This
167 // occurs when a function passes an argument straight through to its tail
169 if (!ArgumentPHIs.empty()) {
170 for (unsigned i = 0, e = ArgumentPHIs.size(); i != e; ++i) {
171 PHINode *PN = ArgumentPHIs[i];
173 // If the PHI Node is a dynamic constant, replace it with the value it is.
174 if (Value *PNV = PN->hasConstantValue()) {
175 PN->replaceAllUsesWith(PNV);
176 PN->eraseFromParent();
181 // Finally, if this function contains no non-escaping allocas, mark all calls
182 // in the function as eligible for tail calls (there is no stack memory for
184 if (!FunctionContainsEscapingAllocas)
185 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
186 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I)
187 if (CallInst *CI = dyn_cast<CallInst>(I))
194 /// CanMoveAboveCall - Return true if it is safe to move the specified
195 /// instruction from after the call to before the call, assuming that all
196 /// instructions between the call and this instruction are movable.
198 bool TailCallElim::CanMoveAboveCall(Instruction *I, CallInst *CI) {
199 // FIXME: We can move load/store/call/free instructions above the call if the
200 // call does not mod/ref the memory location being processed.
201 if (I->mayWriteToMemory() || isa<LoadInst>(I))
204 // Otherwise, if this is a side-effect free instruction, check to make sure
205 // that it does not use the return value of the call. If it doesn't use the
206 // return value of the call, it must only use things that are defined before
207 // the call, or movable instructions between the call and the instruction
209 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
210 if (I->getOperand(i) == CI)
215 // isDynamicConstant - Return true if the specified value is the same when the
216 // return would exit as it was when the initial iteration of the recursive
217 // function was executed.
219 // We currently handle static constants and arguments that are not modified as
220 // part of the recursion.
222 static bool isDynamicConstant(Value *V, CallInst *CI) {
223 if (isa<Constant>(V)) return true; // Static constants are always dyn consts
225 // Check to see if this is an immutable argument, if so, the value
226 // will be available to initialize the accumulator.
227 if (Argument *Arg = dyn_cast<Argument>(V)) {
228 // Figure out which argument number this is...
230 Function *F = CI->getParent()->getParent();
231 for (Function::arg_iterator AI = F->arg_begin(); &*AI != Arg; ++AI)
234 // If we are passing this argument into call as the corresponding
235 // argument operand, then the argument is dynamically constant.
236 // Otherwise, we cannot transform this function safely.
237 if (CI->getOperand(ArgNo+1) == Arg)
240 // Not a constant or immutable argument, we can't safely transform.
244 // getCommonReturnValue - Check to see if the function containing the specified
245 // return instruction and tail call consistently returns the same
246 // runtime-constant value at all exit points. If so, return the returned value.
248 static Value *getCommonReturnValue(ReturnInst *TheRI, CallInst *CI) {
249 Function *F = TheRI->getParent()->getParent();
250 Value *ReturnedValue = 0;
252 for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI)
253 if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator()))
255 Value *RetOp = RI->getOperand(0);
257 // We can only perform this transformation if the value returned is
258 // evaluatable at the start of the initial invocation of the function,
259 // instead of at the end of the evaluation.
261 if (!isDynamicConstant(RetOp, CI))
264 if (ReturnedValue && RetOp != ReturnedValue)
265 return 0; // Cannot transform if differing values are returned.
266 ReturnedValue = RetOp;
268 return ReturnedValue;
271 /// CanTransformAccumulatorRecursion - If the specified instruction can be
272 /// transformed using accumulator recursion elimination, return the constant
273 /// which is the start of the accumulator value. Otherwise return null.
275 Value *TailCallElim::CanTransformAccumulatorRecursion(Instruction *I,
277 if (!I->isAssociative()) return 0;
278 assert(I->getNumOperands() == 2 &&
279 "Associative operations should have 2 args!");
281 // Exactly one operand should be the result of the call instruction...
282 if (I->getOperand(0) == CI && I->getOperand(1) == CI ||
283 I->getOperand(0) != CI && I->getOperand(1) != CI)
286 // The only user of this instruction we allow is a single return instruction.
287 if (!I->hasOneUse() || !isa<ReturnInst>(I->use_back()))
290 // Ok, now we have to check all of the other return instructions in this
291 // function. If they return non-constants or differing values, then we cannot
292 // transform the function safely.
293 return getCommonReturnValue(cast<ReturnInst>(I->use_back()), CI);
296 bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry,
297 bool &TailCallsAreMarkedTail,
298 std::vector<PHINode*> &ArgumentPHIs,
299 bool CannotTailCallElimCallsMarkedTail) {
300 BasicBlock *BB = Ret->getParent();
301 Function *F = BB->getParent();
303 if (&BB->front() == Ret) // Make sure there is something before the ret...
306 // If the return is in the entry block, then making this transformation would
307 // turn infinite recursion into an infinite loop. This transformation is ok
308 // in theory, but breaks some code like:
309 // double fabs(double f) { return __builtin_fabs(f); } // a 'fabs' call
310 // disable this xform in this case, because the code generator will lower the
311 // call to fabs into inline code.
312 if (BB == &F->getEntryBlock())
315 // Scan backwards from the return, checking to see if there is a tail call in
316 // this block. If so, set CI to it.
318 BasicBlock::iterator BBI = Ret;
320 CI = dyn_cast<CallInst>(BBI);
321 if (CI && CI->getCalledFunction() == F)
324 if (BBI == BB->begin())
325 return false; // Didn't find a potential tail call.
329 // If this call is marked as a tail call, and if there are dynamic allocas in
330 // the function, we cannot perform this optimization.
331 if (CI->isTailCall() && CannotTailCallElimCallsMarkedTail)
334 // If we are introducing accumulator recursion to eliminate associative
335 // operations after the call instruction, this variable contains the initial
336 // value for the accumulator. If this value is set, we actually perform
337 // accumulator recursion elimination instead of simple tail recursion
339 Value *AccumulatorRecursionEliminationInitVal = 0;
340 Instruction *AccumulatorRecursionInstr = 0;
342 // Ok, we found a potential tail call. We can currently only transform the
343 // tail call if all of the instructions between the call and the return are
344 // movable to above the call itself, leaving the call next to the return.
345 // Check that this is the case now.
346 for (BBI = CI, ++BBI; &*BBI != Ret; ++BBI)
347 if (!CanMoveAboveCall(BBI, CI)) {
348 // If we can't move the instruction above the call, it might be because it
349 // is an associative operation that could be tranformed using accumulator
350 // recursion elimination. Check to see if this is the case, and if so,
351 // remember the initial accumulator value for later.
352 if ((AccumulatorRecursionEliminationInitVal =
353 CanTransformAccumulatorRecursion(BBI, CI))) {
354 // Yes, this is accumulator recursion. Remember which instruction
356 AccumulatorRecursionInstr = BBI;
358 return false; // Otherwise, we cannot eliminate the tail recursion!
362 // We can only transform call/return pairs that either ignore the return value
363 // of the call and return void, ignore the value of the call and return a
364 // constant, return the value returned by the tail call, or that are being
365 // accumulator recursion variable eliminated.
366 if (Ret->getNumOperands() != 0 && Ret->getReturnValue() != CI &&
367 !isa<UndefValue>(Ret->getReturnValue()) &&
368 AccumulatorRecursionEliminationInitVal == 0 &&
369 !getCommonReturnValue(Ret, CI))
372 // OK! We can transform this tail call. If this is the first one found,
373 // create the new entry block, allowing us to branch back to the old entry.
375 OldEntry = &F->getEntryBlock();
376 BasicBlock *NewEntry = new BasicBlock("", F, OldEntry);
377 NewEntry->takeName(OldEntry);
378 OldEntry->setName("tailrecurse");
379 new BranchInst(OldEntry, NewEntry);
381 // If this tail call is marked 'tail' and if there are any allocas in the
382 // entry block, move them up to the new entry block.
383 TailCallsAreMarkedTail = CI->isTailCall();
384 if (TailCallsAreMarkedTail)
385 // Move all fixed sized allocas from OldEntry to NewEntry.
386 for (BasicBlock::iterator OEBI = OldEntry->begin(), E = OldEntry->end(),
387 NEBI = NewEntry->begin(); OEBI != E; )
388 if (AllocaInst *AI = dyn_cast<AllocaInst>(OEBI++))
389 if (isa<ConstantInt>(AI->getArraySize()))
390 AI->moveBefore(NEBI);
392 // Now that we have created a new block, which jumps to the entry
393 // block, insert a PHI node for each argument of the function.
394 // For now, we initialize each PHI to only have the real arguments
395 // which are passed in.
396 Instruction *InsertPos = OldEntry->begin();
397 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
399 PHINode *PN = new PHINode(I->getType(), I->getName()+".tr", InsertPos);
400 I->replaceAllUsesWith(PN); // Everyone use the PHI node now!
401 PN->addIncoming(I, NewEntry);
402 ArgumentPHIs.push_back(PN);
406 // If this function has self recursive calls in the tail position where some
407 // are marked tail and some are not, only transform one flavor or another. We
408 // have to choose whether we move allocas in the entry block to the new entry
409 // block or not, so we can't make a good choice for both. NOTE: We could do
410 // slightly better here in the case that the function has no entry block
412 if (TailCallsAreMarkedTail && !CI->isTailCall())
415 // Ok, now that we know we have a pseudo-entry block WITH all of the
416 // required PHI nodes, add entries into the PHI node for the actual
417 // parameters passed into the tail-recursive call.
418 for (unsigned i = 0, e = CI->getNumOperands()-1; i != e; ++i)
419 ArgumentPHIs[i]->addIncoming(CI->getOperand(i+1), BB);
421 // If we are introducing an accumulator variable to eliminate the recursion,
422 // do so now. Note that we _know_ that no subsequent tail recursion
423 // eliminations will happen on this function because of the way the
424 // accumulator recursion predicate is set up.
426 if (AccumulatorRecursionEliminationInitVal) {
427 Instruction *AccRecInstr = AccumulatorRecursionInstr;
428 // Start by inserting a new PHI node for the accumulator.
429 PHINode *AccPN = new PHINode(AccRecInstr->getType(), "accumulator.tr",
432 // Loop over all of the predecessors of the tail recursion block. For the
433 // real entry into the function we seed the PHI with the initial value,
434 // computed earlier. For any other existing branches to this block (due to
435 // other tail recursions eliminated) the accumulator is not modified.
436 // Because we haven't added the branch in the current block to OldEntry yet,
437 // it will not show up as a predecessor.
438 for (pred_iterator PI = pred_begin(OldEntry), PE = pred_end(OldEntry);
440 if (*PI == &F->getEntryBlock())
441 AccPN->addIncoming(AccumulatorRecursionEliminationInitVal, *PI);
443 AccPN->addIncoming(AccPN, *PI);
446 // Add an incoming argument for the current block, which is computed by our
447 // associative accumulator instruction.
448 AccPN->addIncoming(AccRecInstr, BB);
450 // Next, rewrite the accumulator recursion instruction so that it does not
451 // use the result of the call anymore, instead, use the PHI node we just
453 AccRecInstr->setOperand(AccRecInstr->getOperand(0) != CI, AccPN);
455 // Finally, rewrite any return instructions in the program to return the PHI
456 // node instead of the "initval" that they do currently. This loop will
457 // actually rewrite the return value we are destroying, but that's ok.
458 for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI)
459 if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator()))
460 RI->setOperand(0, AccPN);
464 // Now that all of the PHI nodes are in place, remove the call and
465 // ret instructions, replacing them with an unconditional branch.
466 new BranchInst(OldEntry, Ret);
467 BB->getInstList().erase(Ret); // Remove return.
468 BB->getInstList().erase(CI); // Remove call.