LLVM API Documentation
00001 //===- TailRecursionElimination.cpp - Eliminate Tail Calls ----------------===// 00002 // 00003 // The LLVM Compiler Infrastructure 00004 // 00005 // This file was developed by the LLVM research group and is distributed under 00006 // the University of Illinois Open Source License. See LICENSE.TXT for details. 00007 // 00008 //===----------------------------------------------------------------------===// 00009 // 00010 // This file transforms calls of the current function (self recursion) followed 00011 // by a return instruction with a branch to the entry of the function, creating 00012 // a loop. This pass also implements the following extensions to the basic 00013 // algorithm: 00014 // 00015 // 1. Trivial instructions between the call and return do not prevent the 00016 // transformation from taking place, though currently the analysis cannot 00017 // support moving any really useful instructions (only dead ones). 00018 // 2. This pass transforms functions that are prevented from being tail 00019 // recursive by an associative expression to use an accumulator variable, 00020 // thus compiling the typical naive factorial or 'fib' implementation into 00021 // efficient code. 00022 // 3. TRE is performed if the function returns void, if the return 00023 // returns the result returned by the call, or if the function returns a 00024 // run-time constant on all exits from the function. It is possible, though 00025 // unlikely, that the return returns something else (like constant 0), and 00026 // can still be TRE'd. It can be TRE'd if ALL OTHER return instructions in 00027 // the function return the exact same value. 00028 // 4. If it can prove that callees do not access theier caller stack frame, 00029 // they are marked as eligible for tail call elimination (by the code 00030 // generator). 00031 // 00032 // There are several improvements that could be made: 00033 // 00034 // 1. If the function has any alloca instructions, these instructions will be 00035 // moved out of the entry block of the function, causing them to be 00036 // evaluated each time through the tail recursion. Safely keeping allocas 00037 // in the entry block requires analysis to proves that the tail-called 00038 // function does not read or write the stack object. 00039 // 2. Tail recursion is only performed if the call immediately preceeds the 00040 // return instruction. It's possible that there could be a jump between 00041 // the call and the return. 00042 // 3. There can be intervening operations between the call and the return that 00043 // prevent the TRE from occurring. For example, there could be GEP's and 00044 // stores to memory that will not be read or written by the call. This 00045 // requires some substantial analysis (such as with DSA) to prove safe to 00046 // move ahead of the call, but doing so could allow many more TREs to be 00047 // performed, for example in TreeAdd/TreeAlloc from the treeadd benchmark. 00048 // 4. The algorithm we use to detect if callees access their caller stack 00049 // frames is very primitive. 00050 // 00051 //===----------------------------------------------------------------------===// 00052 00053 #include "llvm/Transforms/Scalar.h" 00054 #include "llvm/Constants.h" 00055 #include "llvm/DerivedTypes.h" 00056 #include "llvm/Function.h" 00057 #include "llvm/Instructions.h" 00058 #include "llvm/Pass.h" 00059 #include "llvm/Support/CFG.h" 00060 #include "llvm/ADT/Statistic.h" 00061 using namespace llvm; 00062 00063 namespace { 00064 Statistic<> NumEliminated("tailcallelim", "Number of tail calls removed"); 00065 Statistic<> NumAccumAdded("tailcallelim","Number of accumulators introduced"); 00066 00067 struct TailCallElim : public FunctionPass { 00068 virtual bool runOnFunction(Function &F); 00069 00070 private: 00071 bool ProcessReturningBlock(ReturnInst *RI, BasicBlock *&OldEntry, 00072 bool &TailCallsAreMarkedTail, 00073 std::vector<PHINode*> &ArgumentPHIs, 00074 bool CannotTailCallElimCallsMarkedTail); 00075 bool CanMoveAboveCall(Instruction *I, CallInst *CI); 00076 Value *CanTransformAccumulatorRecursion(Instruction *I, CallInst *CI); 00077 }; 00078 RegisterOpt<TailCallElim> X("tailcallelim", "Tail Call Elimination"); 00079 } 00080 00081 // Public interface to the TailCallElimination pass 00082 FunctionPass *llvm::createTailCallEliminationPass() { 00083 return new TailCallElim(); 00084 } 00085 00086 00087 /// AllocaMightEscapeToCalls - Return true if this alloca may be accessed by 00088 /// callees of this function. We only do very simple analysis right now, this 00089 /// could be expanded in the future to use mod/ref information for particular 00090 /// call sites if desired. 00091 static bool AllocaMightEscapeToCalls(AllocaInst *AI) { 00092 // FIXME: do simple 'address taken' analysis. 00093 return true; 00094 } 00095 00096 /// FunctionContainsAllocas - Scan the specified basic block for alloca 00097 /// instructions. If it contains any that might be accessed by calls, return 00098 /// true. 00099 static bool CheckForEscapingAllocas(BasicBlock *BB, 00100 bool &CannotTCETailMarkedCall) { 00101 bool RetVal = false; 00102 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) 00103 if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) { 00104 RetVal |= AllocaMightEscapeToCalls(AI); 00105 00106 // If this alloca is in the body of the function, or if it is a variable 00107 // sized allocation, we cannot tail call eliminate calls marked 'tail' 00108 // with this mechanism. 00109 if (BB != &BB->getParent()->front() || 00110 !isa<ConstantInt>(AI->getArraySize())) 00111 CannotTCETailMarkedCall = true; 00112 } 00113 return RetVal; 00114 } 00115 00116 bool TailCallElim::runOnFunction(Function &F) { 00117 // If this function is a varargs function, we won't be able to PHI the args 00118 // right, so don't even try to convert it... 00119 if (F.getFunctionType()->isVarArg()) return false; 00120 00121 BasicBlock *OldEntry = 0; 00122 bool TailCallsAreMarkedTail = false; 00123 std::vector<PHINode*> ArgumentPHIs; 00124 bool MadeChange = false; 00125 00126 bool FunctionContainsEscapingAllocas = false; 00127 00128 // CannotTCETailMarkedCall - If true, we cannot perform TCE on tail calls 00129 // marked with the 'tail' attribute, because doing so would cause the stack 00130 // size to increase (real TCE would deallocate variable sized allocas, TCE 00131 // doesn't). 00132 bool CannotTCETailMarkedCall = false; 00133 00134 // Loop over the function, looking for any returning blocks, and keeping track 00135 // of whether this function has any non-trivially used allocas. 00136 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) { 00137 if (FunctionContainsEscapingAllocas && CannotTCETailMarkedCall) 00138 break; 00139 00140 FunctionContainsEscapingAllocas |= 00141 CheckForEscapingAllocas(BB, CannotTCETailMarkedCall); 00142 } 00143 00144 // Second pass, change any tail calls to loops. 00145 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) 00146 if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator())) 00147 MadeChange |= ProcessReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail, 00148 ArgumentPHIs,CannotTCETailMarkedCall); 00149 00150 // If we eliminated any tail recursions, it's possible that we inserted some 00151 // silly PHI nodes which just merge an initial value (the incoming operand) 00152 // with themselves. Check to see if we did and clean up our mess if so. This 00153 // occurs when a function passes an argument straight through to its tail 00154 // call. 00155 if (!ArgumentPHIs.empty()) { 00156 unsigned NumIncoming = ArgumentPHIs[0]->getNumIncomingValues(); 00157 for (unsigned i = 0, e = ArgumentPHIs.size(); i != e; ++i) { 00158 PHINode *PN = ArgumentPHIs[i]; 00159 00160 // If the PHI Node is a dynamic constant, replace it with the value it is. 00161 if (Value *PNV = PN->hasConstantValue()) { 00162 PN->replaceAllUsesWith(PNV); 00163 PN->eraseFromParent(); 00164 } 00165 } 00166 } 00167 00168 // Finally, if this function contains no non-escaping allocas, mark all calls 00169 // in the function as eligible for tail calls (there is no stack memory for 00170 // them to access). 00171 if (!FunctionContainsEscapingAllocas) 00172 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) 00173 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) 00174 if (CallInst *CI = dyn_cast<CallInst>(I)) 00175 CI->setTailCall(); 00176 00177 return MadeChange; 00178 } 00179 00180 00181 /// CanMoveAboveCall - Return true if it is safe to move the specified 00182 /// instruction from after the call to before the call, assuming that all 00183 /// instructions between the call and this instruction are movable. 00184 /// 00185 bool TailCallElim::CanMoveAboveCall(Instruction *I, CallInst *CI) { 00186 // FIXME: We can move load/store/call/free instructions above the call if the 00187 // call does not mod/ref the memory location being processed. 00188 if (I->mayWriteToMemory() || isa<LoadInst>(I)) 00189 return false; 00190 00191 // Otherwise, if this is a side-effect free instruction, check to make sure 00192 // that it does not use the return value of the call. If it doesn't use the 00193 // return value of the call, it must only use things that are defined before 00194 // the call, or movable instructions between the call and the instruction 00195 // itself. 00196 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) 00197 if (I->getOperand(i) == CI) 00198 return false; 00199 return true; 00200 } 00201 00202 // isDynamicConstant - Return true if the specified value is the same when the 00203 // return would exit as it was when the initial iteration of the recursive 00204 // function was executed. 00205 // 00206 // We currently handle static constants and arguments that are not modified as 00207 // part of the recursion. 00208 // 00209 static bool isDynamicConstant(Value *V, CallInst *CI) { 00210 if (isa<Constant>(V)) return true; // Static constants are always dyn consts 00211 00212 // Check to see if this is an immutable argument, if so, the value 00213 // will be available to initialize the accumulator. 00214 if (Argument *Arg = dyn_cast<Argument>(V)) { 00215 // Figure out which argument number this is... 00216 unsigned ArgNo = 0; 00217 Function *F = CI->getParent()->getParent(); 00218 for (Function::arg_iterator AI = F->arg_begin(); &*AI != Arg; ++AI) 00219 ++ArgNo; 00220 00221 // If we are passing this argument into call as the corresponding 00222 // argument operand, then the argument is dynamically constant. 00223 // Otherwise, we cannot transform this function safely. 00224 if (CI->getOperand(ArgNo+1) == Arg) 00225 return true; 00226 } 00227 // Not a constant or immutable argument, we can't safely transform. 00228 return false; 00229 } 00230 00231 // getCommonReturnValue - Check to see if the function containing the specified 00232 // return instruction and tail call consistently returns the same 00233 // runtime-constant value at all exit points. If so, return the returned value. 00234 // 00235 static Value *getCommonReturnValue(ReturnInst *TheRI, CallInst *CI) { 00236 Function *F = TheRI->getParent()->getParent(); 00237 Value *ReturnedValue = 0; 00238 00239 for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI) 00240 if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator())) 00241 if (RI != TheRI) { 00242 Value *RetOp = RI->getOperand(0); 00243 00244 // We can only perform this transformation if the value returned is 00245 // evaluatable at the start of the initial invocation of the function, 00246 // instead of at the end of the evaluation. 00247 // 00248 if (!isDynamicConstant(RetOp, CI)) 00249 return 0; 00250 00251 if (ReturnedValue && RetOp != ReturnedValue) 00252 return 0; // Cannot transform if differing values are returned. 00253 ReturnedValue = RetOp; 00254 } 00255 return ReturnedValue; 00256 } 00257 00258 /// CanTransformAccumulatorRecursion - If the specified instruction can be 00259 /// transformed using accumulator recursion elimination, return the constant 00260 /// which is the start of the accumulator value. Otherwise return null. 00261 /// 00262 Value *TailCallElim::CanTransformAccumulatorRecursion(Instruction *I, 00263 CallInst *CI) { 00264 if (!I->isAssociative()) return 0; 00265 assert(I->getNumOperands() == 2 && 00266 "Associative operations should have 2 args!"); 00267 00268 // Exactly one operand should be the result of the call instruction... 00269 if (I->getOperand(0) == CI && I->getOperand(1) == CI || 00270 I->getOperand(0) != CI && I->getOperand(1) != CI) 00271 return 0; 00272 00273 // The only user of this instruction we allow is a single return instruction. 00274 if (!I->hasOneUse() || !isa<ReturnInst>(I->use_back())) 00275 return 0; 00276 00277 // Ok, now we have to check all of the other return instructions in this 00278 // function. If they return non-constants or differing values, then we cannot 00279 // transform the function safely. 00280 return getCommonReturnValue(cast<ReturnInst>(I->use_back()), CI); 00281 } 00282 00283 bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry, 00284 bool &TailCallsAreMarkedTail, 00285 std::vector<PHINode*> &ArgumentPHIs, 00286 bool CannotTailCallElimCallsMarkedTail) { 00287 BasicBlock *BB = Ret->getParent(); 00288 Function *F = BB->getParent(); 00289 00290 if (&BB->front() == Ret) // Make sure there is something before the ret... 00291 return false; 00292 00293 // Scan backwards from the return, checking to see if there is a tail call in 00294 // this block. If so, set CI to it. 00295 CallInst *CI; 00296 BasicBlock::iterator BBI = Ret; 00297 while (1) { 00298 CI = dyn_cast<CallInst>(BBI); 00299 if (CI && CI->getCalledFunction() == F) 00300 break; 00301 00302 if (BBI == BB->begin()) 00303 return false; // Didn't find a potential tail call. 00304 --BBI; 00305 } 00306 00307 // If this call is marked as a tail call, and if there are dynamic allocas in 00308 // the function, we cannot perform this optimization. 00309 if (CI->isTailCall() && CannotTailCallElimCallsMarkedTail) 00310 return false; 00311 00312 // If we are introducing accumulator recursion to eliminate associative 00313 // operations after the call instruction, this variable contains the initial 00314 // value for the accumulator. If this value is set, we actually perform 00315 // accumulator recursion elimination instead of simple tail recursion 00316 // elimination. 00317 Value *AccumulatorRecursionEliminationInitVal = 0; 00318 Instruction *AccumulatorRecursionInstr = 0; 00319 00320 // Ok, we found a potential tail call. We can currently only transform the 00321 // tail call if all of the instructions between the call and the return are 00322 // movable to above the call itself, leaving the call next to the return. 00323 // Check that this is the case now. 00324 for (BBI = CI, ++BBI; &*BBI != Ret; ++BBI) 00325 if (!CanMoveAboveCall(BBI, CI)) { 00326 // If we can't move the instruction above the call, it might be because it 00327 // is an associative operation that could be tranformed using accumulator 00328 // recursion elimination. Check to see if this is the case, and if so, 00329 // remember the initial accumulator value for later. 00330 if ((AccumulatorRecursionEliminationInitVal = 00331 CanTransformAccumulatorRecursion(BBI, CI))) { 00332 // Yes, this is accumulator recursion. Remember which instruction 00333 // accumulates. 00334 AccumulatorRecursionInstr = BBI; 00335 } else { 00336 return false; // Otherwise, we cannot eliminate the tail recursion! 00337 } 00338 } 00339 00340 // We can only transform call/return pairs that either ignore the return value 00341 // of the call and return void, ignore the value of the call and return a 00342 // constant, return the value returned by the tail call, or that are being 00343 // accumulator recursion variable eliminated. 00344 if (Ret->getNumOperands() != 0 && Ret->getReturnValue() != CI && 00345 !isa<UndefValue>(Ret->getReturnValue()) && 00346 AccumulatorRecursionEliminationInitVal == 0 && 00347 !getCommonReturnValue(Ret, CI)) 00348 return false; 00349 00350 // OK! We can transform this tail call. If this is the first one found, 00351 // create the new entry block, allowing us to branch back to the old entry. 00352 if (OldEntry == 0) { 00353 OldEntry = &F->getEntryBlock(); 00354 std::string OldName = OldEntry->getName(); OldEntry->setName("tailrecurse"); 00355 BasicBlock *NewEntry = new BasicBlock(OldName, F, OldEntry); 00356 new BranchInst(OldEntry, NewEntry); 00357 00358 // If this tail call is marked 'tail' and if there are any allocas in the 00359 // entry block, move them up to the new entry block. 00360 TailCallsAreMarkedTail = CI->isTailCall(); 00361 if (TailCallsAreMarkedTail) 00362 // Move all fixed sized allocas from OldEntry to NewEntry. 00363 for (BasicBlock::iterator OEBI = OldEntry->begin(), E = OldEntry->end(), 00364 NEBI = NewEntry->begin(); OEBI != E; ) 00365 if (AllocaInst *AI = dyn_cast<AllocaInst>(OEBI++)) 00366 if (isa<ConstantInt>(AI->getArraySize())) 00367 AI->moveBefore(NEBI); 00368 00369 // Now that we have created a new block, which jumps to the entry 00370 // block, insert a PHI node for each argument of the function. 00371 // For now, we initialize each PHI to only have the real arguments 00372 // which are passed in. 00373 Instruction *InsertPos = OldEntry->begin(); 00374 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); 00375 I != E; ++I) { 00376 PHINode *PN = new PHINode(I->getType(), I->getName()+".tr", InsertPos); 00377 I->replaceAllUsesWith(PN); // Everyone use the PHI node now! 00378 PN->addIncoming(I, NewEntry); 00379 ArgumentPHIs.push_back(PN); 00380 } 00381 } 00382 00383 // If this function has self recursive calls in the tail position where some 00384 // are marked tail and some are not, only transform one flavor or another. We 00385 // have to choose whether we move allocas in the entry block to the new entry 00386 // block or not, so we can't make a good choice for both. NOTE: We could do 00387 // slightly better here in the case that the function has no entry block 00388 // allocas. 00389 if (TailCallsAreMarkedTail && !CI->isTailCall()) 00390 return false; 00391 00392 // Ok, now that we know we have a pseudo-entry block WITH all of the 00393 // required PHI nodes, add entries into the PHI node for the actual 00394 // parameters passed into the tail-recursive call. 00395 for (unsigned i = 0, e = CI->getNumOperands()-1; i != e; ++i) 00396 ArgumentPHIs[i]->addIncoming(CI->getOperand(i+1), BB); 00397 00398 // If we are introducing an accumulator variable to eliminate the recursion, 00399 // do so now. Note that we _know_ that no subsequent tail recursion 00400 // eliminations will happen on this function because of the way the 00401 // accumulator recursion predicate is set up. 00402 // 00403 if (AccumulatorRecursionEliminationInitVal) { 00404 Instruction *AccRecInstr = AccumulatorRecursionInstr; 00405 // Start by inserting a new PHI node for the accumulator. 00406 PHINode *AccPN = new PHINode(AccRecInstr->getType(), "accumulator.tr", 00407 OldEntry->begin()); 00408 00409 // Loop over all of the predecessors of the tail recursion block. For the 00410 // real entry into the function we seed the PHI with the initial value, 00411 // computed earlier. For any other existing branches to this block (due to 00412 // other tail recursions eliminated) the accumulator is not modified. 00413 // Because we haven't added the branch in the current block to OldEntry yet, 00414 // it will not show up as a predecessor. 00415 for (pred_iterator PI = pred_begin(OldEntry), PE = pred_end(OldEntry); 00416 PI != PE; ++PI) { 00417 if (*PI == &F->getEntryBlock()) 00418 AccPN->addIncoming(AccumulatorRecursionEliminationInitVal, *PI); 00419 else 00420 AccPN->addIncoming(AccPN, *PI); 00421 } 00422 00423 // Add an incoming argument for the current block, which is computed by our 00424 // associative accumulator instruction. 00425 AccPN->addIncoming(AccRecInstr, BB); 00426 00427 // Next, rewrite the accumulator recursion instruction so that it does not 00428 // use the result of the call anymore, instead, use the PHI node we just 00429 // inserted. 00430 AccRecInstr->setOperand(AccRecInstr->getOperand(0) != CI, AccPN); 00431 00432 // Finally, rewrite any return instructions in the program to return the PHI 00433 // node instead of the "initval" that they do currently. This loop will 00434 // actually rewrite the return value we are destroying, but that's ok. 00435 for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI) 00436 if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator())) 00437 RI->setOperand(0, AccPN); 00438 ++NumAccumAdded; 00439 } 00440 00441 // Now that all of the PHI nodes are in place, remove the call and 00442 // ret instructions, replacing them with an unconditional branch. 00443 new BranchInst(OldEntry, Ret); 00444 BB->getInstList().erase(Ret); // Remove return. 00445 BB->getInstList().erase(CI); // Remove call. 00446 ++NumEliminated; 00447 return true; 00448 }