TailRecursionElimination.cpp revision ac0b6ae358944ae8b2b5a11dc08f52c3ed89f2da
1//===- TailRecursionElimination.cpp - Eliminate Tail Calls ----------------===// 2// 3// The LLVM Compiler Infrastructure 4// 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. 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 expression to use an accumulator variable, 20// thus compiling the typical naive factorial or 'fib' implementation into 21// 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 theier 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 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. 50// 51//===----------------------------------------------------------------------===// 52 53#include "llvm/Transforms/Scalar.h" 54#include "llvm/Constants.h" 55#include "llvm/DerivedTypes.h" 56#include "llvm/Function.h" 57#include "llvm/Instructions.h" 58#include "llvm/Pass.h" 59#include "llvm/Support/CFG.h" 60#include "llvm/ADT/Statistic.h" 61using namespace llvm; 62 63namespace { 64 Statistic NumEliminated("tailcallelim", "Number of tail calls removed"); 65 Statistic NumAccumAdded("tailcallelim","Number of accumulators introduced"); 66 67 struct TailCallElim : public FunctionPass { 68 virtual bool runOnFunction(Function &F); 69 70 private: 71 bool ProcessReturningBlock(ReturnInst *RI, BasicBlock *&OldEntry, 72 bool &TailCallsAreMarkedTail, 73 std::vector<PHINode*> &ArgumentPHIs, 74 bool CannotTailCallElimCallsMarkedTail); 75 bool CanMoveAboveCall(Instruction *I, CallInst *CI); 76 Value *CanTransformAccumulatorRecursion(Instruction *I, CallInst *CI); 77 }; 78 RegisterPass<TailCallElim> X("tailcallelim", "Tail Call Elimination"); 79} 80 81// Public interface to the TailCallElimination pass 82FunctionPass *llvm::createTailCallEliminationPass() { 83 return new TailCallElim(); 84} 85 86 87/// AllocaMightEscapeToCalls - Return true if this alloca may be accessed by 88/// callees of this function. We only do very simple analysis right now, this 89/// could be expanded in the future to use mod/ref information for particular 90/// call sites if desired. 91static bool AllocaMightEscapeToCalls(AllocaInst *AI) { 92 // FIXME: do simple 'address taken' analysis. 93 return true; 94} 95 96/// FunctionContainsAllocas - Scan the specified basic block for alloca 97/// instructions. If it contains any that might be accessed by calls, return 98/// true. 99static bool CheckForEscapingAllocas(BasicBlock *BB, 100 bool &CannotTCETailMarkedCall) { 101 bool RetVal = false; 102 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) 103 if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) { 104 RetVal |= AllocaMightEscapeToCalls(AI); 105 106 // If this alloca is in the body of the function, or if it is a variable 107 // sized allocation, we cannot tail call eliminate calls marked 'tail' 108 // with this mechanism. 109 if (BB != &BB->getParent()->front() || 110 !isa<ConstantInt>(AI->getArraySize())) 111 CannotTCETailMarkedCall = true; 112 } 113 return RetVal; 114} 115 116bool TailCallElim::runOnFunction(Function &F) { 117 // If this function is a varargs function, we won't be able to PHI the args 118 // right, so don't even try to convert it... 119 if (F.getFunctionType()->isVarArg()) return false; 120 121 BasicBlock *OldEntry = 0; 122 bool TailCallsAreMarkedTail = false; 123 std::vector<PHINode*> ArgumentPHIs; 124 bool MadeChange = false; 125 126 bool FunctionContainsEscapingAllocas = false; 127 128 // CannotTCETailMarkedCall - If true, we cannot perform TCE on tail calls 129 // marked with the 'tail' attribute, because doing so would cause the stack 130 // size to increase (real TCE would deallocate variable sized allocas, TCE 131 // doesn't). 132 bool CannotTCETailMarkedCall = false; 133 134 // Loop over the function, looking for any returning blocks, and keeping track 135 // of whether this function has any non-trivially used allocas. 136 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) { 137 if (FunctionContainsEscapingAllocas && CannotTCETailMarkedCall) 138 break; 139 140 FunctionContainsEscapingAllocas |= 141 CheckForEscapingAllocas(BB, CannotTCETailMarkedCall); 142 } 143 144 /// FIXME: The code generator produces really bad code when an 'escaping 145 /// alloca' is changed from being a static alloca to being a dynamic alloca. 146 /// Until this is resolved, disable this transformation if that would ever 147 /// happen. This bug is PR962. 148 if (FunctionContainsEscapingAllocas) 149 return false; 150 151 152 // Second pass, change any tail calls to loops. 153 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) 154 if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator())) 155 MadeChange |= ProcessReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail, 156 ArgumentPHIs,CannotTCETailMarkedCall); 157 158 // If we eliminated any tail recursions, it's possible that we inserted some 159 // silly PHI nodes which just merge an initial value (the incoming operand) 160 // with themselves. Check to see if we did and clean up our mess if so. This 161 // occurs when a function passes an argument straight through to its tail 162 // call. 163 if (!ArgumentPHIs.empty()) { 164 for (unsigned i = 0, e = ArgumentPHIs.size(); i != e; ++i) { 165 PHINode *PN = ArgumentPHIs[i]; 166 167 // If the PHI Node is a dynamic constant, replace it with the value it is. 168 if (Value *PNV = PN->hasConstantValue()) { 169 PN->replaceAllUsesWith(PNV); 170 PN->eraseFromParent(); 171 } 172 } 173 } 174 175 // Finally, if this function contains no non-escaping allocas, mark all calls 176 // in the function as eligible for tail calls (there is no stack memory for 177 // them to access). 178 if (!FunctionContainsEscapingAllocas) 179 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) 180 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) 181 if (CallInst *CI = dyn_cast<CallInst>(I)) 182 CI->setTailCall(); 183 184 return MadeChange; 185} 186 187 188/// CanMoveAboveCall - Return true if it is safe to move the specified 189/// instruction from after the call to before the call, assuming that all 190/// instructions between the call and this instruction are movable. 191/// 192bool TailCallElim::CanMoveAboveCall(Instruction *I, CallInst *CI) { 193 // FIXME: We can move load/store/call/free instructions above the call if the 194 // call does not mod/ref the memory location being processed. 195 if (I->mayWriteToMemory() || isa<LoadInst>(I)) 196 return false; 197 198 // Otherwise, if this is a side-effect free instruction, check to make sure 199 // that it does not use the return value of the call. If it doesn't use the 200 // return value of the call, it must only use things that are defined before 201 // the call, or movable instructions between the call and the instruction 202 // itself. 203 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) 204 if (I->getOperand(i) == CI) 205 return false; 206 return true; 207} 208 209// isDynamicConstant - Return true if the specified value is the same when the 210// return would exit as it was when the initial iteration of the recursive 211// function was executed. 212// 213// We currently handle static constants and arguments that are not modified as 214// part of the recursion. 215// 216static bool isDynamicConstant(Value *V, CallInst *CI) { 217 if (isa<Constant>(V)) return true; // Static constants are always dyn consts 218 219 // Check to see if this is an immutable argument, if so, the value 220 // will be available to initialize the accumulator. 221 if (Argument *Arg = dyn_cast<Argument>(V)) { 222 // Figure out which argument number this is... 223 unsigned ArgNo = 0; 224 Function *F = CI->getParent()->getParent(); 225 for (Function::arg_iterator AI = F->arg_begin(); &*AI != Arg; ++AI) 226 ++ArgNo; 227 228 // If we are passing this argument into call as the corresponding 229 // argument operand, then the argument is dynamically constant. 230 // Otherwise, we cannot transform this function safely. 231 if (CI->getOperand(ArgNo+1) == Arg) 232 return true; 233 } 234 // Not a constant or immutable argument, we can't safely transform. 235 return false; 236} 237 238// getCommonReturnValue - Check to see if the function containing the specified 239// return instruction and tail call consistently returns the same 240// runtime-constant value at all exit points. If so, return the returned value. 241// 242static Value *getCommonReturnValue(ReturnInst *TheRI, CallInst *CI) { 243 Function *F = TheRI->getParent()->getParent(); 244 Value *ReturnedValue = 0; 245 246 for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI) 247 if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator())) 248 if (RI != TheRI) { 249 Value *RetOp = RI->getOperand(0); 250 251 // We can only perform this transformation if the value returned is 252 // evaluatable at the start of the initial invocation of the function, 253 // instead of at the end of the evaluation. 254 // 255 if (!isDynamicConstant(RetOp, CI)) 256 return 0; 257 258 if (ReturnedValue && RetOp != ReturnedValue) 259 return 0; // Cannot transform if differing values are returned. 260 ReturnedValue = RetOp; 261 } 262 return ReturnedValue; 263} 264 265/// CanTransformAccumulatorRecursion - If the specified instruction can be 266/// transformed using accumulator recursion elimination, return the constant 267/// which is the start of the accumulator value. Otherwise return null. 268/// 269Value *TailCallElim::CanTransformAccumulatorRecursion(Instruction *I, 270 CallInst *CI) { 271 if (!I->isAssociative()) return 0; 272 assert(I->getNumOperands() == 2 && 273 "Associative operations should have 2 args!"); 274 275 // Exactly one operand should be the result of the call instruction... 276 if (I->getOperand(0) == CI && I->getOperand(1) == CI || 277 I->getOperand(0) != CI && I->getOperand(1) != CI) 278 return 0; 279 280 // The only user of this instruction we allow is a single return instruction. 281 if (!I->hasOneUse() || !isa<ReturnInst>(I->use_back())) 282 return 0; 283 284 // Ok, now we have to check all of the other return instructions in this 285 // function. If they return non-constants or differing values, then we cannot 286 // transform the function safely. 287 return getCommonReturnValue(cast<ReturnInst>(I->use_back()), CI); 288} 289 290bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry, 291 bool &TailCallsAreMarkedTail, 292 std::vector<PHINode*> &ArgumentPHIs, 293 bool CannotTailCallElimCallsMarkedTail) { 294 BasicBlock *BB = Ret->getParent(); 295 Function *F = BB->getParent(); 296 297 if (&BB->front() == Ret) // Make sure there is something before the ret... 298 return false; 299 300 // Scan backwards from the return, checking to see if there is a tail call in 301 // this block. If so, set CI to it. 302 CallInst *CI; 303 BasicBlock::iterator BBI = Ret; 304 while (1) { 305 CI = dyn_cast<CallInst>(BBI); 306 if (CI && CI->getCalledFunction() == F) 307 break; 308 309 if (BBI == BB->begin()) 310 return false; // Didn't find a potential tail call. 311 --BBI; 312 } 313 314 // If this call is marked as a tail call, and if there are dynamic allocas in 315 // the function, we cannot perform this optimization. 316 if (CI->isTailCall() && CannotTailCallElimCallsMarkedTail) 317 return false; 318 319 // If we are introducing accumulator recursion to eliminate associative 320 // operations after the call instruction, this variable contains the initial 321 // value for the accumulator. If this value is set, we actually perform 322 // accumulator recursion elimination instead of simple tail recursion 323 // elimination. 324 Value *AccumulatorRecursionEliminationInitVal = 0; 325 Instruction *AccumulatorRecursionInstr = 0; 326 327 // Ok, we found a potential tail call. We can currently only transform the 328 // tail call if all of the instructions between the call and the return are 329 // movable to above the call itself, leaving the call next to the return. 330 // Check that this is the case now. 331 for (BBI = CI, ++BBI; &*BBI != Ret; ++BBI) 332 if (!CanMoveAboveCall(BBI, CI)) { 333 // If we can't move the instruction above the call, it might be because it 334 // is an associative operation that could be tranformed using accumulator 335 // recursion elimination. Check to see if this is the case, and if so, 336 // remember the initial accumulator value for later. 337 if ((AccumulatorRecursionEliminationInitVal = 338 CanTransformAccumulatorRecursion(BBI, CI))) { 339 // Yes, this is accumulator recursion. Remember which instruction 340 // accumulates. 341 AccumulatorRecursionInstr = BBI; 342 } else { 343 return false; // Otherwise, we cannot eliminate the tail recursion! 344 } 345 } 346 347 // We can only transform call/return pairs that either ignore the return value 348 // of the call and return void, ignore the value of the call and return a 349 // constant, return the value returned by the tail call, or that are being 350 // accumulator recursion variable eliminated. 351 if (Ret->getNumOperands() != 0 && Ret->getReturnValue() != CI && 352 !isa<UndefValue>(Ret->getReturnValue()) && 353 AccumulatorRecursionEliminationInitVal == 0 && 354 !getCommonReturnValue(Ret, CI)) 355 return false; 356 357 // OK! We can transform this tail call. If this is the first one found, 358 // create the new entry block, allowing us to branch back to the old entry. 359 if (OldEntry == 0) { 360 OldEntry = &F->getEntryBlock(); 361 std::string OldName = OldEntry->getName(); OldEntry->setName("tailrecurse"); 362 BasicBlock *NewEntry = new BasicBlock(OldName, F, OldEntry); 363 new BranchInst(OldEntry, NewEntry); 364 365 // If this tail call is marked 'tail' and if there are any allocas in the 366 // entry block, move them up to the new entry block. 367 TailCallsAreMarkedTail = CI->isTailCall(); 368 if (TailCallsAreMarkedTail) 369 // Move all fixed sized allocas from OldEntry to NewEntry. 370 for (BasicBlock::iterator OEBI = OldEntry->begin(), E = OldEntry->end(), 371 NEBI = NewEntry->begin(); OEBI != E; ) 372 if (AllocaInst *AI = dyn_cast<AllocaInst>(OEBI++)) 373 if (isa<ConstantInt>(AI->getArraySize())) 374 AI->moveBefore(NEBI); 375 376 // Now that we have created a new block, which jumps to the entry 377 // block, insert a PHI node for each argument of the function. 378 // For now, we initialize each PHI to only have the real arguments 379 // which are passed in. 380 Instruction *InsertPos = OldEntry->begin(); 381 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); 382 I != E; ++I) { 383 PHINode *PN = new PHINode(I->getType(), I->getName()+".tr", InsertPos); 384 I->replaceAllUsesWith(PN); // Everyone use the PHI node now! 385 PN->addIncoming(I, NewEntry); 386 ArgumentPHIs.push_back(PN); 387 } 388 } 389 390 // If this function has self recursive calls in the tail position where some 391 // are marked tail and some are not, only transform one flavor or another. We 392 // have to choose whether we move allocas in the entry block to the new entry 393 // block or not, so we can't make a good choice for both. NOTE: We could do 394 // slightly better here in the case that the function has no entry block 395 // allocas. 396 if (TailCallsAreMarkedTail && !CI->isTailCall()) 397 return false; 398 399 // Ok, now that we know we have a pseudo-entry block WITH all of the 400 // required PHI nodes, add entries into the PHI node for the actual 401 // parameters passed into the tail-recursive call. 402 for (unsigned i = 0, e = CI->getNumOperands()-1; i != e; ++i) 403 ArgumentPHIs[i]->addIncoming(CI->getOperand(i+1), BB); 404 405 // If we are introducing an accumulator variable to eliminate the recursion, 406 // do so now. Note that we _know_ that no subsequent tail recursion 407 // eliminations will happen on this function because of the way the 408 // accumulator recursion predicate is set up. 409 // 410 if (AccumulatorRecursionEliminationInitVal) { 411 Instruction *AccRecInstr = AccumulatorRecursionInstr; 412 // Start by inserting a new PHI node for the accumulator. 413 PHINode *AccPN = new PHINode(AccRecInstr->getType(), "accumulator.tr", 414 OldEntry->begin()); 415 416 // Loop over all of the predecessors of the tail recursion block. For the 417 // real entry into the function we seed the PHI with the initial value, 418 // computed earlier. For any other existing branches to this block (due to 419 // other tail recursions eliminated) the accumulator is not modified. 420 // Because we haven't added the branch in the current block to OldEntry yet, 421 // it will not show up as a predecessor. 422 for (pred_iterator PI = pred_begin(OldEntry), PE = pred_end(OldEntry); 423 PI != PE; ++PI) { 424 if (*PI == &F->getEntryBlock()) 425 AccPN->addIncoming(AccumulatorRecursionEliminationInitVal, *PI); 426 else 427 AccPN->addIncoming(AccPN, *PI); 428 } 429 430 // Add an incoming argument for the current block, which is computed by our 431 // associative accumulator instruction. 432 AccPN->addIncoming(AccRecInstr, BB); 433 434 // Next, rewrite the accumulator recursion instruction so that it does not 435 // use the result of the call anymore, instead, use the PHI node we just 436 // inserted. 437 AccRecInstr->setOperand(AccRecInstr->getOperand(0) != CI, AccPN); 438 439 // Finally, rewrite any return instructions in the program to return the PHI 440 // node instead of the "initval" that they do currently. This loop will 441 // actually rewrite the return value we are destroying, but that's ok. 442 for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI) 443 if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator())) 444 RI->setOperand(0, AccPN); 445 ++NumAccumAdded; 446 } 447 448 // Now that all of the PHI nodes are in place, remove the call and 449 // ret instructions, replacing them with an unconditional branch. 450 new BranchInst(OldEntry, Ret); 451 BB->getInstList().erase(Ret); // Remove return. 452 BB->getInstList().erase(CI); // Remove call. 453 ++NumEliminated; 454 return true; 455} 456