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 #include "llvm/Transforms/Scalar.h" 54 #include "llvm/ADT/STLExtras.h" 55 #include "llvm/ADT/SmallPtrSet.h" 56 #include "llvm/ADT/Statistic.h" 57 #include "llvm/Analysis/CaptureTracking.h" 58 #include "llvm/Analysis/CFG.h" 59 #include "llvm/Analysis/InlineCost.h" 60 #include "llvm/Analysis/InstructionSimplify.h" 61 #include "llvm/Analysis/Loads.h" 62 #include "llvm/Analysis/TargetTransformInfo.h" 63 #include "llvm/IR/CFG.h" 64 #include "llvm/IR/CallSite.h" 65 #include "llvm/IR/Constants.h" 66 #include "llvm/IR/DerivedTypes.h" 67 #include "llvm/IR/DiagnosticInfo.h" 68 #include "llvm/IR/Function.h" 69 #include "llvm/IR/Instructions.h" 70 #include "llvm/IR/IntrinsicInst.h" 71 #include "llvm/IR/Module.h" 72 #include "llvm/IR/ValueHandle.h" 73 #include "llvm/Pass.h" 74 #include "llvm/Support/Debug.h" 75 #include "llvm/Support/raw_ostream.h" 76 #include "llvm/Transforms/Utils/BasicBlockUtils.h" 77 #include "llvm/Transforms/Utils/Local.h" 78 using namespace llvm; 79 80 #define DEBUG_TYPE "tailcallelim" 81 82 STATISTIC(NumEliminated, "Number of tail calls removed"); 83 STATISTIC(NumRetDuped, "Number of return duplicated"); 84 STATISTIC(NumAccumAdded, "Number of accumulators introduced"); 85 86 namespace { 87 struct TailCallElim : public FunctionPass { 88 const TargetTransformInfo *TTI; 89 90 static char ID; // Pass identification, replacement for typeid 91 TailCallElim() : FunctionPass(ID) { 92 initializeTailCallElimPass(*PassRegistry::getPassRegistry()); 93 } 94 95 void getAnalysisUsage(AnalysisUsage &AU) const override; 96 97 bool runOnFunction(Function &F) override; 98 99 private: 100 bool runTRE(Function &F); 101 bool markTails(Function &F, bool &AllCallsAreTailCalls); 102 103 CallInst *FindTRECandidate(Instruction *I, 104 bool CannotTailCallElimCallsMarkedTail); 105 bool EliminateRecursiveTailCall(CallInst *CI, ReturnInst *Ret, 106 BasicBlock *&OldEntry, 107 bool &TailCallsAreMarkedTail, 108 SmallVectorImpl<PHINode *> &ArgumentPHIs, 109 bool CannotTailCallElimCallsMarkedTail); 110 bool FoldReturnAndProcessPred(BasicBlock *BB, 111 ReturnInst *Ret, BasicBlock *&OldEntry, 112 bool &TailCallsAreMarkedTail, 113 SmallVectorImpl<PHINode *> &ArgumentPHIs, 114 bool CannotTailCallElimCallsMarkedTail); 115 bool ProcessReturningBlock(ReturnInst *RI, BasicBlock *&OldEntry, 116 bool &TailCallsAreMarkedTail, 117 SmallVectorImpl<PHINode *> &ArgumentPHIs, 118 bool CannotTailCallElimCallsMarkedTail); 119 bool CanMoveAboveCall(Instruction *I, CallInst *CI); 120 Value *CanTransformAccumulatorRecursion(Instruction *I, CallInst *CI); 121 }; 122 } 123 124 char TailCallElim::ID = 0; 125 INITIALIZE_PASS_BEGIN(TailCallElim, "tailcallelim", 126 "Tail Call Elimination", false, false) 127 INITIALIZE_AG_DEPENDENCY(TargetTransformInfo) 128 INITIALIZE_PASS_END(TailCallElim, "tailcallelim", 129 "Tail Call Elimination", false, false) 130 131 // Public interface to the TailCallElimination pass 132 FunctionPass *llvm::createTailCallEliminationPass() { 133 return new TailCallElim(); 134 } 135 136 void TailCallElim::getAnalysisUsage(AnalysisUsage &AU) const { 137 AU.addRequired<TargetTransformInfo>(); 138 } 139 140 /// \brief Scan the specified function for alloca instructions. 141 /// If it contains any dynamic allocas, returns false. 142 static bool CanTRE(Function &F) { 143 // Because of PR962, we don't TRE dynamic allocas. 144 for (auto &BB : F) { 145 for (auto &I : BB) { 146 if (AllocaInst *AI = dyn_cast<AllocaInst>(&I)) { 147 if (!AI->isStaticAlloca()) 148 return false; 149 } 150 } 151 } 152 153 return true; 154 } 155 156 bool TailCallElim::runOnFunction(Function &F) { 157 if (skipOptnoneFunction(F)) 158 return false; 159 160 bool AllCallsAreTailCalls = false; 161 bool Modified = markTails(F, AllCallsAreTailCalls); 162 if (AllCallsAreTailCalls) 163 Modified |= runTRE(F); 164 return Modified; 165 } 166 167 namespace { 168 struct AllocaDerivedValueTracker { 169 // Start at a root value and walk its use-def chain to mark calls that use the 170 // value or a derived value in AllocaUsers, and places where it may escape in 171 // EscapePoints. 172 void walk(Value *Root) { 173 SmallVector<Use *, 32> Worklist; 174 SmallPtrSet<Use *, 32> Visited; 175 176 auto AddUsesToWorklist = [&](Value *V) { 177 for (auto &U : V->uses()) { 178 if (!Visited.insert(&U)) 179 continue; 180 Worklist.push_back(&U); 181 } 182 }; 183 184 AddUsesToWorklist(Root); 185 186 while (!Worklist.empty()) { 187 Use *U = Worklist.pop_back_val(); 188 Instruction *I = cast<Instruction>(U->getUser()); 189 190 switch (I->getOpcode()) { 191 case Instruction::Call: 192 case Instruction::Invoke: { 193 CallSite CS(I); 194 bool IsNocapture = !CS.isCallee(U) && 195 CS.doesNotCapture(CS.getArgumentNo(U)); 196 callUsesLocalStack(CS, IsNocapture); 197 if (IsNocapture) { 198 // If the alloca-derived argument is passed in as nocapture, then it 199 // can't propagate to the call's return. That would be capturing. 200 continue; 201 } 202 break; 203 } 204 case Instruction::Load: { 205 // The result of a load is not alloca-derived (unless an alloca has 206 // otherwise escaped, but this is a local analysis). 207 continue; 208 } 209 case Instruction::Store: { 210 if (U->getOperandNo() == 0) 211 EscapePoints.insert(I); 212 continue; // Stores have no users to analyze. 213 } 214 case Instruction::BitCast: 215 case Instruction::GetElementPtr: 216 case Instruction::PHI: 217 case Instruction::Select: 218 case Instruction::AddrSpaceCast: 219 break; 220 default: 221 EscapePoints.insert(I); 222 break; 223 } 224 225 AddUsesToWorklist(I); 226 } 227 } 228 229 void callUsesLocalStack(CallSite CS, bool IsNocapture) { 230 // Add it to the list of alloca users. If it's already there, skip further 231 // processing. 232 if (!AllocaUsers.insert(CS.getInstruction())) 233 return; 234 235 // If it's nocapture then it can't capture the alloca. 236 if (IsNocapture) 237 return; 238 239 // If it can write to memory, it can leak the alloca value. 240 if (!CS.onlyReadsMemory()) 241 EscapePoints.insert(CS.getInstruction()); 242 } 243 244 SmallPtrSet<Instruction *, 32> AllocaUsers; 245 SmallPtrSet<Instruction *, 32> EscapePoints; 246 }; 247 } 248 249 bool TailCallElim::markTails(Function &F, bool &AllCallsAreTailCalls) { 250 if (F.callsFunctionThatReturnsTwice()) 251 return false; 252 AllCallsAreTailCalls = true; 253 254 // The local stack holds all alloca instructions and all byval arguments. 255 AllocaDerivedValueTracker Tracker; 256 for (Argument &Arg : F.args()) { 257 if (Arg.hasByValAttr()) 258 Tracker.walk(&Arg); 259 } 260 for (auto &BB : F) { 261 for (auto &I : BB) 262 if (AllocaInst *AI = dyn_cast<AllocaInst>(&I)) 263 Tracker.walk(AI); 264 } 265 266 bool Modified = false; 267 268 // Track whether a block is reachable after an alloca has escaped. Blocks that 269 // contain the escaping instruction will be marked as being visited without an 270 // escaped alloca, since that is how the block began. 271 enum VisitType { 272 UNVISITED, 273 UNESCAPED, 274 ESCAPED 275 }; 276 DenseMap<BasicBlock *, VisitType> Visited; 277 278 // We propagate the fact that an alloca has escaped from block to successor. 279 // Visit the blocks that are propagating the escapedness first. To do this, we 280 // maintain two worklists. 281 SmallVector<BasicBlock *, 32> WorklistUnescaped, WorklistEscaped; 282 283 // We may enter a block and visit it thinking that no alloca has escaped yet, 284 // then see an escape point and go back around a loop edge and come back to 285 // the same block twice. Because of this, we defer setting tail on calls when 286 // we first encounter them in a block. Every entry in this list does not 287 // statically use an alloca via use-def chain analysis, but may find an alloca 288 // through other means if the block turns out to be reachable after an escape 289 // point. 290 SmallVector<CallInst *, 32> DeferredTails; 291 292 BasicBlock *BB = &F.getEntryBlock(); 293 VisitType Escaped = UNESCAPED; 294 do { 295 for (auto &I : *BB) { 296 if (Tracker.EscapePoints.count(&I)) 297 Escaped = ESCAPED; 298 299 CallInst *CI = dyn_cast<CallInst>(&I); 300 if (!CI || CI->isTailCall()) 301 continue; 302 303 if (CI->doesNotAccessMemory()) { 304 // A call to a readnone function whose arguments are all things computed 305 // outside this function can be marked tail. Even if you stored the 306 // alloca address into a global, a readnone function can't load the 307 // global anyhow. 308 // 309 // Note that this runs whether we know an alloca has escaped or not. If 310 // it has, then we can't trust Tracker.AllocaUsers to be accurate. 311 bool SafeToTail = true; 312 for (auto &Arg : CI->arg_operands()) { 313 if (isa<Constant>(Arg.getUser())) 314 continue; 315 if (Argument *A = dyn_cast<Argument>(Arg.getUser())) 316 if (!A->hasByValAttr()) 317 continue; 318 SafeToTail = false; 319 break; 320 } 321 if (SafeToTail) { 322 emitOptimizationRemark( 323 F.getContext(), "tailcallelim", F, CI->getDebugLoc(), 324 "marked this readnone call a tail call candidate"); 325 CI->setTailCall(); 326 Modified = true; 327 continue; 328 } 329 } 330 331 if (Escaped == UNESCAPED && !Tracker.AllocaUsers.count(CI)) { 332 DeferredTails.push_back(CI); 333 } else { 334 AllCallsAreTailCalls = false; 335 } 336 } 337 338 for (auto *SuccBB : make_range(succ_begin(BB), succ_end(BB))) { 339 auto &State = Visited[SuccBB]; 340 if (State < Escaped) { 341 State = Escaped; 342 if (State == ESCAPED) 343 WorklistEscaped.push_back(SuccBB); 344 else 345 WorklistUnescaped.push_back(SuccBB); 346 } 347 } 348 349 if (!WorklistEscaped.empty()) { 350 BB = WorklistEscaped.pop_back_val(); 351 Escaped = ESCAPED; 352 } else { 353 BB = nullptr; 354 while (!WorklistUnescaped.empty()) { 355 auto *NextBB = WorklistUnescaped.pop_back_val(); 356 if (Visited[NextBB] == UNESCAPED) { 357 BB = NextBB; 358 Escaped = UNESCAPED; 359 break; 360 } 361 } 362 } 363 } while (BB); 364 365 for (CallInst *CI : DeferredTails) { 366 if (Visited[CI->getParent()] != ESCAPED) { 367 // If the escape point was part way through the block, calls after the 368 // escape point wouldn't have been put into DeferredTails. 369 emitOptimizationRemark(F.getContext(), "tailcallelim", F, 370 CI->getDebugLoc(), 371 "marked this call a tail call candidate"); 372 CI->setTailCall(); 373 Modified = true; 374 } else { 375 AllCallsAreTailCalls = false; 376 } 377 } 378 379 return Modified; 380 } 381 382 bool TailCallElim::runTRE(Function &F) { 383 // If this function is a varargs function, we won't be able to PHI the args 384 // right, so don't even try to convert it... 385 if (F.getFunctionType()->isVarArg()) return false; 386 387 TTI = &getAnalysis<TargetTransformInfo>(); 388 BasicBlock *OldEntry = nullptr; 389 bool TailCallsAreMarkedTail = false; 390 SmallVector<PHINode*, 8> ArgumentPHIs; 391 bool MadeChange = false; 392 393 // CanTRETailMarkedCall - If false, we cannot perform TRE on tail calls 394 // marked with the 'tail' attribute, because doing so would cause the stack 395 // size to increase (real TRE would deallocate variable sized allocas, TRE 396 // doesn't). 397 bool CanTRETailMarkedCall = CanTRE(F); 398 399 // Change any tail recursive calls to loops. 400 // 401 // FIXME: The code generator produces really bad code when an 'escaping 402 // alloca' is changed from being a static alloca to being a dynamic alloca. 403 // Until this is resolved, disable this transformation if that would ever 404 // happen. This bug is PR962. 405 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) { 406 if (ReturnInst *Ret = dyn_cast<ReturnInst>(BB->getTerminator())) { 407 bool Change = ProcessReturningBlock(Ret, OldEntry, TailCallsAreMarkedTail, 408 ArgumentPHIs, !CanTRETailMarkedCall); 409 if (!Change && BB->getFirstNonPHIOrDbg() == Ret) 410 Change = FoldReturnAndProcessPred(BB, Ret, OldEntry, 411 TailCallsAreMarkedTail, ArgumentPHIs, 412 !CanTRETailMarkedCall); 413 MadeChange |= Change; 414 } 415 } 416 417 // If we eliminated any tail recursions, it's possible that we inserted some 418 // silly PHI nodes which just merge an initial value (the incoming operand) 419 // with themselves. Check to see if we did and clean up our mess if so. This 420 // occurs when a function passes an argument straight through to its tail 421 // call. 422 for (unsigned i = 0, e = ArgumentPHIs.size(); i != e; ++i) { 423 PHINode *PN = ArgumentPHIs[i]; 424 425 // If the PHI Node is a dynamic constant, replace it with the value it is. 426 if (Value *PNV = SimplifyInstruction(PN)) { 427 PN->replaceAllUsesWith(PNV); 428 PN->eraseFromParent(); 429 } 430 } 431 432 return MadeChange; 433 } 434 435 436 /// CanMoveAboveCall - Return true if it is safe to move the specified 437 /// instruction from after the call to before the call, assuming that all 438 /// instructions between the call and this instruction are movable. 439 /// 440 bool TailCallElim::CanMoveAboveCall(Instruction *I, CallInst *CI) { 441 // FIXME: We can move load/store/call/free instructions above the call if the 442 // call does not mod/ref the memory location being processed. 443 if (I->mayHaveSideEffects()) // This also handles volatile loads. 444 return false; 445 446 if (LoadInst *L = dyn_cast<LoadInst>(I)) { 447 // Loads may always be moved above calls without side effects. 448 if (CI->mayHaveSideEffects()) { 449 // Non-volatile loads may be moved above a call with side effects if it 450 // does not write to memory and the load provably won't trap. 451 // FIXME: Writes to memory only matter if they may alias the pointer 452 // being loaded from. 453 if (CI->mayWriteToMemory() || 454 !isSafeToLoadUnconditionally(L->getPointerOperand(), L, 455 L->getAlignment())) 456 return false; 457 } 458 } 459 460 // Otherwise, if this is a side-effect free instruction, check to make sure 461 // that it does not use the return value of the call. If it doesn't use the 462 // return value of the call, it must only use things that are defined before 463 // the call, or movable instructions between the call and the instruction 464 // itself. 465 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) 466 if (I->getOperand(i) == CI) 467 return false; 468 return true; 469 } 470 471 // isDynamicConstant - Return true if the specified value is the same when the 472 // return would exit as it was when the initial iteration of the recursive 473 // function was executed. 474 // 475 // We currently handle static constants and arguments that are not modified as 476 // part of the recursion. 477 // 478 static bool isDynamicConstant(Value *V, CallInst *CI, ReturnInst *RI) { 479 if (isa<Constant>(V)) return true; // Static constants are always dyn consts 480 481 // Check to see if this is an immutable argument, if so, the value 482 // will be available to initialize the accumulator. 483 if (Argument *Arg = dyn_cast<Argument>(V)) { 484 // Figure out which argument number this is... 485 unsigned ArgNo = 0; 486 Function *F = CI->getParent()->getParent(); 487 for (Function::arg_iterator AI = F->arg_begin(); &*AI != Arg; ++AI) 488 ++ArgNo; 489 490 // If we are passing this argument into call as the corresponding 491 // argument operand, then the argument is dynamically constant. 492 // Otherwise, we cannot transform this function safely. 493 if (CI->getArgOperand(ArgNo) == Arg) 494 return true; 495 } 496 497 // Switch cases are always constant integers. If the value is being switched 498 // on and the return is only reachable from one of its cases, it's 499 // effectively constant. 500 if (BasicBlock *UniquePred = RI->getParent()->getUniquePredecessor()) 501 if (SwitchInst *SI = dyn_cast<SwitchInst>(UniquePred->getTerminator())) 502 if (SI->getCondition() == V) 503 return SI->getDefaultDest() != RI->getParent(); 504 505 // Not a constant or immutable argument, we can't safely transform. 506 return false; 507 } 508 509 // getCommonReturnValue - Check to see if the function containing the specified 510 // tail call consistently returns the same runtime-constant value at all exit 511 // points except for IgnoreRI. If so, return the returned value. 512 // 513 static Value *getCommonReturnValue(ReturnInst *IgnoreRI, CallInst *CI) { 514 Function *F = CI->getParent()->getParent(); 515 Value *ReturnedValue = nullptr; 516 517 for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI) { 518 ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator()); 519 if (RI == nullptr || RI == IgnoreRI) continue; 520 521 // We can only perform this transformation if the value returned is 522 // evaluatable at the start of the initial invocation of the function, 523 // instead of at the end of the evaluation. 524 // 525 Value *RetOp = RI->getOperand(0); 526 if (!isDynamicConstant(RetOp, CI, RI)) 527 return nullptr; 528 529 if (ReturnedValue && RetOp != ReturnedValue) 530 return nullptr; // Cannot transform if differing values are returned. 531 ReturnedValue = RetOp; 532 } 533 return ReturnedValue; 534 } 535 536 /// CanTransformAccumulatorRecursion - If the specified instruction can be 537 /// transformed using accumulator recursion elimination, return the constant 538 /// which is the start of the accumulator value. Otherwise return null. 539 /// 540 Value *TailCallElim::CanTransformAccumulatorRecursion(Instruction *I, 541 CallInst *CI) { 542 if (!I->isAssociative() || !I->isCommutative()) return nullptr; 543 assert(I->getNumOperands() == 2 && 544 "Associative/commutative operations should have 2 args!"); 545 546 // Exactly one operand should be the result of the call instruction. 547 if ((I->getOperand(0) == CI && I->getOperand(1) == CI) || 548 (I->getOperand(0) != CI && I->getOperand(1) != CI)) 549 return nullptr; 550 551 // The only user of this instruction we allow is a single return instruction. 552 if (!I->hasOneUse() || !isa<ReturnInst>(I->user_back())) 553 return nullptr; 554 555 // Ok, now we have to check all of the other return instructions in this 556 // function. If they return non-constants or differing values, then we cannot 557 // transform the function safely. 558 return getCommonReturnValue(cast<ReturnInst>(I->user_back()), CI); 559 } 560 561 static Instruction *FirstNonDbg(BasicBlock::iterator I) { 562 while (isa<DbgInfoIntrinsic>(I)) 563 ++I; 564 return &*I; 565 } 566 567 CallInst* 568 TailCallElim::FindTRECandidate(Instruction *TI, 569 bool CannotTailCallElimCallsMarkedTail) { 570 BasicBlock *BB = TI->getParent(); 571 Function *F = BB->getParent(); 572 573 if (&BB->front() == TI) // Make sure there is something before the terminator. 574 return nullptr; 575 576 // Scan backwards from the return, checking to see if there is a tail call in 577 // this block. If so, set CI to it. 578 CallInst *CI = nullptr; 579 BasicBlock::iterator BBI = TI; 580 while (true) { 581 CI = dyn_cast<CallInst>(BBI); 582 if (CI && CI->getCalledFunction() == F) 583 break; 584 585 if (BBI == BB->begin()) 586 return nullptr; // Didn't find a potential tail call. 587 --BBI; 588 } 589 590 // If this call is marked as a tail call, and if there are dynamic allocas in 591 // the function, we cannot perform this optimization. 592 if (CI->isTailCall() && CannotTailCallElimCallsMarkedTail) 593 return nullptr; 594 595 // As a special case, detect code like this: 596 // double fabs(double f) { return __builtin_fabs(f); } // a 'fabs' call 597 // and disable this xform in this case, because the code generator will 598 // lower the call to fabs into inline code. 599 if (BB == &F->getEntryBlock() && 600 FirstNonDbg(BB->front()) == CI && 601 FirstNonDbg(std::next(BB->begin())) == TI && 602 CI->getCalledFunction() && 603 !TTI->isLoweredToCall(CI->getCalledFunction())) { 604 // A single-block function with just a call and a return. Check that 605 // the arguments match. 606 CallSite::arg_iterator I = CallSite(CI).arg_begin(), 607 E = CallSite(CI).arg_end(); 608 Function::arg_iterator FI = F->arg_begin(), 609 FE = F->arg_end(); 610 for (; I != E && FI != FE; ++I, ++FI) 611 if (*I != &*FI) break; 612 if (I == E && FI == FE) 613 return nullptr; 614 } 615 616 return CI; 617 } 618 619 bool TailCallElim::EliminateRecursiveTailCall(CallInst *CI, ReturnInst *Ret, 620 BasicBlock *&OldEntry, 621 bool &TailCallsAreMarkedTail, 622 SmallVectorImpl<PHINode *> &ArgumentPHIs, 623 bool CannotTailCallElimCallsMarkedTail) { 624 // If we are introducing accumulator recursion to eliminate operations after 625 // the call instruction that are both associative and commutative, the initial 626 // value for the accumulator is placed in this variable. If this value is set 627 // then we actually perform accumulator recursion elimination instead of 628 // simple tail recursion elimination. If the operation is an LLVM instruction 629 // (eg: "add") then it is recorded in AccumulatorRecursionInstr. If not, then 630 // we are handling the case when the return instruction returns a constant C 631 // which is different to the constant returned by other return instructions 632 // (which is recorded in AccumulatorRecursionEliminationInitVal). This is a 633 // special case of accumulator recursion, the operation being "return C". 634 Value *AccumulatorRecursionEliminationInitVal = nullptr; 635 Instruction *AccumulatorRecursionInstr = nullptr; 636 637 // Ok, we found a potential tail call. We can currently only transform the 638 // tail call if all of the instructions between the call and the return are 639 // movable to above the call itself, leaving the call next to the return. 640 // Check that this is the case now. 641 BasicBlock::iterator BBI = CI; 642 for (++BBI; &*BBI != Ret; ++BBI) { 643 if (CanMoveAboveCall(BBI, CI)) continue; 644 645 // If we can't move the instruction above the call, it might be because it 646 // is an associative and commutative operation that could be transformed 647 // using accumulator recursion elimination. Check to see if this is the 648 // case, and if so, remember the initial accumulator value for later. 649 if ((AccumulatorRecursionEliminationInitVal = 650 CanTransformAccumulatorRecursion(BBI, CI))) { 651 // Yes, this is accumulator recursion. Remember which instruction 652 // accumulates. 653 AccumulatorRecursionInstr = BBI; 654 } else { 655 return false; // Otherwise, we cannot eliminate the tail recursion! 656 } 657 } 658 659 // We can only transform call/return pairs that either ignore the return value 660 // of the call and return void, ignore the value of the call and return a 661 // constant, return the value returned by the tail call, or that are being 662 // accumulator recursion variable eliminated. 663 if (Ret->getNumOperands() == 1 && Ret->getReturnValue() != CI && 664 !isa<UndefValue>(Ret->getReturnValue()) && 665 AccumulatorRecursionEliminationInitVal == nullptr && 666 !getCommonReturnValue(nullptr, CI)) { 667 // One case remains that we are able to handle: the current return 668 // instruction returns a constant, and all other return instructions 669 // return a different constant. 670 if (!isDynamicConstant(Ret->getReturnValue(), CI, Ret)) 671 return false; // Current return instruction does not return a constant. 672 // Check that all other return instructions return a common constant. If 673 // so, record it in AccumulatorRecursionEliminationInitVal. 674 AccumulatorRecursionEliminationInitVal = getCommonReturnValue(Ret, CI); 675 if (!AccumulatorRecursionEliminationInitVal) 676 return false; 677 } 678 679 BasicBlock *BB = Ret->getParent(); 680 Function *F = BB->getParent(); 681 682 emitOptimizationRemark(F->getContext(), "tailcallelim", *F, CI->getDebugLoc(), 683 "transforming tail recursion to loop"); 684 685 // OK! We can transform this tail call. If this is the first one found, 686 // create the new entry block, allowing us to branch back to the old entry. 687 if (!OldEntry) { 688 OldEntry = &F->getEntryBlock(); 689 BasicBlock *NewEntry = BasicBlock::Create(F->getContext(), "", F, OldEntry); 690 NewEntry->takeName(OldEntry); 691 OldEntry->setName("tailrecurse"); 692 BranchInst::Create(OldEntry, NewEntry); 693 694 // If this tail call is marked 'tail' and if there are any allocas in the 695 // entry block, move them up to the new entry block. 696 TailCallsAreMarkedTail = CI->isTailCall(); 697 if (TailCallsAreMarkedTail) 698 // Move all fixed sized allocas from OldEntry to NewEntry. 699 for (BasicBlock::iterator OEBI = OldEntry->begin(), E = OldEntry->end(), 700 NEBI = NewEntry->begin(); OEBI != E; ) 701 if (AllocaInst *AI = dyn_cast<AllocaInst>(OEBI++)) 702 if (isa<ConstantInt>(AI->getArraySize())) 703 AI->moveBefore(NEBI); 704 705 // Now that we have created a new block, which jumps to the entry 706 // block, insert a PHI node for each argument of the function. 707 // For now, we initialize each PHI to only have the real arguments 708 // which are passed in. 709 Instruction *InsertPos = OldEntry->begin(); 710 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); 711 I != E; ++I) { 712 PHINode *PN = PHINode::Create(I->getType(), 2, 713 I->getName() + ".tr", InsertPos); 714 I->replaceAllUsesWith(PN); // Everyone use the PHI node now! 715 PN->addIncoming(I, NewEntry); 716 ArgumentPHIs.push_back(PN); 717 } 718 } 719 720 // If this function has self recursive calls in the tail position where some 721 // are marked tail and some are not, only transform one flavor or another. We 722 // have to choose whether we move allocas in the entry block to the new entry 723 // block or not, so we can't make a good choice for both. NOTE: We could do 724 // slightly better here in the case that the function has no entry block 725 // allocas. 726 if (TailCallsAreMarkedTail && !CI->isTailCall()) 727 return false; 728 729 // Ok, now that we know we have a pseudo-entry block WITH all of the 730 // required PHI nodes, add entries into the PHI node for the actual 731 // parameters passed into the tail-recursive call. 732 for (unsigned i = 0, e = CI->getNumArgOperands(); i != e; ++i) 733 ArgumentPHIs[i]->addIncoming(CI->getArgOperand(i), BB); 734 735 // If we are introducing an accumulator variable to eliminate the recursion, 736 // do so now. Note that we _know_ that no subsequent tail recursion 737 // eliminations will happen on this function because of the way the 738 // accumulator recursion predicate is set up. 739 // 740 if (AccumulatorRecursionEliminationInitVal) { 741 Instruction *AccRecInstr = AccumulatorRecursionInstr; 742 // Start by inserting a new PHI node for the accumulator. 743 pred_iterator PB = pred_begin(OldEntry), PE = pred_end(OldEntry); 744 PHINode *AccPN = 745 PHINode::Create(AccumulatorRecursionEliminationInitVal->getType(), 746 std::distance(PB, PE) + 1, 747 "accumulator.tr", OldEntry->begin()); 748 749 // Loop over all of the predecessors of the tail recursion block. For the 750 // real entry into the function we seed the PHI with the initial value, 751 // computed earlier. For any other existing branches to this block (due to 752 // other tail recursions eliminated) the accumulator is not modified. 753 // Because we haven't added the branch in the current block to OldEntry yet, 754 // it will not show up as a predecessor. 755 for (pred_iterator PI = PB; PI != PE; ++PI) { 756 BasicBlock *P = *PI; 757 if (P == &F->getEntryBlock()) 758 AccPN->addIncoming(AccumulatorRecursionEliminationInitVal, P); 759 else 760 AccPN->addIncoming(AccPN, P); 761 } 762 763 if (AccRecInstr) { 764 // Add an incoming argument for the current block, which is computed by 765 // our associative and commutative accumulator instruction. 766 AccPN->addIncoming(AccRecInstr, BB); 767 768 // Next, rewrite the accumulator recursion instruction so that it does not 769 // use the result of the call anymore, instead, use the PHI node we just 770 // inserted. 771 AccRecInstr->setOperand(AccRecInstr->getOperand(0) != CI, AccPN); 772 } else { 773 // Add an incoming argument for the current block, which is just the 774 // constant returned by the current return instruction. 775 AccPN->addIncoming(Ret->getReturnValue(), BB); 776 } 777 778 // Finally, rewrite any return instructions in the program to return the PHI 779 // node instead of the "initval" that they do currently. This loop will 780 // actually rewrite the return value we are destroying, but that's ok. 781 for (Function::iterator BBI = F->begin(), E = F->end(); BBI != E; ++BBI) 782 if (ReturnInst *RI = dyn_cast<ReturnInst>(BBI->getTerminator())) 783 RI->setOperand(0, AccPN); 784 ++NumAccumAdded; 785 } 786 787 // Now that all of the PHI nodes are in place, remove the call and 788 // ret instructions, replacing them with an unconditional branch. 789 BranchInst *NewBI = BranchInst::Create(OldEntry, Ret); 790 NewBI->setDebugLoc(CI->getDebugLoc()); 791 792 BB->getInstList().erase(Ret); // Remove return. 793 BB->getInstList().erase(CI); // Remove call. 794 ++NumEliminated; 795 return true; 796 } 797 798 bool TailCallElim::FoldReturnAndProcessPred(BasicBlock *BB, 799 ReturnInst *Ret, BasicBlock *&OldEntry, 800 bool &TailCallsAreMarkedTail, 801 SmallVectorImpl<PHINode *> &ArgumentPHIs, 802 bool CannotTailCallElimCallsMarkedTail) { 803 bool Change = false; 804 805 // If the return block contains nothing but the return and PHI's, 806 // there might be an opportunity to duplicate the return in its 807 // predecessors and perform TRC there. Look for predecessors that end 808 // in unconditional branch and recursive call(s). 809 SmallVector<BranchInst*, 8> UncondBranchPreds; 810 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { 811 BasicBlock *Pred = *PI; 812 TerminatorInst *PTI = Pred->getTerminator(); 813 if (BranchInst *BI = dyn_cast<BranchInst>(PTI)) 814 if (BI->isUnconditional()) 815 UncondBranchPreds.push_back(BI); 816 } 817 818 while (!UncondBranchPreds.empty()) { 819 BranchInst *BI = UncondBranchPreds.pop_back_val(); 820 BasicBlock *Pred = BI->getParent(); 821 if (CallInst *CI = FindTRECandidate(BI, CannotTailCallElimCallsMarkedTail)){ 822 DEBUG(dbgs() << "FOLDING: " << *BB 823 << "INTO UNCOND BRANCH PRED: " << *Pred); 824 EliminateRecursiveTailCall(CI, FoldReturnIntoUncondBranch(Ret, BB, Pred), 825 OldEntry, TailCallsAreMarkedTail, ArgumentPHIs, 826 CannotTailCallElimCallsMarkedTail); 827 ++NumRetDuped; 828 Change = true; 829 } 830 } 831 832 return Change; 833 } 834 835 bool 836 TailCallElim::ProcessReturningBlock(ReturnInst *Ret, BasicBlock *&OldEntry, 837 bool &TailCallsAreMarkedTail, 838 SmallVectorImpl<PHINode *> &ArgumentPHIs, 839 bool CannotTailCallElimCallsMarkedTail) { 840 CallInst *CI = FindTRECandidate(Ret, CannotTailCallElimCallsMarkedTail); 841 if (!CI) 842 return false; 843 844 return EliminateRecursiveTailCall(CI, Ret, OldEntry, TailCallsAreMarkedTail, 845 ArgumentPHIs, 846 CannotTailCallElimCallsMarkedTail); 847 } 848