DeadArgumentElimination.cpp revision 207618
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/LLVMContext.h" 28#include "llvm/Module.h" 29#include "llvm/Pass.h" 30#include "llvm/Support/CallSite.h" 31#include "llvm/Support/Debug.h" 32#include "llvm/Support/raw_ostream.h" 33#include "llvm/ADT/SmallVector.h" 34#include "llvm/ADT/Statistic.h" 35#include "llvm/ADT/StringExtras.h" 36#include <map> 37#include <set> 38using namespace llvm; 39 40STATISTIC(NumArgumentsEliminated, "Number of unread args removed"); 41STATISTIC(NumRetValsEliminated , "Number of unused return values removed"); 42 43namespace { 44 /// DAE - The dead argument elimination pass. 45 /// 46 class DAE : public ModulePass { 47 public: 48 49 /// Struct that represents (part of) either a return value or a function 50 /// argument. Used so that arguments and return values can be used 51 /// interchangably. 52 struct RetOrArg { 53 RetOrArg(const Function *F, unsigned Idx, bool IsArg) : F(F), Idx(Idx), 54 IsArg(IsArg) {} 55 const Function *F; 56 unsigned Idx; 57 bool IsArg; 58 59 /// Make RetOrArg comparable, so we can put it into a map. 60 bool operator<(const RetOrArg &O) const { 61 if (F != O.F) 62 return F < O.F; 63 else if (Idx != O.Idx) 64 return Idx < O.Idx; 65 else 66 return IsArg < O.IsArg; 67 } 68 69 /// Make RetOrArg comparable, so we can easily iterate the multimap. 70 bool operator==(const RetOrArg &O) const { 71 return F == O.F && Idx == O.Idx && IsArg == O.IsArg; 72 } 73 74 std::string getDescription() const { 75 return std::string((IsArg ? "Argument #" : "Return value #")) 76 + utostr(Idx) + " of function " + F->getNameStr(); 77 } 78 }; 79 80 /// Liveness enum - During our initial pass over the program, we determine 81 /// that things are either alive or maybe alive. We don't mark anything 82 /// explicitly dead (even if we know they are), since anything not alive 83 /// with no registered uses (in Uses) will never be marked alive and will 84 /// thus become dead in the end. 85 enum Liveness { Live, MaybeLive }; 86 87 /// Convenience wrapper 88 RetOrArg CreateRet(const Function *F, unsigned Idx) { 89 return RetOrArg(F, Idx, false); 90 } 91 /// Convenience wrapper 92 RetOrArg CreateArg(const Function *F, unsigned Idx) { 93 return RetOrArg(F, Idx, true); 94 } 95 96 typedef std::multimap<RetOrArg, RetOrArg> UseMap; 97 /// This maps a return value or argument to any MaybeLive return values or 98 /// arguments it uses. This allows the MaybeLive values to be marked live 99 /// when any of its users is marked live. 100 /// For example (indices are left out for clarity): 101 /// - Uses[ret F] = ret G 102 /// This means that F calls G, and F returns the value returned by G. 103 /// - Uses[arg F] = ret G 104 /// This means that some function calls G and passes its result as an 105 /// argument to F. 106 /// - Uses[ret F] = arg F 107 /// This means that F returns one of its own arguments. 108 /// - Uses[arg F] = arg G 109 /// This means that G calls F and passes one of its own (G's) arguments 110 /// directly to F. 111 UseMap Uses; 112 113 typedef std::set<RetOrArg> LiveSet; 114 typedef std::set<const Function*> LiveFuncSet; 115 116 /// This set contains all values that have been determined to be live. 117 LiveSet LiveValues; 118 /// This set contains all values that are cannot be changed in any way. 119 LiveFuncSet LiveFunctions; 120 121 typedef SmallVector<RetOrArg, 5> UseVector; 122 123 public: 124 static char ID; // Pass identification, replacement for typeid 125 DAE() : ModulePass(&ID) {} 126 bool runOnModule(Module &M); 127 128 virtual bool ShouldHackArguments() const { return false; } 129 130 private: 131 Liveness MarkIfNotLive(RetOrArg Use, UseVector &MaybeLiveUses); 132 Liveness SurveyUse(Value::const_use_iterator U, UseVector &MaybeLiveUses, 133 unsigned RetValNum = 0); 134 Liveness SurveyUses(const Value *V, UseVector &MaybeLiveUses); 135 136 void SurveyFunction(const Function &F); 137 void MarkValue(const RetOrArg &RA, Liveness L, 138 const UseVector &MaybeLiveUses); 139 void MarkLive(const RetOrArg &RA); 140 void MarkLive(const Function &F); 141 void PropagateLiveness(const RetOrArg &RA); 142 bool RemoveDeadStuffFromFunction(Function *F); 143 bool DeleteDeadVarargs(Function &Fn); 144 }; 145} 146 147 148char DAE::ID = 0; 149static RegisterPass<DAE> 150X("deadargelim", "Dead Argument Elimination"); 151 152namespace { 153 /// DAH - DeadArgumentHacking pass - Same as dead argument elimination, but 154 /// deletes arguments to functions which are external. This is only for use 155 /// by bugpoint. 156 struct DAH : public DAE { 157 static char ID; 158 virtual bool ShouldHackArguments() const { return true; } 159 }; 160} 161 162char DAH::ID = 0; 163static RegisterPass<DAH> 164Y("deadarghaX0r", "Dead Argument Hacking (BUGPOINT USE ONLY; DO NOT USE)"); 165 166/// createDeadArgEliminationPass - This pass removes arguments from functions 167/// which are not used by the body of the function. 168/// 169ModulePass *llvm::createDeadArgEliminationPass() { return new DAE(); } 170ModulePass *llvm::createDeadArgHackingPass() { return new DAH(); } 171 172/// DeleteDeadVarargs - If this is an function that takes a ... list, and if 173/// llvm.vastart is never called, the varargs list is dead for the function. 174bool DAE::DeleteDeadVarargs(Function &Fn) { 175 assert(Fn.getFunctionType()->isVarArg() && "Function isn't varargs!"); 176 if (Fn.isDeclaration() || !Fn.hasLocalLinkage()) return false; 177 178 // Ensure that the function is only directly called. 179 if (Fn.hasAddressTaken()) 180 return false; 181 182 // Okay, we know we can transform this function if safe. Scan its body 183 // looking for calls to llvm.vastart. 184 for (Function::iterator BB = Fn.begin(), E = Fn.end(); BB != E; ++BB) { 185 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) { 186 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) { 187 if (II->getIntrinsicID() == Intrinsic::vastart) 188 return false; 189 } 190 } 191 } 192 193 // If we get here, there are no calls to llvm.vastart in the function body, 194 // remove the "..." and adjust all the calls. 195 196 // Start by computing a new prototype for the function, which is the same as 197 // the old function, but doesn't have isVarArg set. 198 const FunctionType *FTy = Fn.getFunctionType(); 199 200 std::vector<const Type*> Params(FTy->param_begin(), FTy->param_end()); 201 FunctionType *NFTy = FunctionType::get(FTy->getReturnType(), 202 Params, false); 203 unsigned NumArgs = Params.size(); 204 205 // Create the new function body and insert it into the module... 206 Function *NF = Function::Create(NFTy, Fn.getLinkage()); 207 NF->copyAttributesFrom(&Fn); 208 Fn.getParent()->getFunctionList().insert(&Fn, NF); 209 NF->takeName(&Fn); 210 211 // Loop over all of the callers of the function, transforming the call sites 212 // to pass in a smaller number of arguments into the new function. 213 // 214 std::vector<Value*> Args; 215 while (!Fn.use_empty()) { 216 CallSite CS = CallSite::get(Fn.use_back()); 217 Instruction *Call = CS.getInstruction(); 218 219 // Pass all the same arguments. 220 Args.assign(CS.arg_begin(), CS.arg_begin()+NumArgs); 221 222 // Drop any attributes that were on the vararg arguments. 223 AttrListPtr PAL = CS.getAttributes(); 224 if (!PAL.isEmpty() && PAL.getSlot(PAL.getNumSlots() - 1).Index > NumArgs) { 225 SmallVector<AttributeWithIndex, 8> AttributesVec; 226 for (unsigned i = 0; PAL.getSlot(i).Index <= NumArgs; ++i) 227 AttributesVec.push_back(PAL.getSlot(i)); 228 if (Attributes FnAttrs = PAL.getFnAttributes()) 229 AttributesVec.push_back(AttributeWithIndex::get(~0, FnAttrs)); 230 PAL = AttrListPtr::get(AttributesVec.begin(), AttributesVec.end()); 231 } 232 233 Instruction *New; 234 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) { 235 New = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(), 236 Args.begin(), Args.end(), "", Call); 237 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv()); 238 cast<InvokeInst>(New)->setAttributes(PAL); 239 } else { 240 New = CallInst::Create(NF, Args.begin(), Args.end(), "", Call); 241 cast<CallInst>(New)->setCallingConv(CS.getCallingConv()); 242 cast<CallInst>(New)->setAttributes(PAL); 243 if (cast<CallInst>(Call)->isTailCall()) 244 cast<CallInst>(New)->setTailCall(); 245 } 246 if (MDNode *N = Call->getDbgMetadata()) 247 New->setDbgMetadata(N); 248 249 Args.clear(); 250 251 if (!Call->use_empty()) 252 Call->replaceAllUsesWith(New); 253 254 New->takeName(Call); 255 256 // Finally, remove the old call from the program, reducing the use-count of 257 // F. 258 Call->eraseFromParent(); 259 } 260 261 // Since we have now created the new function, splice the body of the old 262 // function right into the new function, leaving the old rotting hulk of the 263 // function empty. 264 NF->getBasicBlockList().splice(NF->begin(), Fn.getBasicBlockList()); 265 266 // Loop over the argument list, transfering uses of the old arguments over to 267 // the new arguments, also transfering over the names as well. While we're at 268 // it, remove the dead arguments from the DeadArguments list. 269 // 270 for (Function::arg_iterator I = Fn.arg_begin(), E = Fn.arg_end(), 271 I2 = NF->arg_begin(); I != E; ++I, ++I2) { 272 // Move the name and users over to the new version. 273 I->replaceAllUsesWith(I2); 274 I2->takeName(I); 275 } 276 277 // Finally, nuke the old function. 278 Fn.eraseFromParent(); 279 return true; 280} 281 282/// Convenience function that returns the number of return values. It returns 0 283/// for void functions and 1 for functions not returning a struct. It returns 284/// the number of struct elements for functions returning a struct. 285static unsigned NumRetVals(const Function *F) { 286 if (F->getReturnType()->isVoidTy()) 287 return 0; 288 else if (const StructType *STy = dyn_cast<StructType>(F->getReturnType())) 289 return STy->getNumElements(); 290 else 291 return 1; 292} 293 294/// MarkIfNotLive - This checks Use for liveness in LiveValues. If Use is not 295/// live, it adds Use to the MaybeLiveUses argument. Returns the determined 296/// liveness of Use. 297DAE::Liveness DAE::MarkIfNotLive(RetOrArg Use, UseVector &MaybeLiveUses) { 298 // We're live if our use or its Function is already marked as live. 299 if (LiveFunctions.count(Use.F) || LiveValues.count(Use)) 300 return Live; 301 302 // We're maybe live otherwise, but remember that we must become live if 303 // Use becomes live. 304 MaybeLiveUses.push_back(Use); 305 return MaybeLive; 306} 307 308 309/// SurveyUse - This looks at a single use of an argument or return value 310/// and determines if it should be alive or not. Adds this use to MaybeLiveUses 311/// if it causes the used value to become MaybeLive. 312/// 313/// RetValNum is the return value number to use when this use is used in a 314/// return instruction. This is used in the recursion, you should always leave 315/// it at 0. 316DAE::Liveness DAE::SurveyUse(Value::const_use_iterator U, 317 UseVector &MaybeLiveUses, unsigned RetValNum) { 318 const User *V = *U; 319 if (const ReturnInst *RI = dyn_cast<ReturnInst>(V)) { 320 // The value is returned from a function. It's only live when the 321 // function's return value is live. We use RetValNum here, for the case 322 // that U is really a use of an insertvalue instruction that uses the 323 // orginal Use. 324 RetOrArg Use = CreateRet(RI->getParent()->getParent(), RetValNum); 325 // We might be live, depending on the liveness of Use. 326 return MarkIfNotLive(Use, MaybeLiveUses); 327 } 328 if (const InsertValueInst *IV = dyn_cast<InsertValueInst>(V)) { 329 if (U.getOperandNo() != InsertValueInst::getAggregateOperandIndex() 330 && IV->hasIndices()) 331 // The use we are examining is inserted into an aggregate. Our liveness 332 // depends on all uses of that aggregate, but if it is used as a return 333 // value, only index at which we were inserted counts. 334 RetValNum = *IV->idx_begin(); 335 336 // Note that if we are used as the aggregate operand to the insertvalue, 337 // we don't change RetValNum, but do survey all our uses. 338 339 Liveness Result = MaybeLive; 340 for (Value::const_use_iterator I = IV->use_begin(), 341 E = V->use_end(); I != E; ++I) { 342 Result = SurveyUse(I, MaybeLiveUses, RetValNum); 343 if (Result == Live) 344 break; 345 } 346 return Result; 347 } 348 349 if (ImmutableCallSite CS = V) { 350 const Function *F = CS.getCalledFunction(); 351 if (F) { 352 // Used in a direct call. 353 354 // Find the argument number. We know for sure that this use is an 355 // argument, since if it was the function argument this would be an 356 // indirect call and the we know can't be looking at a value of the 357 // label type (for the invoke instruction). 358 unsigned ArgNo = CS.getArgumentNo(U); 359 360 if (ArgNo >= F->getFunctionType()->getNumParams()) 361 // The value is passed in through a vararg! Must be live. 362 return Live; 363 364 assert(CS.getArgument(ArgNo) 365 == CS->getOperand(U.getOperandNo()) 366 && "Argument is not where we expected it"); 367 368 // Value passed to a normal call. It's only live when the corresponding 369 // argument to the called function turns out live. 370 RetOrArg Use = CreateArg(F, ArgNo); 371 return MarkIfNotLive(Use, MaybeLiveUses); 372 } 373 } 374 // Used in any other way? Value must be live. 375 return Live; 376} 377 378/// SurveyUses - This looks at all the uses of the given value 379/// Returns the Liveness deduced from the uses of this value. 380/// 381/// Adds all uses that cause the result to be MaybeLive to MaybeLiveRetUses. If 382/// the result is Live, MaybeLiveUses might be modified but its content should 383/// be ignored (since it might not be complete). 384DAE::Liveness DAE::SurveyUses(const Value *V, UseVector &MaybeLiveUses) { 385 // Assume it's dead (which will only hold if there are no uses at all..). 386 Liveness Result = MaybeLive; 387 // Check each use. 388 for (Value::const_use_iterator I = V->use_begin(), 389 E = V->use_end(); I != E; ++I) { 390 Result = SurveyUse(I, MaybeLiveUses); 391 if (Result == Live) 392 break; 393 } 394 return Result; 395} 396 397// SurveyFunction - This performs the initial survey of the specified function, 398// checking out whether or not it uses any of its incoming arguments or whether 399// any callers use the return value. This fills in the LiveValues set and Uses 400// map. 401// 402// We consider arguments of non-internal functions to be intrinsically alive as 403// well as arguments to functions which have their "address taken". 404// 405void DAE::SurveyFunction(const Function &F) { 406 unsigned RetCount = NumRetVals(&F); 407 // Assume all return values are dead 408 typedef SmallVector<Liveness, 5> RetVals; 409 RetVals RetValLiveness(RetCount, MaybeLive); 410 411 typedef SmallVector<UseVector, 5> RetUses; 412 // These vectors map each return value to the uses that make it MaybeLive, so 413 // we can add those to the Uses map if the return value really turns out to be 414 // MaybeLive. Initialized to a list of RetCount empty lists. 415 RetUses MaybeLiveRetUses(RetCount); 416 417 for (Function::const_iterator BB = F.begin(), E = F.end(); BB != E; ++BB) 418 if (const ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) 419 if (RI->getNumOperands() != 0 && RI->getOperand(0)->getType() 420 != F.getFunctionType()->getReturnType()) { 421 // We don't support old style multiple return values. 422 MarkLive(F); 423 return; 424 } 425 426 if (!F.hasLocalLinkage() && (!ShouldHackArguments() || F.isIntrinsic())) { 427 MarkLive(F); 428 return; 429 } 430 431 DEBUG(dbgs() << "DAE - Inspecting callers for fn: " << F.getName() << "\n"); 432 // Keep track of the number of live retvals, so we can skip checks once all 433 // of them turn out to be live. 434 unsigned NumLiveRetVals = 0; 435 const Type *STy = dyn_cast<StructType>(F.getReturnType()); 436 // Loop all uses of the function. 437 for (Value::const_use_iterator I = F.use_begin(), E = F.use_end(); 438 I != E; ++I) { 439 // If the function is PASSED IN as an argument, its address has been 440 // taken. 441 ImmutableCallSite CS(*I); 442 if (!CS || !CS.isCallee(I)) { 443 MarkLive(F); 444 return; 445 } 446 447 // If this use is anything other than a call site, the function is alive. 448 const Instruction *TheCall = CS.getInstruction(); 449 if (!TheCall) { // Not a direct call site? 450 MarkLive(F); 451 return; 452 } 453 454 // If we end up here, we are looking at a direct call to our function. 455 456 // Now, check how our return value(s) is/are used in this caller. Don't 457 // bother checking return values if all of them are live already. 458 if (NumLiveRetVals != RetCount) { 459 if (STy) { 460 // Check all uses of the return value. 461 for (Value::const_use_iterator I = TheCall->use_begin(), 462 E = TheCall->use_end(); I != E; ++I) { 463 const ExtractValueInst *Ext = dyn_cast<ExtractValueInst>(*I); 464 if (Ext && Ext->hasIndices()) { 465 // This use uses a part of our return value, survey the uses of 466 // that part and store the results for this index only. 467 unsigned Idx = *Ext->idx_begin(); 468 if (RetValLiveness[Idx] != Live) { 469 RetValLiveness[Idx] = SurveyUses(Ext, MaybeLiveRetUses[Idx]); 470 if (RetValLiveness[Idx] == Live) 471 NumLiveRetVals++; 472 } 473 } else { 474 // Used by something else than extractvalue. Mark all return 475 // values as live. 476 for (unsigned i = 0; i != RetCount; ++i ) 477 RetValLiveness[i] = Live; 478 NumLiveRetVals = RetCount; 479 break; 480 } 481 } 482 } else { 483 // Single return value 484 RetValLiveness[0] = SurveyUses(TheCall, MaybeLiveRetUses[0]); 485 if (RetValLiveness[0] == Live) 486 NumLiveRetVals = RetCount; 487 } 488 } 489 } 490 491 // Now we've inspected all callers, record the liveness of our return values. 492 for (unsigned i = 0; i != RetCount; ++i) 493 MarkValue(CreateRet(&F, i), RetValLiveness[i], MaybeLiveRetUses[i]); 494 495 DEBUG(dbgs() << "DAE - Inspecting args for fn: " << F.getName() << "\n"); 496 497 // Now, check all of our arguments. 498 unsigned i = 0; 499 UseVector MaybeLiveArgUses; 500 for (Function::const_arg_iterator AI = F.arg_begin(), 501 E = F.arg_end(); AI != E; ++AI, ++i) { 502 // See what the effect of this use is (recording any uses that cause 503 // MaybeLive in MaybeLiveArgUses). 504 Liveness Result = SurveyUses(AI, MaybeLiveArgUses); 505 // Mark the result. 506 MarkValue(CreateArg(&F, i), Result, MaybeLiveArgUses); 507 // Clear the vector again for the next iteration. 508 MaybeLiveArgUses.clear(); 509 } 510} 511 512/// MarkValue - This function marks the liveness of RA depending on L. If L is 513/// MaybeLive, it also takes all uses in MaybeLiveUses and records them in Uses, 514/// such that RA will be marked live if any use in MaybeLiveUses gets marked 515/// live later on. 516void DAE::MarkValue(const RetOrArg &RA, Liveness L, 517 const UseVector &MaybeLiveUses) { 518 switch (L) { 519 case Live: MarkLive(RA); break; 520 case MaybeLive: 521 { 522 // Note any uses of this value, so this return value can be 523 // marked live whenever one of the uses becomes live. 524 for (UseVector::const_iterator UI = MaybeLiveUses.begin(), 525 UE = MaybeLiveUses.end(); UI != UE; ++UI) 526 Uses.insert(std::make_pair(*UI, RA)); 527 break; 528 } 529 } 530} 531 532/// MarkLive - Mark the given Function as alive, meaning that it cannot be 533/// changed in any way. Additionally, 534/// mark any values that are used as this function's parameters or by its return 535/// values (according to Uses) live as well. 536void DAE::MarkLive(const Function &F) { 537 DEBUG(dbgs() << "DAE - Intrinsically live fn: " << F.getName() << "\n"); 538 // Mark the function as live. 539 LiveFunctions.insert(&F); 540 // Mark all arguments as live. 541 for (unsigned i = 0, e = F.arg_size(); i != e; ++i) 542 PropagateLiveness(CreateArg(&F, i)); 543 // Mark all return values as live. 544 for (unsigned i = 0, e = NumRetVals(&F); i != e; ++i) 545 PropagateLiveness(CreateRet(&F, i)); 546} 547 548/// MarkLive - Mark the given return value or argument as live. Additionally, 549/// mark any values that are used by this value (according to Uses) live as 550/// well. 551void DAE::MarkLive(const RetOrArg &RA) { 552 if (LiveFunctions.count(RA.F)) 553 return; // Function was already marked Live. 554 555 if (!LiveValues.insert(RA).second) 556 return; // We were already marked Live. 557 558 DEBUG(dbgs() << "DAE - Marking " << RA.getDescription() << " live\n"); 559 PropagateLiveness(RA); 560} 561 562/// PropagateLiveness - Given that RA is a live value, propagate it's liveness 563/// to any other values it uses (according to Uses). 564void DAE::PropagateLiveness(const RetOrArg &RA) { 565 // We don't use upper_bound (or equal_range) here, because our recursive call 566 // to ourselves is likely to cause the upper_bound (which is the first value 567 // not belonging to RA) to become erased and the iterator invalidated. 568 UseMap::iterator Begin = Uses.lower_bound(RA); 569 UseMap::iterator E = Uses.end(); 570 UseMap::iterator I; 571 for (I = Begin; I != E && I->first == RA; ++I) 572 MarkLive(I->second); 573 574 // Erase RA from the Uses map (from the lower bound to wherever we ended up 575 // after the loop). 576 Uses.erase(Begin, I); 577} 578 579// RemoveDeadStuffFromFunction - Remove any arguments and return values from F 580// that are not in LiveValues. Transform the function and all of the callees of 581// the function to not have these arguments and return values. 582// 583bool DAE::RemoveDeadStuffFromFunction(Function *F) { 584 // Don't modify fully live functions 585 if (LiveFunctions.count(F)) 586 return false; 587 588 // Start by computing a new prototype for the function, which is the same as 589 // the old function, but has fewer arguments and a different return type. 590 const FunctionType *FTy = F->getFunctionType(); 591 std::vector<const Type*> Params; 592 593 // Set up to build a new list of parameter attributes. 594 SmallVector<AttributeWithIndex, 8> AttributesVec; 595 const AttrListPtr &PAL = F->getAttributes(); 596 597 // The existing function return attributes. 598 Attributes RAttrs = PAL.getRetAttributes(); 599 Attributes FnAttrs = PAL.getFnAttributes(); 600 601 // Find out the new return value. 602 603 const Type *RetTy = FTy->getReturnType(); 604 const Type *NRetTy = NULL; 605 unsigned RetCount = NumRetVals(F); 606 607 // -1 means unused, other numbers are the new index 608 SmallVector<int, 5> NewRetIdxs(RetCount, -1); 609 std::vector<const Type*> RetTypes; 610 if (RetTy->isVoidTy()) { 611 NRetTy = RetTy; 612 } else { 613 const StructType *STy = dyn_cast<StructType>(RetTy); 614 if (STy) 615 // Look at each of the original return values individually. 616 for (unsigned i = 0; i != RetCount; ++i) { 617 RetOrArg Ret = CreateRet(F, i); 618 if (LiveValues.erase(Ret)) { 619 RetTypes.push_back(STy->getElementType(i)); 620 NewRetIdxs[i] = RetTypes.size() - 1; 621 } else { 622 ++NumRetValsEliminated; 623 DEBUG(dbgs() << "DAE - Removing return value " << i << " from " 624 << F->getName() << "\n"); 625 } 626 } 627 else 628 // We used to return a single value. 629 if (LiveValues.erase(CreateRet(F, 0))) { 630 RetTypes.push_back(RetTy); 631 NewRetIdxs[0] = 0; 632 } else { 633 DEBUG(dbgs() << "DAE - Removing return value from " << F->getName() 634 << "\n"); 635 ++NumRetValsEliminated; 636 } 637 if (RetTypes.size() > 1) 638 // More than one return type? Return a struct with them. Also, if we used 639 // to return a struct and didn't change the number of return values, 640 // return a struct again. This prevents changing {something} into 641 // something and {} into void. 642 // Make the new struct packed if we used to return a packed struct 643 // already. 644 NRetTy = StructType::get(STy->getContext(), RetTypes, STy->isPacked()); 645 else if (RetTypes.size() == 1) 646 // One return type? Just a simple value then, but only if we didn't use to 647 // return a struct with that simple value before. 648 NRetTy = RetTypes.front(); 649 else if (RetTypes.size() == 0) 650 // No return types? Make it void, but only if we didn't use to return {}. 651 NRetTy = Type::getVoidTy(F->getContext()); 652 } 653 654 assert(NRetTy && "No new return type found?"); 655 656 // Remove any incompatible attributes, but only if we removed all return 657 // values. Otherwise, ensure that we don't have any conflicting attributes 658 // here. Currently, this should not be possible, but special handling might be 659 // required when new return value attributes are added. 660 if (NRetTy->isVoidTy()) 661 RAttrs &= ~Attribute::typeIncompatible(NRetTy); 662 else 663 assert((RAttrs & Attribute::typeIncompatible(NRetTy)) == 0 664 && "Return attributes no longer compatible?"); 665 666 if (RAttrs) 667 AttributesVec.push_back(AttributeWithIndex::get(0, RAttrs)); 668 669 // Remember which arguments are still alive. 670 SmallVector<bool, 10> ArgAlive(FTy->getNumParams(), false); 671 // Construct the new parameter list from non-dead arguments. Also construct 672 // a new set of parameter attributes to correspond. Skip the first parameter 673 // attribute, since that belongs to the return value. 674 unsigned i = 0; 675 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); 676 I != E; ++I, ++i) { 677 RetOrArg Arg = CreateArg(F, i); 678 if (LiveValues.erase(Arg)) { 679 Params.push_back(I->getType()); 680 ArgAlive[i] = true; 681 682 // Get the original parameter attributes (skipping the first one, that is 683 // for the return value. 684 if (Attributes Attrs = PAL.getParamAttributes(i + 1)) 685 AttributesVec.push_back(AttributeWithIndex::get(Params.size(), Attrs)); 686 } else { 687 ++NumArgumentsEliminated; 688 DEBUG(dbgs() << "DAE - Removing argument " << i << " (" << I->getName() 689 << ") from " << F->getName() << "\n"); 690 } 691 } 692 693 if (FnAttrs != Attribute::None) 694 AttributesVec.push_back(AttributeWithIndex::get(~0, FnAttrs)); 695 696 // Reconstruct the AttributesList based on the vector we constructed. 697 AttrListPtr NewPAL = AttrListPtr::get(AttributesVec.begin(), 698 AttributesVec.end()); 699 700 // Create the new function type based on the recomputed parameters. 701 FunctionType *NFTy = FunctionType::get(NRetTy, Params, FTy->isVarArg()); 702 703 // No change? 704 if (NFTy == FTy) 705 return false; 706 707 // Create the new function body and insert it into the module... 708 Function *NF = Function::Create(NFTy, F->getLinkage()); 709 NF->copyAttributesFrom(F); 710 NF->setAttributes(NewPAL); 711 // Insert the new function before the old function, so we won't be processing 712 // it again. 713 F->getParent()->getFunctionList().insert(F, NF); 714 NF->takeName(F); 715 716 // Loop over all of the callers of the function, transforming the call sites 717 // to pass in a smaller number of arguments into the new function. 718 // 719 std::vector<Value*> Args; 720 while (!F->use_empty()) { 721 CallSite CS = CallSite::get(F->use_back()); 722 Instruction *Call = CS.getInstruction(); 723 724 AttributesVec.clear(); 725 const AttrListPtr &CallPAL = CS.getAttributes(); 726 727 // The call return attributes. 728 Attributes RAttrs = CallPAL.getRetAttributes(); 729 Attributes FnAttrs = CallPAL.getFnAttributes(); 730 // Adjust in case the function was changed to return void. 731 RAttrs &= ~Attribute::typeIncompatible(NF->getReturnType()); 732 if (RAttrs) 733 AttributesVec.push_back(AttributeWithIndex::get(0, RAttrs)); 734 735 // Declare these outside of the loops, so we can reuse them for the second 736 // loop, which loops the varargs. 737 CallSite::arg_iterator I = CS.arg_begin(); 738 unsigned i = 0; 739 // Loop over those operands, corresponding to the normal arguments to the 740 // original function, and add those that are still alive. 741 for (unsigned e = FTy->getNumParams(); i != e; ++I, ++i) 742 if (ArgAlive[i]) { 743 Args.push_back(*I); 744 // Get original parameter attributes, but skip return attributes. 745 if (Attributes Attrs = CallPAL.getParamAttributes(i + 1)) 746 AttributesVec.push_back(AttributeWithIndex::get(Args.size(), Attrs)); 747 } 748 749 // Push any varargs arguments on the list. Don't forget their attributes. 750 for (CallSite::arg_iterator E = CS.arg_end(); I != E; ++I, ++i) { 751 Args.push_back(*I); 752 if (Attributes Attrs = CallPAL.getParamAttributes(i + 1)) 753 AttributesVec.push_back(AttributeWithIndex::get(Args.size(), Attrs)); 754 } 755 756 if (FnAttrs != Attribute::None) 757 AttributesVec.push_back(AttributeWithIndex::get(~0, FnAttrs)); 758 759 // Reconstruct the AttributesList based on the vector we constructed. 760 AttrListPtr NewCallPAL = AttrListPtr::get(AttributesVec.begin(), 761 AttributesVec.end()); 762 763 Instruction *New; 764 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) { 765 New = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(), 766 Args.begin(), Args.end(), "", Call); 767 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv()); 768 cast<InvokeInst>(New)->setAttributes(NewCallPAL); 769 } else { 770 New = CallInst::Create(NF, Args.begin(), Args.end(), "", Call); 771 cast<CallInst>(New)->setCallingConv(CS.getCallingConv()); 772 cast<CallInst>(New)->setAttributes(NewCallPAL); 773 if (cast<CallInst>(Call)->isTailCall()) 774 cast<CallInst>(New)->setTailCall(); 775 } 776 if (MDNode *N = Call->getDbgMetadata()) 777 New->setDbgMetadata(N); 778 779 Args.clear(); 780 781 if (!Call->use_empty()) { 782 if (New->getType() == Call->getType()) { 783 // Return type not changed? Just replace users then. 784 Call->replaceAllUsesWith(New); 785 New->takeName(Call); 786 } else if (New->getType()->isVoidTy()) { 787 // Our return value has uses, but they will get removed later on. 788 // Replace by null for now. 789 Call->replaceAllUsesWith(Constant::getNullValue(Call->getType())); 790 } else { 791 assert(RetTy->isStructTy() && 792 "Return type changed, but not into a void. The old return type" 793 " must have been a struct!"); 794 Instruction *InsertPt = Call; 795 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) { 796 BasicBlock::iterator IP = II->getNormalDest()->begin(); 797 while (isa<PHINode>(IP)) ++IP; 798 InsertPt = IP; 799 } 800 801 // We used to return a struct. Instead of doing smart stuff with all the 802 // uses of this struct, we will just rebuild it using 803 // extract/insertvalue chaining and let instcombine clean that up. 804 // 805 // Start out building up our return value from undef 806 Value *RetVal = UndefValue::get(RetTy); 807 for (unsigned i = 0; i != RetCount; ++i) 808 if (NewRetIdxs[i] != -1) { 809 Value *V; 810 if (RetTypes.size() > 1) 811 // We are still returning a struct, so extract the value from our 812 // return value 813 V = ExtractValueInst::Create(New, NewRetIdxs[i], "newret", 814 InsertPt); 815 else 816 // We are now returning a single element, so just insert that 817 V = New; 818 // Insert the value at the old position 819 RetVal = InsertValueInst::Create(RetVal, V, i, "oldret", InsertPt); 820 } 821 // Now, replace all uses of the old call instruction with the return 822 // struct we built 823 Call->replaceAllUsesWith(RetVal); 824 New->takeName(Call); 825 } 826 } 827 828 // Finally, remove the old call from the program, reducing the use-count of 829 // F. 830 Call->eraseFromParent(); 831 } 832 833 // Since we have now created the new function, splice the body of the old 834 // function right into the new function, leaving the old rotting hulk of the 835 // function empty. 836 NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList()); 837 838 // Loop over the argument list, transfering uses of the old arguments over to 839 // the new arguments, also transfering over the names as well. 840 i = 0; 841 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(), 842 I2 = NF->arg_begin(); I != E; ++I, ++i) 843 if (ArgAlive[i]) { 844 // If this is a live argument, move the name and users over to the new 845 // version. 846 I->replaceAllUsesWith(I2); 847 I2->takeName(I); 848 ++I2; 849 } else { 850 // If this argument is dead, replace any uses of it with null constants 851 // (these are guaranteed to become unused later on). 852 I->replaceAllUsesWith(Constant::getNullValue(I->getType())); 853 } 854 855 // If we change the return value of the function we must rewrite any return 856 // instructions. Check this now. 857 if (F->getReturnType() != NF->getReturnType()) 858 for (Function::iterator BB = NF->begin(), E = NF->end(); BB != E; ++BB) 859 if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator())) { 860 Value *RetVal; 861 862 if (NFTy->getReturnType() == Type::getVoidTy(F->getContext())) { 863 RetVal = 0; 864 } else { 865 assert (RetTy->isStructTy()); 866 // The original return value was a struct, insert 867 // extractvalue/insertvalue chains to extract only the values we need 868 // to return and insert them into our new result. 869 // This does generate messy code, but we'll let it to instcombine to 870 // clean that up. 871 Value *OldRet = RI->getOperand(0); 872 // Start out building up our return value from undef 873 RetVal = UndefValue::get(NRetTy); 874 for (unsigned i = 0; i != RetCount; ++i) 875 if (NewRetIdxs[i] != -1) { 876 ExtractValueInst *EV = ExtractValueInst::Create(OldRet, i, 877 "oldret", RI); 878 if (RetTypes.size() > 1) { 879 // We're still returning a struct, so reinsert the value into 880 // our new return value at the new index 881 882 RetVal = InsertValueInst::Create(RetVal, EV, NewRetIdxs[i], 883 "newret", RI); 884 } else { 885 // We are now only returning a simple value, so just return the 886 // extracted value. 887 RetVal = EV; 888 } 889 } 890 } 891 // Replace the return instruction with one returning the new return 892 // value (possibly 0 if we became void). 893 ReturnInst::Create(F->getContext(), RetVal, RI); 894 BB->getInstList().erase(RI); 895 } 896 897 // Now that the old function is dead, delete it. 898 F->eraseFromParent(); 899 900 return true; 901} 902 903bool DAE::runOnModule(Module &M) { 904 bool Changed = false; 905 906 // First pass: Do a simple check to see if any functions can have their "..." 907 // removed. We can do this if they never call va_start. This loop cannot be 908 // fused with the next loop, because deleting a function invalidates 909 // information computed while surveying other functions. 910 DEBUG(dbgs() << "DAE - Deleting dead varargs\n"); 911 for (Module::iterator I = M.begin(), E = M.end(); I != E; ) { 912 Function &F = *I++; 913 if (F.getFunctionType()->isVarArg()) 914 Changed |= DeleteDeadVarargs(F); 915 } 916 917 // Second phase:loop through the module, determining which arguments are live. 918 // We assume all arguments are dead unless proven otherwise (allowing us to 919 // determine that dead arguments passed into recursive functions are dead). 920 // 921 DEBUG(dbgs() << "DAE - Determining liveness\n"); 922 for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) 923 SurveyFunction(*I); 924 925 // Now, remove all dead arguments and return values from each function in 926 // turn. 927 for (Module::iterator I = M.begin(), E = M.end(); I != E; ) { 928 // Increment now, because the function will probably get removed (ie. 929 // replaced by a new one). 930 Function *F = I++; 931 Changed |= RemoveDeadStuffFromFunction(F); 932 } 933 return Changed; 934} 935