DeadArgumentElimination.cpp revision 194178
1//===-- DeadArgumentElimination.cpp - Eliminate dead arguments ------------===// 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 pass deletes dead arguments from internal functions. Dead argument 11// elimination removes arguments which are directly dead, as well as arguments 12// only passed into function calls as dead arguments of other functions. This 13// pass also deletes dead return values in a similar way. 14// 15// This pass is often useful as a cleanup pass to run after aggressive 16// interprocedural passes, which add possibly-dead arguments or return values. 17// 18//===----------------------------------------------------------------------===// 19 20#define DEBUG_TYPE "deadargelim" 21#include "llvm/Transforms/IPO.h" 22#include "llvm/CallingConv.h" 23#include "llvm/Constant.h" 24#include "llvm/DerivedTypes.h" 25#include "llvm/Instructions.h" 26#include "llvm/IntrinsicInst.h" 27#include "llvm/Module.h" 28#include "llvm/Pass.h" 29#include "llvm/Support/CallSite.h" 30#include "llvm/Support/Debug.h" 31#include "llvm/ADT/SmallVector.h" 32#include "llvm/ADT/Statistic.h" 33#include "llvm/ADT/StringExtras.h" 34#include "llvm/Support/Compiler.h" 35#include <map> 36#include <set> 37using namespace llvm; 38 39STATISTIC(NumArgumentsEliminated, "Number of unread args removed"); 40STATISTIC(NumRetValsEliminated , "Number of unused return values removed"); 41 42namespace { 43 /// DAE - The dead argument elimination pass. 44 /// 45 class VISIBILITY_HIDDEN DAE : public ModulePass { 46 public: 47 48 /// Struct that represents (part of) either a return value or a function 49 /// argument. Used so that arguments and return values can be used 50 /// interchangably. 51 struct RetOrArg { 52 RetOrArg(const Function* F, unsigned Idx, bool IsArg) : F(F), Idx(Idx), 53 IsArg(IsArg) {} 54 const Function *F; 55 unsigned Idx; 56 bool IsArg; 57 58 /// Make RetOrArg comparable, so we can put it into a map. 59 bool operator<(const RetOrArg &O) const { 60 if (F != O.F) 61 return F < O.F; 62 else if (Idx != O.Idx) 63 return Idx < O.Idx; 64 else 65 return IsArg < O.IsArg; 66 } 67 68 /// Make RetOrArg comparable, so we can easily iterate the multimap. 69 bool operator==(const RetOrArg &O) const { 70 return F == O.F && Idx == O.Idx && IsArg == O.IsArg; 71 } 72 73 std::string getDescription() const { 74 return std::string((IsArg ? "Argument #" : "Return value #")) 75 + utostr(Idx) + " of function " + F->getName(); 76 } 77 }; 78 79 /// Liveness enum - During our initial pass over the program, we determine 80 /// that things are either alive or maybe alive. We don't mark anything 81 /// explicitly dead (even if we know they are), since anything not alive 82 /// with no registered uses (in Uses) will never be marked alive and will 83 /// thus become dead in the end. 84 enum Liveness { Live, MaybeLive }; 85 86 /// Convenience wrapper 87 RetOrArg CreateRet(const Function *F, unsigned Idx) { 88 return RetOrArg(F, Idx, false); 89 } 90 /// Convenience wrapper 91 RetOrArg CreateArg(const Function *F, unsigned Idx) { 92 return RetOrArg(F, Idx, true); 93 } 94 95 typedef std::multimap<RetOrArg, RetOrArg> UseMap; 96 /// This maps a return value or argument to any MaybeLive return values or 97 /// arguments it uses. This allows the MaybeLive values to be marked live 98 /// when any of its users is marked live. 99 /// For example (indices are left out for clarity): 100 /// - Uses[ret F] = ret G 101 /// This means that F calls G, and F returns the value returned by G. 102 /// - Uses[arg F] = ret G 103 /// This means that some function calls G and passes its result as an 104 /// argument to F. 105 /// - Uses[ret F] = arg F 106 /// This means that F returns one of its own arguments. 107 /// - Uses[arg F] = arg G 108 /// This means that G calls F and passes one of its own (G's) arguments 109 /// directly to F. 110 UseMap Uses; 111 112 typedef std::set<RetOrArg> LiveSet; 113 typedef std::set<const Function*> LiveFuncSet; 114 115 /// This set contains all values that have been determined to be live. 116 LiveSet LiveValues; 117 /// This set contains all values that are cannot be changed in any way. 118 LiveFuncSet LiveFunctions; 119 120 typedef SmallVector<RetOrArg, 5> UseVector; 121 122 public: 123 static char ID; // Pass identification, replacement for typeid 124 DAE() : ModulePass(&ID) {} 125 bool runOnModule(Module &M); 126 127 virtual bool ShouldHackArguments() const { return false; } 128 129 private: 130 Liveness MarkIfNotLive(RetOrArg Use, UseVector &MaybeLiveUses); 131 Liveness SurveyUse(Value::use_iterator U, UseVector &MaybeLiveUses, 132 unsigned RetValNum = 0); 133 Liveness SurveyUses(Value *V, UseVector &MaybeLiveUses); 134 135 void SurveyFunction(Function &F); 136 void MarkValue(const RetOrArg &RA, Liveness L, 137 const UseVector &MaybeLiveUses); 138 void MarkLive(const RetOrArg &RA); 139 void MarkLive(const Function &F); 140 void PropagateLiveness(const RetOrArg &RA); 141 bool RemoveDeadStuffFromFunction(Function *F); 142 bool DeleteDeadVarargs(Function &Fn); 143 }; 144} 145 146 147char DAE::ID = 0; 148static RegisterPass<DAE> 149X("deadargelim", "Dead Argument Elimination"); 150 151namespace { 152 /// DAH - DeadArgumentHacking pass - Same as dead argument elimination, but 153 /// deletes arguments to functions which are external. This is only for use 154 /// by bugpoint. 155 struct DAH : public DAE { 156 static char ID; 157 virtual bool ShouldHackArguments() const { return true; } 158 }; 159} 160 161char DAH::ID = 0; 162static RegisterPass<DAH> 163Y("deadarghaX0r", "Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)"); 164 165/// createDeadArgEliminationPass - This pass removes arguments from functions 166/// which are not used by the body of the function. 167/// 168ModulePass *llvm::createDeadArgEliminationPass() { return new DAE(); } 169ModulePass *llvm::createDeadArgHackingPass() { return new DAH(); } 170 171/// DeleteDeadVarargs - If this is an function that takes a ... list, and if 172/// llvm.vastart is never called, the varargs list is dead for the function. 173bool DAE::DeleteDeadVarargs(Function &Fn) { 174 assert(Fn.getFunctionType()->isVarArg() && "Function isn't varargs!"); 175 if (Fn.isDeclaration() || !Fn.hasLocalLinkage()) return false; 176 177 // Ensure that the function is only directly called. 178 if (Fn.hasAddressTaken()) 179 return false; 180 181 // Okay, we know we can transform this function if safe. Scan its body 182 // looking for calls to llvm.vastart. 183 for (Function::iterator BB = Fn.begin(), E = Fn.end(); BB != E; ++BB) { 184 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) { 185 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) { 186 if (II->getIntrinsicID() == Intrinsic::vastart) 187 return false; 188 } 189 } 190 } 191 192 // If we get here, there are no calls to llvm.vastart in the function body, 193 // remove the "..." and adjust all the calls. 194 195 // Start by computing a new prototype for the function, which is the same as 196 // the old function, but doesn't have isVarArg set. 197 const FunctionType *FTy = Fn.getFunctionType(); 198 std::vector<const Type*> Params(FTy->param_begin(), FTy->param_end()); 199 FunctionType *NFTy = FunctionType::get(FTy->getReturnType(), Params, false); 200 unsigned NumArgs = Params.size(); 201 202 // Create the new function body and insert it into the module... 203 Function *NF = Function::Create(NFTy, Fn.getLinkage()); 204 NF->copyAttributesFrom(&Fn); 205 Fn.getParent()->getFunctionList().insert(&Fn, NF); 206 NF->takeName(&Fn); 207 208 // Loop over all of the callers of the function, transforming the call sites 209 // to pass in a smaller number of arguments into the new function. 210 // 211 std::vector<Value*> Args; 212 while (!Fn.use_empty()) { 213 CallSite CS = CallSite::get(Fn.use_back()); 214 Instruction *Call = CS.getInstruction(); 215 216 // Pass all the same arguments. 217 Args.assign(CS.arg_begin(), CS.arg_begin()+NumArgs); 218 219 // Drop any attributes that were on the vararg arguments. 220 AttrListPtr PAL = CS.getAttributes(); 221 if (!PAL.isEmpty() && PAL.getSlot(PAL.getNumSlots() - 1).Index > NumArgs) { 222 SmallVector<AttributeWithIndex, 8> AttributesVec; 223 for (unsigned i = 0; PAL.getSlot(i).Index <= NumArgs; ++i) 224 AttributesVec.push_back(PAL.getSlot(i)); 225 if (Attributes FnAttrs = PAL.getFnAttributes()) 226 AttributesVec.push_back(AttributeWithIndex::get(~0, FnAttrs)); 227 PAL = AttrListPtr::get(AttributesVec.begin(), AttributesVec.end()); 228 } 229 230 Instruction *New; 231 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) { 232 New = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(), 233 Args.begin(), Args.end(), "", Call); 234 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv()); 235 cast<InvokeInst>(New)->setAttributes(PAL); 236 } else { 237 New = CallInst::Create(NF, Args.begin(), Args.end(), "", Call); 238 cast<CallInst>(New)->setCallingConv(CS.getCallingConv()); 239 cast<CallInst>(New)->setAttributes(PAL); 240 if (cast<CallInst>(Call)->isTailCall()) 241 cast<CallInst>(New)->setTailCall(); 242 } 243 Args.clear(); 244 245 if (!Call->use_empty()) 246 Call->replaceAllUsesWith(New); 247 248 New->takeName(Call); 249 250 // Finally, remove the old call from the program, reducing the use-count of 251 // F. 252 Call->eraseFromParent(); 253 } 254 255 // Since we have now created the new function, splice the body of the old 256 // function right into the new function, leaving the old rotting hulk of the 257 // function empty. 258 NF->getBasicBlockList().splice(NF->begin(), Fn.getBasicBlockList()); 259 260 // Loop over the argument list, transfering uses of the old arguments over to 261 // the new arguments, also transfering over the names as well. While we're at 262 // it, remove the dead arguments from the DeadArguments list. 263 // 264 for (Function::arg_iterator I = Fn.arg_begin(), E = Fn.arg_end(), 265 I2 = NF->arg_begin(); I != E; ++I, ++I2) { 266 // Move the name and users over to the new version. 267 I->replaceAllUsesWith(I2); 268 I2->takeName(I); 269 } 270 271 // Finally, nuke the old function. 272 Fn.eraseFromParent(); 273 return true; 274} 275 276/// Convenience function that returns the number of return values. It returns 0 277/// for void functions and 1 for functions not returning a struct. It returns 278/// the number of struct elements for functions returning a struct. 279static unsigned NumRetVals(const Function *F) { 280 if (F->getReturnType() == Type::VoidTy) 281 return 0; 282 else if (const StructType *STy = dyn_cast<StructType>(F->getReturnType())) 283 return STy->getNumElements(); 284 else 285 return 1; 286} 287 288/// MarkIfNotLive - This checks Use for liveness in LiveValues. If Use is not 289/// live, it adds Use to the MaybeLiveUses argument. Returns the determined 290/// liveness of Use. 291DAE::Liveness DAE::MarkIfNotLive(RetOrArg Use, UseVector &MaybeLiveUses) { 292 // We're live if our use or its Function is already marked as live. 293 if (LiveFunctions.count(Use.F) || LiveValues.count(Use)) 294 return Live; 295 296 // We're maybe live otherwise, but remember that we must become live if 297 // Use becomes live. 298 MaybeLiveUses.push_back(Use); 299 return MaybeLive; 300} 301 302 303/// SurveyUse - This looks at a single use of an argument or return value 304/// and determines if it should be alive or not. Adds this use to MaybeLiveUses 305/// if it causes the used value to become MaybeAlive. 306/// 307/// RetValNum is the return value number to use when this use is used in a 308/// return instruction. This is used in the recursion, you should always leave 309/// it at 0. 310DAE::Liveness DAE::SurveyUse(Value::use_iterator U, UseVector &MaybeLiveUses, 311 unsigned RetValNum) { 312 Value *V = *U; 313 if (ReturnInst *RI = dyn_cast<ReturnInst>(V)) { 314 // The value is returned from a function. It's only live when the 315 // function's return value is live. We use RetValNum here, for the case 316 // that U is really a use of an insertvalue instruction that uses the 317 // orginal Use. 318 RetOrArg Use = CreateRet(RI->getParent()->getParent(), RetValNum); 319 // We might be live, depending on the liveness of Use. 320 return MarkIfNotLive(Use, MaybeLiveUses); 321 } 322 if (InsertValueInst *IV = dyn_cast<InsertValueInst>(V)) { 323 if (U.getOperandNo() != InsertValueInst::getAggregateOperandIndex() 324 && IV->hasIndices()) 325 // The use we are examining is inserted into an aggregate. Our liveness 326 // depends on all uses of that aggregate, but if it is used as a return 327 // value, only index at which we were inserted counts. 328 RetValNum = *IV->idx_begin(); 329 330 // Note that if we are used as the aggregate operand to the insertvalue, 331 // we don't change RetValNum, but do survey all our uses. 332 333 Liveness Result = MaybeLive; 334 for (Value::use_iterator I = IV->use_begin(), 335 E = V->use_end(); I != E; ++I) { 336 Result = SurveyUse(I, MaybeLiveUses, RetValNum); 337 if (Result == Live) 338 break; 339 } 340 return Result; 341 } 342 CallSite CS = CallSite::get(V); 343 if (CS.getInstruction()) { 344 Function *F = CS.getCalledFunction(); 345 if (F) { 346 // Used in a direct call. 347 348 // Find the argument number. We know for sure that this use is an 349 // argument, since if it was the function argument this would be an 350 // indirect call and the we know can't be looking at a value of the 351 // label type (for the invoke instruction). 352 unsigned ArgNo = CS.getArgumentNo(U.getOperandNo()); 353 354 if (ArgNo >= F->getFunctionType()->getNumParams()) 355 // The value is passed in through a vararg! Must be live. 356 return Live; 357 358 assert(CS.getArgument(ArgNo) 359 == CS.getInstruction()->getOperand(U.getOperandNo()) 360 && "Argument is not where we expected it"); 361 362 // Value passed to a normal call. It's only live when the corresponding 363 // argument to the called function turns out live. 364 RetOrArg Use = CreateArg(F, ArgNo); 365 return MarkIfNotLive(Use, MaybeLiveUses); 366 } 367 } 368 // Used in any other way? Value must be live. 369 return Live; 370} 371 372/// SurveyUses - This looks at all the uses of the given value 373/// Returns the Liveness deduced from the uses of this value. 374/// 375/// Adds all uses that cause the result to be MaybeLive to MaybeLiveRetUses. If 376/// the result is Live, MaybeLiveUses might be modified but its content should 377/// be ignored (since it might not be complete). 378DAE::Liveness DAE::SurveyUses(Value *V, UseVector &MaybeLiveUses) { 379 // Assume it's dead (which will only hold if there are no uses at all..). 380 Liveness Result = MaybeLive; 381 // Check each use. 382 for (Value::use_iterator I = V->use_begin(), 383 E = V->use_end(); I != E; ++I) { 384 Result = SurveyUse(I, MaybeLiveUses); 385 if (Result == Live) 386 break; 387 } 388 return Result; 389} 390 391// SurveyFunction - This performs the initial survey of the specified function, 392// checking out whether or not it uses any of its incoming arguments or whether 393// any callers use the return value. This fills in the LiveValues set and Uses 394// map. 395// 396// We consider arguments of non-internal functions to be intrinsically alive as 397// well as arguments to functions which have their "address taken". 398// 399void DAE::SurveyFunction(Function &F) { 400 unsigned RetCount = NumRetVals(&F); 401 // Assume all return values are dead 402 typedef SmallVector<Liveness, 5> RetVals; 403 RetVals RetValLiveness(RetCount, MaybeLive); 404 405 typedef SmallVector<UseVector, 5> RetUses; 406 // These vectors map each return value to the uses that make it MaybeLive, so 407 // we can add those to the Uses map if the return value really turns out to be 408 // MaybeLive. Initialized to a list of RetCount empty lists. 409 RetUses MaybeLiveRetUses(RetCount); 410 411 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) 412 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) 413 if (RI->getNumOperands() != 0 && RI->getOperand(0)->getType() 414 != F.getFunctionType()->getReturnType()) { 415 // We don't support old style multiple return values. 416 MarkLive(F); 417 return; 418 } 419 420 if (!F.hasLocalLinkage() && (!ShouldHackArguments() || F.isIntrinsic())) { 421 MarkLive(F); 422 return; 423 } 424 425 DOUT << "DAE - Inspecting callers for fn: " << F.getName() << "\n"; 426 // Keep track of the number of live retvals, so we can skip checks once all 427 // of them turn out to be live. 428 unsigned NumLiveRetVals = 0; 429 const Type *STy = dyn_cast<StructType>(F.getReturnType()); 430 // Loop all uses of the function. 431 for (Value::use_iterator I = F.use_begin(), E = F.use_end(); I != E; ++I) { 432 // If the function is PASSED IN as an argument, its address has been 433 // taken. 434 CallSite CS = CallSite::get(*I); 435 if (!CS.getInstruction() || !CS.isCallee(I)) { 436 MarkLive(F); 437 return; 438 } 439 440 // If this use is anything other than a call site, the function is alive. 441 Instruction *TheCall = CS.getInstruction(); 442 if (!TheCall) { // Not a direct call site? 443 MarkLive(F); 444 return; 445 } 446 447 // If we end up here, we are looking at a direct call to our function. 448 449 // Now, check how our return value(s) is/are used in this caller. Don't 450 // bother checking return values if all of them are live already. 451 if (NumLiveRetVals != RetCount) { 452 if (STy) { 453 // Check all uses of the return value. 454 for (Value::use_iterator I = TheCall->use_begin(), 455 E = TheCall->use_end(); I != E; ++I) { 456 ExtractValueInst *Ext = dyn_cast<ExtractValueInst>(*I); 457 if (Ext && Ext->hasIndices()) { 458 // This use uses a part of our return value, survey the uses of 459 // that part and store the results for this index only. 460 unsigned Idx = *Ext->idx_begin(); 461 if (RetValLiveness[Idx] != Live) { 462 RetValLiveness[Idx] = SurveyUses(Ext, MaybeLiveRetUses[Idx]); 463 if (RetValLiveness[Idx] == Live) 464 NumLiveRetVals++; 465 } 466 } else { 467 // Used by something else than extractvalue. Mark all return 468 // values as live. 469 for (unsigned i = 0; i != RetCount; ++i ) 470 RetValLiveness[i] = Live; 471 NumLiveRetVals = RetCount; 472 break; 473 } 474 } 475 } else { 476 // Single return value 477 RetValLiveness[0] = SurveyUses(TheCall, MaybeLiveRetUses[0]); 478 if (RetValLiveness[0] == Live) 479 NumLiveRetVals = RetCount; 480 } 481 } 482 } 483 484 // Now we've inspected all callers, record the liveness of our return values. 485 for (unsigned i = 0; i != RetCount; ++i) 486 MarkValue(CreateRet(&F, i), RetValLiveness[i], MaybeLiveRetUses[i]); 487 488 DOUT << "DAE - Inspecting args for fn: " << F.getName() << "\n"; 489 490 // Now, check all of our arguments. 491 unsigned i = 0; 492 UseVector MaybeLiveArgUses; 493 for (Function::arg_iterator AI = F.arg_begin(), 494 E = F.arg_end(); AI != E; ++AI, ++i) { 495 // See what the effect of this use is (recording any uses that cause 496 // MaybeLive in MaybeLiveArgUses). 497 Liveness Result = SurveyUses(AI, MaybeLiveArgUses); 498 // Mark the result. 499 MarkValue(CreateArg(&F, i), Result, MaybeLiveArgUses); 500 // Clear the vector again for the next iteration. 501 MaybeLiveArgUses.clear(); 502 } 503} 504 505/// MarkValue - This function marks the liveness of RA depending on L. If L is 506/// MaybeLive, it also takes all uses in MaybeLiveUses and records them in Uses, 507/// such that RA will be marked live if any use in MaybeLiveUses gets marked 508/// live later on. 509void DAE::MarkValue(const RetOrArg &RA, Liveness L, 510 const UseVector &MaybeLiveUses) { 511 switch (L) { 512 case Live: MarkLive(RA); break; 513 case MaybeLive: 514 { 515 // Note any uses of this value, so this return value can be 516 // marked live whenever one of the uses becomes live. 517 for (UseVector::const_iterator UI = MaybeLiveUses.begin(), 518 UE = MaybeLiveUses.end(); UI != UE; ++UI) 519 Uses.insert(std::make_pair(*UI, RA)); 520 break; 521 } 522 } 523} 524 525/// MarkLive - Mark the given Function as alive, meaning that it cannot be 526/// changed in any way. Additionally, 527/// mark any values that are used as this function's parameters or by its return 528/// values (according to Uses) live as well. 529void DAE::MarkLive(const Function &F) { 530 DOUT << "DAE - Intrinsically live fn: " << F.getName() << "\n"; 531 // Mark the function as live. 532 LiveFunctions.insert(&F); 533 // Mark all arguments as live. 534 for (unsigned i = 0, e = F.arg_size(); i != e; ++i) 535 PropagateLiveness(CreateArg(&F, i)); 536 // Mark all return values as live. 537 for (unsigned i = 0, e = NumRetVals(&F); i != e; ++i) 538 PropagateLiveness(CreateRet(&F, i)); 539} 540 541/// MarkLive - Mark the given return value or argument as live. Additionally, 542/// mark any values that are used by this value (according to Uses) live as 543/// well. 544void DAE::MarkLive(const RetOrArg &RA) { 545 if (LiveFunctions.count(RA.F)) 546 return; // Function was already marked Live. 547 548 if (!LiveValues.insert(RA).second) 549 return; // We were already marked Live. 550 551 DOUT << "DAE - Marking " << RA.getDescription() << " live\n"; 552 PropagateLiveness(RA); 553} 554 555/// PropagateLiveness - Given that RA is a live value, propagate it's liveness 556/// to any other values it uses (according to Uses). 557void DAE::PropagateLiveness(const RetOrArg &RA) { 558 // We don't use upper_bound (or equal_range) here, because our recursive call 559 // to ourselves is likely to cause the upper_bound (which is the first value 560 // not belonging to RA) to become erased and the iterator invalidated. 561 UseMap::iterator Begin = Uses.lower_bound(RA); 562 UseMap::iterator E = Uses.end(); 563 UseMap::iterator I; 564 for (I = Begin; I != E && I->first == RA; ++I) 565 MarkLive(I->second); 566 567 // Erase RA from the Uses map (from the lower bound to wherever we ended up 568 // after the loop). 569 Uses.erase(Begin, I); 570} 571 572// RemoveDeadStuffFromFunction - Remove any arguments and return values from F 573// that are not in LiveValues. Transform the function and all of the callees of 574// the function to not have these arguments and return values. 575// 576bool DAE::RemoveDeadStuffFromFunction(Function *F) { 577 // Don't modify fully live functions 578 if (LiveFunctions.count(F)) 579 return false; 580 581 // Start by computing a new prototype for the function, which is the same as 582 // the old function, but has fewer arguments and a different return type. 583 const FunctionType *FTy = F->getFunctionType(); 584 std::vector<const Type*> Params; 585 586 // Set up to build a new list of parameter attributes. 587 SmallVector<AttributeWithIndex, 8> AttributesVec; 588 const AttrListPtr &PAL = F->getAttributes(); 589 590 // The existing function return attributes. 591 Attributes RAttrs = PAL.getRetAttributes(); 592 Attributes FnAttrs = PAL.getFnAttributes(); 593 594 // Find out the new return value. 595 596 const Type *RetTy = FTy->getReturnType(); 597 const Type *NRetTy = NULL; 598 unsigned RetCount = NumRetVals(F); 599 // -1 means unused, other numbers are the new index 600 SmallVector<int, 5> NewRetIdxs(RetCount, -1); 601 std::vector<const Type*> RetTypes; 602 if (RetTy == Type::VoidTy) { 603 NRetTy = Type::VoidTy; 604 } else { 605 const StructType *STy = dyn_cast<StructType>(RetTy); 606 if (STy) 607 // Look at each of the original return values individually. 608 for (unsigned i = 0; i != RetCount; ++i) { 609 RetOrArg Ret = CreateRet(F, i); 610 if (LiveValues.erase(Ret)) { 611 RetTypes.push_back(STy->getElementType(i)); 612 NewRetIdxs[i] = RetTypes.size() - 1; 613 } else { 614 ++NumRetValsEliminated; 615 DOUT << "DAE - Removing return value " << i << " from " 616 << F->getNameStart() << "\n"; 617 } 618 } 619 else 620 // We used to return a single value. 621 if (LiveValues.erase(CreateRet(F, 0))) { 622 RetTypes.push_back(RetTy); 623 NewRetIdxs[0] = 0; 624 } else { 625 DOUT << "DAE - Removing return value from " << F->getNameStart() 626 << "\n"; 627 ++NumRetValsEliminated; 628 } 629 if (RetTypes.size() > 1) 630 // More than one return type? Return a struct with them. Also, if we used 631 // to return a struct and didn't change the number of return values, 632 // return a struct again. This prevents changing {something} into 633 // something and {} into void. 634 // Make the new struct packed if we used to return a packed struct 635 // already. 636 NRetTy = StructType::get(RetTypes, STy->isPacked()); 637 else if (RetTypes.size() == 1) 638 // One return type? Just a simple value then, but only if we didn't use to 639 // return a struct with that simple value before. 640 NRetTy = RetTypes.front(); 641 else if (RetTypes.size() == 0) 642 // No return types? Make it void, but only if we didn't use to return {}. 643 NRetTy = Type::VoidTy; 644 } 645 646 assert(NRetTy && "No new return type found?"); 647 648 // Remove any incompatible attributes, but only if we removed all return 649 // values. Otherwise, ensure that we don't have any conflicting attributes 650 // here. Currently, this should not be possible, but special handling might be 651 // required when new return value attributes are added. 652 if (NRetTy == Type::VoidTy) 653 RAttrs &= ~Attribute::typeIncompatible(NRetTy); 654 else 655 assert((RAttrs & Attribute::typeIncompatible(NRetTy)) == 0 656 && "Return attributes no longer compatible?"); 657 658 if (RAttrs) 659 AttributesVec.push_back(AttributeWithIndex::get(0, RAttrs)); 660 661 // Remember which arguments are still alive. 662 SmallVector<bool, 10> ArgAlive(FTy->getNumParams(), false); 663 // Construct the new parameter list from non-dead arguments. Also construct 664 // a new set of parameter attributes to correspond. Skip the first parameter 665 // attribute, since that belongs to the return value. 666 unsigned i = 0; 667 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); 668 I != E; ++I, ++i) { 669 RetOrArg Arg = CreateArg(F, i); 670 if (LiveValues.erase(Arg)) { 671 Params.push_back(I->getType()); 672 ArgAlive[i] = true; 673 674 // Get the original parameter attributes (skipping the first one, that is 675 // for the return value. 676 if (Attributes Attrs = PAL.getParamAttributes(i + 1)) 677 AttributesVec.push_back(AttributeWithIndex::get(Params.size(), Attrs)); 678 } else { 679 ++NumArgumentsEliminated; 680 DOUT << "DAE - Removing argument " << i << " (" << I->getNameStart() 681 << ") from " << F->getNameStart() << "\n"; 682 } 683 } 684 685 if (FnAttrs != Attribute::None) 686 AttributesVec.push_back(AttributeWithIndex::get(~0, FnAttrs)); 687 688 // Reconstruct the AttributesList based on the vector we constructed. 689 AttrListPtr NewPAL = AttrListPtr::get(AttributesVec.begin(), AttributesVec.end()); 690 691 // Work around LLVM bug PR56: the CWriter cannot emit varargs functions which 692 // have zero fixed arguments. 693 // 694 // Note that we apply this hack for a vararg fuction that does not have any 695 // arguments anymore, but did have them before (so don't bother fixing 696 // functions that were already broken wrt CWriter). 697 bool ExtraArgHack = false; 698 if (Params.empty() && FTy->isVarArg() && FTy->getNumParams() != 0) { 699 ExtraArgHack = true; 700 Params.push_back(Type::Int32Ty); 701 } 702 703 // Create the new function type based on the recomputed parameters. 704 FunctionType *NFTy = FunctionType::get(NRetTy, Params, FTy->isVarArg()); 705 706 // No change? 707 if (NFTy == FTy) 708 return false; 709 710 // Create the new function body and insert it into the module... 711 Function *NF = Function::Create(NFTy, F->getLinkage()); 712 NF->copyAttributesFrom(F); 713 NF->setAttributes(NewPAL); 714 // Insert the new function before the old function, so we won't be processing 715 // it again. 716 F->getParent()->getFunctionList().insert(F, NF); 717 NF->takeName(F); 718 719 // Loop over all of the callers of the function, transforming the call sites 720 // to pass in a smaller number of arguments into the new function. 721 // 722 std::vector<Value*> Args; 723 while (!F->use_empty()) { 724 CallSite CS = CallSite::get(F->use_back()); 725 Instruction *Call = CS.getInstruction(); 726 727 AttributesVec.clear(); 728 const AttrListPtr &CallPAL = CS.getAttributes(); 729 730 // The call return attributes. 731 Attributes RAttrs = CallPAL.getRetAttributes(); 732 Attributes FnAttrs = CallPAL.getFnAttributes(); 733 // Adjust in case the function was changed to return void. 734 RAttrs &= ~Attribute::typeIncompatible(NF->getReturnType()); 735 if (RAttrs) 736 AttributesVec.push_back(AttributeWithIndex::get(0, RAttrs)); 737 738 // Declare these outside of the loops, so we can reuse them for the second 739 // loop, which loops the varargs. 740 CallSite::arg_iterator I = CS.arg_begin(); 741 unsigned i = 0; 742 // Loop over those operands, corresponding to the normal arguments to the 743 // original function, and add those that are still alive. 744 for (unsigned e = FTy->getNumParams(); i != e; ++I, ++i) 745 if (ArgAlive[i]) { 746 Args.push_back(*I); 747 // Get original parameter attributes, but skip return attributes. 748 if (Attributes Attrs = CallPAL.getParamAttributes(i + 1)) 749 AttributesVec.push_back(AttributeWithIndex::get(Args.size(), Attrs)); 750 } 751 752 if (ExtraArgHack) 753 Args.push_back(UndefValue::get(Type::Int32Ty)); 754 755 // Push any varargs arguments on the list. Don't forget their attributes. 756 for (CallSite::arg_iterator E = CS.arg_end(); I != E; ++I, ++i) { 757 Args.push_back(*I); 758 if (Attributes Attrs = CallPAL.getParamAttributes(i + 1)) 759 AttributesVec.push_back(AttributeWithIndex::get(Args.size(), Attrs)); 760 } 761 762 if (FnAttrs != Attribute::None) 763 AttributesVec.push_back(AttributeWithIndex::get(~0, FnAttrs)); 764 765 // Reconstruct the AttributesList based on the vector we constructed. 766 AttrListPtr NewCallPAL = AttrListPtr::get(AttributesVec.begin(), 767 AttributesVec.end()); 768 769 Instruction *New; 770 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) { 771 New = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(), 772 Args.begin(), Args.end(), "", Call); 773 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv()); 774 cast<InvokeInst>(New)->setAttributes(NewCallPAL); 775 } else { 776 New = CallInst::Create(NF, Args.begin(), Args.end(), "", Call); 777 cast<CallInst>(New)->setCallingConv(CS.getCallingConv()); 778 cast<CallInst>(New)->setAttributes(NewCallPAL); 779 if (cast<CallInst>(Call)->isTailCall()) 780 cast<CallInst>(New)->setTailCall(); 781 } 782 Args.clear(); 783 784 if (!Call->use_empty()) { 785 if (New->getType() == Call->getType()) { 786 // Return type not changed? Just replace users then. 787 Call->replaceAllUsesWith(New); 788 New->takeName(Call); 789 } else if (New->getType() == Type::VoidTy) { 790 // Our return value has uses, but they will get removed later on. 791 // Replace by null for now. 792 Call->replaceAllUsesWith(Constant::getNullValue(Call->getType())); 793 } else { 794 assert(isa<StructType>(RetTy) && 795 "Return type changed, but not into a void. The old return type" 796 " must have been a struct!"); 797 Instruction *InsertPt = Call; 798 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) { 799 BasicBlock::iterator IP = II->getNormalDest()->begin(); 800 while (isa<PHINode>(IP)) ++IP; 801 InsertPt = IP; 802 } 803 804 // We used to return a struct. Instead of doing smart stuff with all the 805 // uses of this struct, we will just rebuild it using 806 // extract/insertvalue chaining and let instcombine clean that up. 807 // 808 // Start out building up our return value from undef 809 Value *RetVal = llvm::UndefValue::get(RetTy); 810 for (unsigned i = 0; i != RetCount; ++i) 811 if (NewRetIdxs[i] != -1) { 812 Value *V; 813 if (RetTypes.size() > 1) 814 // We are still returning a struct, so extract the value from our 815 // return value 816 V = ExtractValueInst::Create(New, NewRetIdxs[i], "newret", 817 InsertPt); 818 else 819 // We are now returning a single element, so just insert that 820 V = New; 821 // Insert the value at the old position 822 RetVal = InsertValueInst::Create(RetVal, V, i, "oldret", InsertPt); 823 } 824 // Now, replace all uses of the old call instruction with the return 825 // struct we built 826 Call->replaceAllUsesWith(RetVal); 827 New->takeName(Call); 828 } 829 } 830 831 // Finally, remove the old call from the program, reducing the use-count of 832 // F. 833 Call->eraseFromParent(); 834 } 835 836 // Since we have now created the new function, splice the body of the old 837 // function right into the new function, leaving the old rotting hulk of the 838 // function empty. 839 NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList()); 840 841 // Loop over the argument list, transfering uses of the old arguments over to 842 // the new arguments, also transfering over the names as well. 843 i = 0; 844 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(), 845 I2 = NF->arg_begin(); I != E; ++I, ++i) 846 if (ArgAlive[i]) { 847 // If this is a live argument, move the name and users over to the new 848 // version. 849 I->replaceAllUsesWith(I2); 850 I2->takeName(I); 851 ++I2; 852 } else { 853 // If this argument is dead, replace any uses of it with null constants 854 // (these are guaranteed to become unused later on). 855 I->replaceAllUsesWith(Constant::getNullValue(I->getType())); 856 } 857 858 // If we change the return value of the function we must rewrite any return 859 // instructions. Check this now. 860 if (F->getReturnType() != NF->getReturnType()) 861 for (Function::iterator BB = NF->begin(), E = NF->end(); BB != E; ++BB) 862 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) { 863 Value *RetVal; 864 865 if (NFTy->getReturnType() == Type::VoidTy) { 866 RetVal = 0; 867 } else { 868 assert (isa<StructType>(RetTy)); 869 // The original return value was a struct, insert 870 // extractvalue/insertvalue chains to extract only the values we need 871 // to return and insert them into our new result. 872 // This does generate messy code, but we'll let it to instcombine to 873 // clean that up. 874 Value *OldRet = RI->getOperand(0); 875 // Start out building up our return value from undef 876 RetVal = llvm::UndefValue::get(NRetTy); 877 for (unsigned i = 0; i != RetCount; ++i) 878 if (NewRetIdxs[i] != -1) { 879 ExtractValueInst *EV = ExtractValueInst::Create(OldRet, i, 880 "oldret", RI); 881 if (RetTypes.size() > 1) { 882 // We're still returning a struct, so reinsert the value into 883 // our new return value at the new index 884 885 RetVal = InsertValueInst::Create(RetVal, EV, NewRetIdxs[i], 886 "newret", RI); 887 } else { 888 // We are now only returning a simple value, so just return the 889 // extracted value. 890 RetVal = EV; 891 } 892 } 893 } 894 // Replace the return instruction with one returning the new return 895 // value (possibly 0 if we became void). 896 ReturnInst::Create(RetVal, RI); 897 BB->getInstList().erase(RI); 898 } 899 900 // Now that the old function is dead, delete it. 901 F->eraseFromParent(); 902 903 return true; 904} 905 906bool DAE::runOnModule(Module &M) { 907 bool Changed = false; 908 909 // First pass: Do a simple check to see if any functions can have their "..." 910 // removed. We can do this if they never call va_start. This loop cannot be 911 // fused with the next loop, because deleting a function invalidates 912 // information computed while surveying other functions. 913 DOUT << "DAE - Deleting dead varargs\n"; 914 for (Module::iterator I = M.begin(), E = M.end(); I != E; ) { 915 Function &F = *I++; 916 if (F.getFunctionType()->isVarArg()) 917 Changed |= DeleteDeadVarargs(F); 918 } 919 920 // Second phase:loop through the module, determining which arguments are live. 921 // We assume all arguments are dead unless proven otherwise (allowing us to 922 // determine that dead arguments passed into recursive functions are dead). 923 // 924 DOUT << "DAE - Determining liveness\n"; 925 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) 926 SurveyFunction(*I); 927 928 // Now, remove all dead arguments and return values from each function in 929 // turn 930 for (Module::iterator I = M.begin(), E = M.end(); I != E; ) { 931 // Increment now, because the function will probably get removed (ie 932 // replaced by a new one). 933 Function *F = I++; 934 Changed |= RemoveDeadStuffFromFunction(F); 935 } 936 return Changed; 937} 938