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/Statistic.h" 57#include "llvm/Analysis/CaptureTracking.h" 58#include "llvm/Analysis/InlineCost.h" 59#include "llvm/Analysis/InstructionSimplify.h" 60#include "llvm/Analysis/Loads.h" 61#include "llvm/Analysis/TargetTransformInfo.h" 62#include "llvm/IR/Constants.h" 63#include "llvm/IR/DerivedTypes.h" 64#include "llvm/IR/Function.h" 65#include "llvm/IR/Instructions.h" 66#include "llvm/IR/IntrinsicInst.h" 67#include "llvm/IR/Module.h" 68#include "llvm/Pass.h" 69#include "llvm/Support/CFG.h" 70#include "llvm/Support/CallSite.h" 71#include "llvm/Support/Debug.h" 72#include "llvm/Support/raw_ostream.h" 73#include "llvm/Transforms/Utils/BasicBlockUtils.h" 74#include "llvm/Transforms/Utils/Local.h" 75using namespace llvm; 76 77STATISTIC(NumEliminated, "Number of tail calls removed"); 78STATISTIC(NumRetDuped, "Number of return duplicated"); 79STATISTIC(NumAccumAdded, "Number of accumulators introduced"); 80 81namespace { 82 struct TailCallElim : public FunctionPass { 83 const TargetTransformInfo *TTI; 84 85 static char ID; // Pass identification, replacement for typeid 86 TailCallElim() : FunctionPass(ID) { 87 initializeTailCallElimPass(*PassRegistry::getPassRegistry()); 88 } 89 90 virtual void getAnalysisUsage(AnalysisUsage &AU) const; 91 92 virtual bool runOnFunction(Function &F); 93 94 private: 95 CallInst *FindTRECandidate(Instruction *I, 96 bool CannotTailCallElimCallsMarkedTail); 97 bool EliminateRecursiveTailCall(CallInst *CI, ReturnInst *Ret, 98 BasicBlock *&OldEntry, 99 bool &TailCallsAreMarkedTail, 100 SmallVector<PHINode*, 8> &ArgumentPHIs, 101 bool CannotTailCallElimCallsMarkedTail); 102 bool FoldReturnAndProcessPred(BasicBlock *BB, 103 ReturnInst *Ret, BasicBlock *&OldEntry, 104 bool &TailCallsAreMarkedTail, 105 SmallVector<PHINode*, 8> &ArgumentPHIs, 106 bool CannotTailCallElimCallsMarkedTail); 107 bool ProcessReturningBlock(ReturnInst *RI, BasicBlock *&OldEntry, 108 bool &TailCallsAreMarkedTail, 109 SmallVector<PHINode*, 8> &ArgumentPHIs, 110 bool CannotTailCallElimCallsMarkedTail); 111 bool CanMoveAboveCall(Instruction *I, CallInst *CI); 112 Value *CanTransformAccumulatorRecursion(Instruction *I, CallInst *CI); 113 }; 114} 115 116char TailCallElim::ID = 0; 117INITIALIZE_PASS_BEGIN(TailCallElim, "tailcallelim", 118 "Tail Call Elimination", false, false) 119INITIALIZE_AG_DEPENDENCY(TargetTransformInfo) 120INITIALIZE_PASS_END(TailCallElim, "tailcallelim", 121 "Tail Call Elimination", false, false) 122 123// Public interface to the TailCallElimination pass 124FunctionPass *llvm::createTailCallEliminationPass() { 125 return new TailCallElim(); 126} 127 128void TailCallElim::getAnalysisUsage(AnalysisUsage &AU) const { 129 AU.addRequired<TargetTransformInfo>(); 130} 131 132/// AllocaMightEscapeToCalls - Return true if this alloca may be accessed by 133/// callees of this function. We only do very simple analysis right now, this 134/// could be expanded in the future to use mod/ref information for particular 135/// call sites if desired. 136static bool AllocaMightEscapeToCalls(AllocaInst *AI) { 137 // FIXME: do simple 'address taken' analysis. 138 return true; 139} 140 141/// CheckForEscapingAllocas - Scan the specified basic block for alloca 142/// instructions. If it contains any that might be accessed by calls, return 143/// true. 144static bool CheckForEscapingAllocas(BasicBlock *BB, 145 bool &CannotTCETailMarkedCall) { 146 bool RetVal = false; 147 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) 148 if (AllocaInst *AI = dyn_cast<AllocaInst>(I)) { 149 RetVal |= AllocaMightEscapeToCalls(AI); 150 151 // If this alloca is in the body of the function, or if it is a variable 152 // sized allocation, we cannot tail call eliminate calls marked 'tail' 153 // with this mechanism. 154 if (BB != &BB->getParent()->getEntryBlock() || 155 !isa<ConstantInt>(AI->getArraySize())) 156 CannotTCETailMarkedCall = true; 157 } 158 return RetVal; 159} 160 161bool TailCallElim::runOnFunction(Function &F) { 162 // If this function is a varargs function, we won't be able to PHI the args 163 // right, so don't even try to convert it... 164 if (F.getFunctionType()->isVarArg()) return false; 165 166 TTI = &getAnalysis<TargetTransformInfo>(); 167 BasicBlock *OldEntry = 0; 168 bool TailCallsAreMarkedTail = false; 169 SmallVector<PHINode*, 8> ArgumentPHIs; 170 bool MadeChange = false; 171 bool FunctionContainsEscapingAllocas = false; 172 173 // CannotTCETailMarkedCall - If true, we cannot perform TCE on tail calls 174 // marked with the 'tail' attribute, because doing so would cause the stack 175 // size to increase (real TCE would deallocate variable sized allocas, TCE 176 // doesn't). 177 bool CannotTCETailMarkedCall = false; 178 179 // Loop over the function, looking for any returning blocks, and keeping track 180 // of whether this function has any non-trivially used allocas. 181 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) { 182 if (FunctionContainsEscapingAllocas && CannotTCETailMarkedCall) 183 break; 184 185 FunctionContainsEscapingAllocas |= 186 CheckForEscapingAllocas(BB, CannotTCETailMarkedCall); 187 } 188 189 /// FIXME: The code generator produces really bad code when an 'escaping 190 /// alloca' is changed from being a static alloca to being a dynamic alloca. 191 /// Until this is resolved, disable this transformation if that would ever 192 /// happen. This bug is PR962. 193 if (FunctionContainsEscapingAllocas) 194 return false; 195 196 // Second pass, change any tail calls to loops. 197 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) { 198 if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator())) { 199 bool Change = ProcessReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail, 200 ArgumentPHIs,CannotTCETailMarkedCall); 201 if (!Change && BB->getFirstNonPHIOrDbg() == Ret) 202 Change = FoldReturnAndProcessPred(BB, Ret, OldEntry, 203 TailCallsAreMarkedTail, ArgumentPHIs, 204 CannotTCETailMarkedCall); 205 MadeChange |= Change; 206 } 207 } 208 209 // If we eliminated any tail recursions, it's possible that we inserted some 210 // silly PHI nodes which just merge an initial value (the incoming operand) 211 // with themselves. Check to see if we did and clean up our mess if so. This 212 // occurs when a function passes an argument straight through to its tail 213 // call. 214 if (!ArgumentPHIs.empty()) { 215 for (unsigned i = 0, e = ArgumentPHIs.size(); i != e; ++i) { 216 PHINode *PN = ArgumentPHIs[i]; 217 218 // If the PHI Node is a dynamic constant, replace it with the value it is. 219 if (Value *PNV = SimplifyInstruction(PN)) { 220 PN->replaceAllUsesWith(PNV); 221 PN->eraseFromParent(); 222 } 223 } 224 } 225 226 // Finally, if this function contains no non-escaping allocas, or calls 227 // setjmp, mark all calls in the function as eligible for tail calls 228 //(there is no stack memory for them to access). 229 if (!FunctionContainsEscapingAllocas && !F.callsFunctionThatReturnsTwice()) 230 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) 231 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) 232 if (CallInst *CI = dyn_cast<CallInst>(I)) { 233 CI->setTailCall(); 234 MadeChange = true; 235 } 236 237 return MadeChange; 238} 239 240 241/// CanMoveAboveCall - Return true if it is safe to move the specified 242/// instruction from after the call to before the call, assuming that all 243/// instructions between the call and this instruction are movable. 244/// 245bool TailCallElim::CanMoveAboveCall(Instruction *I, CallInst *CI) { 246 // FIXME: We can move load/store/call/free instructions above the call if the 247 // call does not mod/ref the memory location being processed. 248 if (I->mayHaveSideEffects()) // This also handles volatile loads. 249 return false; 250 251 if (LoadInst *L = dyn_cast<LoadInst>(I)) { 252 // Loads may always be moved above calls without side effects. 253 if (CI->mayHaveSideEffects()) { 254 // Non-volatile loads may be moved above a call with side effects if it 255 // does not write to memory and the load provably won't trap. 256 // FIXME: Writes to memory only matter if they may alias the pointer 257 // being loaded from. 258 if (CI->mayWriteToMemory() || 259 !isSafeToLoadUnconditionally(L->getPointerOperand(), L, 260 L->getAlignment())) 261 return false; 262 } 263 } 264 265 // Otherwise, if this is a side-effect free instruction, check to make sure 266 // that it does not use the return value of the call. If it doesn't use the 267 // return value of the call, it must only use things that are defined before 268 // the call, or movable instructions between the call and the instruction 269 // itself. 270 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) 271 if (I->getOperand(i) == CI) 272 return false; 273 return true; 274} 275 276// isDynamicConstant - Return true if the specified value is the same when the 277// return would exit as it was when the initial iteration of the recursive 278// function was executed. 279// 280// We currently handle static constants and arguments that are not modified as 281// part of the recursion. 282// 283static bool isDynamicConstant(Value *V, CallInst *CI, ReturnInst *RI) { 284 if (isa<Constant>(V)) return true; // Static constants are always dyn consts 285 286 // Check to see if this is an immutable argument, if so, the value 287 // will be available to initialize the accumulator. 288 if (Argument *Arg = dyn_cast<Argument>(V)) { 289 // Figure out which argument number this is... 290 unsigned ArgNo = 0; 291 Function *F = CI->getParent()->getParent(); 292 for (Function::arg_iterator AI = F->arg_begin(); &*AI != Arg; ++AI) 293 ++ArgNo; 294 295 // If we are passing this argument into call as the corresponding 296 // argument operand, then the argument is dynamically constant. 297 // Otherwise, we cannot transform this function safely. 298 if (CI->getArgOperand(ArgNo) == Arg) 299 return true; 300 } 301 302 // Switch cases are always constant integers. If the value is being switched 303 // on and the return is only reachable from one of its cases, it's 304 // effectively constant. 305 if (BasicBlock *UniquePred = RI->getParent()->getUniquePredecessor()) 306 if (SwitchInst *SI = dyn_cast<SwitchInst>(UniquePred->getTerminator())) 307 if (SI->getCondition() == V) 308 return SI->getDefaultDest() != RI->getParent(); 309 310 // Not a constant or immutable argument, we can't safely transform. 311 return false; 312} 313 314// getCommonReturnValue - Check to see if the function containing the specified 315// tail call consistently returns the same runtime-constant value at all exit 316// points except for IgnoreRI. If so, return the returned value. 317// 318static Value *getCommonReturnValue(ReturnInst *IgnoreRI, CallInst *CI) { 319 Function *F = CI->getParent()->getParent(); 320 Value *ReturnedValue = 0; 321 322 for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI) { 323 ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator()); 324 if (RI == 0 || RI == IgnoreRI) continue; 325 326 // We can only perform this transformation if the value returned is 327 // evaluatable at the start of the initial invocation of the function, 328 // instead of at the end of the evaluation. 329 // 330 Value *RetOp = RI->getOperand(0); 331 if (!isDynamicConstant(RetOp, CI, RI)) 332 return 0; 333 334 if (ReturnedValue && RetOp != ReturnedValue) 335 return 0; // Cannot transform if differing values are returned. 336 ReturnedValue = RetOp; 337 } 338 return ReturnedValue; 339} 340 341/// CanTransformAccumulatorRecursion - If the specified instruction can be 342/// transformed using accumulator recursion elimination, return the constant 343/// which is the start of the accumulator value. Otherwise return null. 344/// 345Value *TailCallElim::CanTransformAccumulatorRecursion(Instruction *I, 346 CallInst *CI) { 347 if (!I->isAssociative() || !I->isCommutative()) return 0; 348 assert(I->getNumOperands() == 2 && 349 "Associative/commutative operations should have 2 args!"); 350 351 // Exactly one operand should be the result of the call instruction. 352 if ((I->getOperand(0) == CI && I->getOperand(1) == CI) || 353 (I->getOperand(0) != CI && I->getOperand(1) != CI)) 354 return 0; 355 356 // The only user of this instruction we allow is a single return instruction. 357 if (!I->hasOneUse() || !isa<ReturnInst>(I->use_back())) 358 return 0; 359 360 // Ok, now we have to check all of the other return instructions in this 361 // function. If they return non-constants or differing values, then we cannot 362 // transform the function safely. 363 return getCommonReturnValue(cast<ReturnInst>(I->use_back()), CI); 364} 365 366static Instruction *FirstNonDbg(BasicBlock::iterator I) { 367 while (isa<DbgInfoIntrinsic>(I)) 368 ++I; 369 return &*I; 370} 371 372CallInst* 373TailCallElim::FindTRECandidate(Instruction *TI, 374 bool CannotTailCallElimCallsMarkedTail) { 375 BasicBlock *BB = TI->getParent(); 376 Function *F = BB->getParent(); 377 378 if (&BB->front() == TI) // Make sure there is something before the terminator. 379 return 0; 380 381 // Scan backwards from the return, checking to see if there is a tail call in 382 // this block. If so, set CI to it. 383 CallInst *CI = 0; 384 BasicBlock::iterator BBI = TI; 385 while (true) { 386 CI = dyn_cast<CallInst>(BBI); 387 if (CI && CI->getCalledFunction() == F) 388 break; 389 390 if (BBI == BB->begin()) 391 return 0; // Didn't find a potential tail call. 392 --BBI; 393 } 394 395 // If this call is marked as a tail call, and if there are dynamic allocas in 396 // the function, we cannot perform this optimization. 397 if (CI->isTailCall() && CannotTailCallElimCallsMarkedTail) 398 return 0; 399 400 // As a special case, detect code like this: 401 // double fabs(double f) { return __builtin_fabs(f); } // a 'fabs' call 402 // and disable this xform in this case, because the code generator will 403 // lower the call to fabs into inline code. 404 if (BB == &F->getEntryBlock() && 405 FirstNonDbg(BB->front()) == CI && 406 FirstNonDbg(llvm::next(BB->begin())) == TI && 407 CI->getCalledFunction() && 408 !TTI->isLoweredToCall(CI->getCalledFunction())) { 409 // A single-block function with just a call and a return. Check that 410 // the arguments match. 411 CallSite::arg_iterator I = CallSite(CI).arg_begin(), 412 E = CallSite(CI).arg_end(); 413 Function::arg_iterator FI = F->arg_begin(), 414 FE = F->arg_end(); 415 for (; I != E && FI != FE; ++I, ++FI) 416 if (*I != &*FI) break; 417 if (I == E && FI == FE) 418 return 0; 419 } 420 421 return CI; 422} 423 424bool TailCallElim::EliminateRecursiveTailCall(CallInst *CI, ReturnInst *Ret, 425 BasicBlock *&OldEntry, 426 bool &TailCallsAreMarkedTail, 427 SmallVector<PHINode*, 8> &ArgumentPHIs, 428 bool CannotTailCallElimCallsMarkedTail) { 429 // If we are introducing accumulator recursion to eliminate operations after 430 // the call instruction that are both associative and commutative, the initial 431 // value for the accumulator is placed in this variable. If this value is set 432 // then we actually perform accumulator recursion elimination instead of 433 // simple tail recursion elimination. If the operation is an LLVM instruction 434 // (eg: "add") then it is recorded in AccumulatorRecursionInstr. If not, then 435 // we are handling the case when the return instruction returns a constant C 436 // which is different to the constant returned by other return instructions 437 // (which is recorded in AccumulatorRecursionEliminationInitVal). This is a 438 // special case of accumulator recursion, the operation being "return C". 439 Value *AccumulatorRecursionEliminationInitVal = 0; 440 Instruction *AccumulatorRecursionInstr = 0; 441 442 // Ok, we found a potential tail call. We can currently only transform the 443 // tail call if all of the instructions between the call and the return are 444 // movable to above the call itself, leaving the call next to the return. 445 // Check that this is the case now. 446 BasicBlock::iterator BBI = CI; 447 for (++BBI; &*BBI != Ret; ++BBI) { 448 if (CanMoveAboveCall(BBI, CI)) continue; 449 450 // If we can't move the instruction above the call, it might be because it 451 // is an associative and commutative operation that could be transformed 452 // using accumulator recursion elimination. Check to see if this is the 453 // case, and if so, remember the initial accumulator value for later. 454 if ((AccumulatorRecursionEliminationInitVal = 455 CanTransformAccumulatorRecursion(BBI, CI))) { 456 // Yes, this is accumulator recursion. Remember which instruction 457 // accumulates. 458 AccumulatorRecursionInstr = BBI; 459 } else { 460 return false; // Otherwise, we cannot eliminate the tail recursion! 461 } 462 } 463 464 // We can only transform call/return pairs that either ignore the return value 465 // of the call and return void, ignore the value of the call and return a 466 // constant, return the value returned by the tail call, or that are being 467 // accumulator recursion variable eliminated. 468 if (Ret->getNumOperands() == 1 && Ret->getReturnValue() != CI && 469 !isa<UndefValue>(Ret->getReturnValue()) && 470 AccumulatorRecursionEliminationInitVal == 0 && 471 !getCommonReturnValue(0, CI)) { 472 // One case remains that we are able to handle: the current return 473 // instruction returns a constant, and all other return instructions 474 // return a different constant. 475 if (!isDynamicConstant(Ret->getReturnValue(), CI, Ret)) 476 return false; // Current return instruction does not return a constant. 477 // Check that all other return instructions return a common constant. If 478 // so, record it in AccumulatorRecursionEliminationInitVal. 479 AccumulatorRecursionEliminationInitVal = getCommonReturnValue(Ret, CI); 480 if (!AccumulatorRecursionEliminationInitVal) 481 return false; 482 } 483 484 BasicBlock *BB = Ret->getParent(); 485 Function *F = BB->getParent(); 486 487 // OK! We can transform this tail call. If this is the first one found, 488 // create the new entry block, allowing us to branch back to the old entry. 489 if (OldEntry == 0) { 490 OldEntry = &F->getEntryBlock(); 491 BasicBlock *NewEntry = BasicBlock::Create(F->getContext(), "", F, OldEntry); 492 NewEntry->takeName(OldEntry); 493 OldEntry->setName("tailrecurse"); 494 BranchInst::Create(OldEntry, NewEntry); 495 496 // If this tail call is marked 'tail' and if there are any allocas in the 497 // entry block, move them up to the new entry block. 498 TailCallsAreMarkedTail = CI->isTailCall(); 499 if (TailCallsAreMarkedTail) 500 // Move all fixed sized allocas from OldEntry to NewEntry. 501 for (BasicBlock::iterator OEBI = OldEntry->begin(), E = OldEntry->end(), 502 NEBI = NewEntry->begin(); OEBI != E; ) 503 if (AllocaInst *AI = dyn_cast<AllocaInst>(OEBI++)) 504 if (isa<ConstantInt>(AI->getArraySize())) 505 AI->moveBefore(NEBI); 506 507 // Now that we have created a new block, which jumps to the entry 508 // block, insert a PHI node for each argument of the function. 509 // For now, we initialize each PHI to only have the real arguments 510 // which are passed in. 511 Instruction *InsertPos = OldEntry->begin(); 512 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); 513 I != E; ++I) { 514 PHINode *PN = PHINode::Create(I->getType(), 2, 515 I->getName() + ".tr", InsertPos); 516 I->replaceAllUsesWith(PN); // Everyone use the PHI node now! 517 PN->addIncoming(I, NewEntry); 518 ArgumentPHIs.push_back(PN); 519 } 520 } 521 522 // If this function has self recursive calls in the tail position where some 523 // are marked tail and some are not, only transform one flavor or another. We 524 // have to choose whether we move allocas in the entry block to the new entry 525 // block or not, so we can't make a good choice for both. NOTE: We could do 526 // slightly better here in the case that the function has no entry block 527 // allocas. 528 if (TailCallsAreMarkedTail && !CI->isTailCall()) 529 return false; 530 531 // Ok, now that we know we have a pseudo-entry block WITH all of the 532 // required PHI nodes, add entries into the PHI node for the actual 533 // parameters passed into the tail-recursive call. 534 for (unsigned i = 0, e = CI->getNumArgOperands(); i != e; ++i) 535 ArgumentPHIs[i]->addIncoming(CI->getArgOperand(i), BB); 536 537 // If we are introducing an accumulator variable to eliminate the recursion, 538 // do so now. Note that we _know_ that no subsequent tail recursion 539 // eliminations will happen on this function because of the way the 540 // accumulator recursion predicate is set up. 541 // 542 if (AccumulatorRecursionEliminationInitVal) { 543 Instruction *AccRecInstr = AccumulatorRecursionInstr; 544 // Start by inserting a new PHI node for the accumulator. 545 pred_iterator PB = pred_begin(OldEntry), PE = pred_end(OldEntry); 546 PHINode *AccPN = 547 PHINode::Create(AccumulatorRecursionEliminationInitVal->getType(), 548 std::distance(PB, PE) + 1, 549 "accumulator.tr", OldEntry->begin()); 550 551 // Loop over all of the predecessors of the tail recursion block. For the 552 // real entry into the function we seed the PHI with the initial value, 553 // computed earlier. For any other existing branches to this block (due to 554 // other tail recursions eliminated) the accumulator is not modified. 555 // Because we haven't added the branch in the current block to OldEntry yet, 556 // it will not show up as a predecessor. 557 for (pred_iterator PI = PB; PI != PE; ++PI) { 558 BasicBlock *P = *PI; 559 if (P == &F->getEntryBlock()) 560 AccPN->addIncoming(AccumulatorRecursionEliminationInitVal, P); 561 else 562 AccPN->addIncoming(AccPN, P); 563 } 564 565 if (AccRecInstr) { 566 // Add an incoming argument for the current block, which is computed by 567 // our associative and commutative accumulator instruction. 568 AccPN->addIncoming(AccRecInstr, BB); 569 570 // Next, rewrite the accumulator recursion instruction so that it does not 571 // use the result of the call anymore, instead, use the PHI node we just 572 // inserted. 573 AccRecInstr->setOperand(AccRecInstr->getOperand(0) != CI, AccPN); 574 } else { 575 // Add an incoming argument for the current block, which is just the 576 // constant returned by the current return instruction. 577 AccPN->addIncoming(Ret->getReturnValue(), BB); 578 } 579 580 // Finally, rewrite any return instructions in the program to return the PHI 581 // node instead of the "initval" that they do currently. This loop will 582 // actually rewrite the return value we are destroying, but that's ok. 583 for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI) 584 if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator())) 585 RI->setOperand(0, AccPN); 586 ++NumAccumAdded; 587 } 588 589 // Now that all of the PHI nodes are in place, remove the call and 590 // ret instructions, replacing them with an unconditional branch. 591 BranchInst *NewBI = BranchInst::Create(OldEntry, Ret); 592 NewBI->setDebugLoc(CI->getDebugLoc()); 593 594 BB->getInstList().erase(Ret); // Remove return. 595 BB->getInstList().erase(CI); // Remove call. 596 ++NumEliminated; 597 return true; 598} 599 600bool TailCallElim::FoldReturnAndProcessPred(BasicBlock *BB, 601 ReturnInst *Ret, BasicBlock *&OldEntry, 602 bool &TailCallsAreMarkedTail, 603 SmallVector<PHINode*, 8> &ArgumentPHIs, 604 bool CannotTailCallElimCallsMarkedTail) { 605 bool Change = false; 606 607 // If the return block contains nothing but the return and PHI's, 608 // there might be an opportunity to duplicate the return in its 609 // predecessors and perform TRC there. Look for predecessors that end 610 // in unconditional branch and recursive call(s). 611 SmallVector<BranchInst*, 8> UncondBranchPreds; 612 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { 613 BasicBlock *Pred = *PI; 614 TerminatorInst *PTI = Pred->getTerminator(); 615 if (BranchInst *BI = dyn_cast<BranchInst>(PTI)) 616 if (BI->isUnconditional()) 617 UncondBranchPreds.push_back(BI); 618 } 619 620 while (!UncondBranchPreds.empty()) { 621 BranchInst *BI = UncondBranchPreds.pop_back_val(); 622 BasicBlock *Pred = BI->getParent(); 623 if (CallInst *CI = FindTRECandidate(BI, CannotTailCallElimCallsMarkedTail)){ 624 DEBUG(dbgs() << "FOLDING: " << *BB 625 << "INTO UNCOND BRANCH PRED: " << *Pred); 626 EliminateRecursiveTailCall(CI, FoldReturnIntoUncondBranch(Ret, BB, Pred), 627 OldEntry, TailCallsAreMarkedTail, ArgumentPHIs, 628 CannotTailCallElimCallsMarkedTail); 629 ++NumRetDuped; 630 Change = true; 631 } 632 } 633 634 return Change; 635} 636 637bool TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry, 638 bool &TailCallsAreMarkedTail, 639 SmallVector<PHINode*, 8> &ArgumentPHIs, 640 bool CannotTailCallElimCallsMarkedTail) { 641 CallInst *CI = FindTRECandidate(Ret, CannotTailCallElimCallsMarkedTail); 642 if (!CI) 643 return false; 644 645 return EliminateRecursiveTailCall(CI, Ret, OldEntry, TailCallsAreMarkedTail, 646 ArgumentPHIs, 647 CannotTailCallElimCallsMarkedTail); 648} 649