ArgumentPromotion.cpp revision ecd94c804a563f2a86572dcf1d2e81f397e19daa
1//===-- ArgumentPromotion.cpp - Promote by-reference arguments ------------===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file was developed by the LLVM research group and is distributed under 6// the University of Illinois Open Source License. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9// 10// This pass promotes "by reference" arguments to be "by value" arguments. In 11// practice, this means looking for internal functions that have pointer 12// arguments. If we can prove, through the use of alias analysis, that an 13// argument is *only* loaded, then we can pass the value into the function 14// instead of the address of the value. This can cause recursive simplification 15// of code and lead to the elimination of allocas (especially in C++ template 16// code like the STL). 17// 18// This pass also handles aggregate arguments that are passed into a function, 19// scalarizing them if the elements of the aggregate are only loaded. Note that 20// we refuse to scalarize aggregates which would require passing in more than 21// three operands to the function, because we don't want to pass thousands of 22// operands for a large array or structure! 23// 24// Note that this transformation could also be done for arguments that are only 25// stored to (returning the value instead), but we do not currently handle that 26// case. This case would be best handled when and if we start supporting 27// multiple return values from functions. 28// 29//===----------------------------------------------------------------------===// 30 31#define DEBUG_TYPE "argpromotion" 32#include "llvm/Transforms/IPO.h" 33#include "llvm/Constants.h" 34#include "llvm/DerivedTypes.h" 35#include "llvm/Module.h" 36#include "llvm/CallGraphSCCPass.h" 37#include "llvm/Instructions.h" 38#include "llvm/Analysis/AliasAnalysis.h" 39#include "llvm/Analysis/CallGraph.h" 40#include "llvm/Target/TargetData.h" 41#include "llvm/Support/CallSite.h" 42#include "llvm/Support/CFG.h" 43#include "llvm/Support/Debug.h" 44#include "llvm/ADT/DepthFirstIterator.h" 45#include "llvm/ADT/Statistic.h" 46#include "llvm/ADT/StringExtras.h" 47#include "llvm/Support/Compiler.h" 48#include <set> 49using namespace llvm; 50 51STATISTIC(NumArgumentsPromoted , "Number of pointer arguments promoted"); 52STATISTIC(NumAggregatesPromoted, "Number of aggregate arguments promoted"); 53STATISTIC(NumArgumentsDead , "Number of dead pointer args eliminated"); 54 55namespace { 56 /// ArgPromotion - The 'by reference' to 'by value' argument promotion pass. 57 /// 58 struct VISIBILITY_HIDDEN ArgPromotion : public CallGraphSCCPass { 59 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 60 AU.addRequired<AliasAnalysis>(); 61 AU.addRequired<TargetData>(); 62 CallGraphSCCPass::getAnalysisUsage(AU); 63 } 64 65 virtual bool runOnSCC(const std::vector<CallGraphNode *> &SCC); 66 static char ID; // Pass identification, replacement for typeid 67 ArgPromotion() : CallGraphSCCPass((intptr_t)&ID) {} 68 69 private: 70 bool PromoteArguments(CallGraphNode *CGN); 71 bool isSafeToPromoteArgument(Argument *Arg) const; 72 Function *DoPromotion(Function *F, std::vector<Argument*> &ArgsToPromote); 73 }; 74 75 char ArgPromotion::ID = 0; 76 RegisterPass<ArgPromotion> X("argpromotion", 77 "Promote 'by reference' arguments to scalars"); 78} 79 80Pass *llvm::createArgumentPromotionPass() { 81 return new ArgPromotion(); 82} 83 84bool ArgPromotion::runOnSCC(const std::vector<CallGraphNode *> &SCC) { 85 bool Changed = false, LocalChange; 86 87 do { // Iterate until we stop promoting from this SCC. 88 LocalChange = false; 89 // Attempt to promote arguments from all functions in this SCC. 90 for (unsigned i = 0, e = SCC.size(); i != e; ++i) 91 LocalChange |= PromoteArguments(SCC[i]); 92 Changed |= LocalChange; // Remember that we changed something. 93 } while (LocalChange); 94 95 return Changed; 96} 97 98/// PromoteArguments - This method checks the specified function to see if there 99/// are any promotable arguments and if it is safe to promote the function (for 100/// example, all callers are direct). If safe to promote some arguments, it 101/// calls the DoPromotion method. 102/// 103bool ArgPromotion::PromoteArguments(CallGraphNode *CGN) { 104 Function *F = CGN->getFunction(); 105 106 // Make sure that it is local to this module. 107 if (!F || !F->hasInternalLinkage()) return false; 108 109 // First check: see if there are any pointer arguments! If not, quick exit. 110 std::vector<Argument*> PointerArgs; 111 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E; ++I) 112 if (isa<PointerType>(I->getType())) 113 PointerArgs.push_back(I); 114 if (PointerArgs.empty()) return false; 115 116 // Second check: make sure that all callers are direct callers. We can't 117 // transform functions that have indirect callers. 118 for (Value::use_iterator UI = F->use_begin(), E = F->use_end(); 119 UI != E; ++UI) { 120 CallSite CS = CallSite::get(*UI); 121 if (!CS.getInstruction()) // "Taking the address" of the function 122 return false; 123 124 // Ensure that this call site is CALLING the function, not passing it as 125 // an argument. 126 for (CallSite::arg_iterator AI = CS.arg_begin(), E = CS.arg_end(); 127 AI != E; ++AI) 128 if (*AI == F) return false; // Passing the function address in! 129 } 130 131 // Check to see which arguments are promotable. If an argument is not 132 // promotable, remove it from the PointerArgs vector. 133 for (unsigned i = 0; i != PointerArgs.size(); ++i) 134 if (!isSafeToPromoteArgument(PointerArgs[i])) { 135 std::swap(PointerArgs[i--], PointerArgs.back()); 136 PointerArgs.pop_back(); 137 } 138 139 // No promotable pointer arguments. 140 if (PointerArgs.empty()) return false; 141 142 // Okay, promote all of the arguments are rewrite the callees! 143 Function *NewF = DoPromotion(F, PointerArgs); 144 145 // Update the call graph to know that the old function is gone. 146 getAnalysis<CallGraph>().changeFunction(F, NewF); 147 return true; 148} 149 150/// IsAlwaysValidPointer - Return true if the specified pointer is always legal 151/// to load. 152static bool IsAlwaysValidPointer(Value *V) { 153 if (isa<AllocaInst>(V) || isa<GlobalVariable>(V)) return true; 154 if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(V)) 155 return IsAlwaysValidPointer(GEP->getOperand(0)); 156 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) 157 if (CE->getOpcode() == Instruction::GetElementPtr) 158 return IsAlwaysValidPointer(CE->getOperand(0)); 159 160 return false; 161} 162 163/// AllCalleesPassInValidPointerForArgument - Return true if we can prove that 164/// all callees pass in a valid pointer for the specified function argument. 165static bool AllCalleesPassInValidPointerForArgument(Argument *Arg) { 166 Function *Callee = Arg->getParent(); 167 168 unsigned ArgNo = std::distance(Callee->arg_begin(), 169 Function::arg_iterator(Arg)); 170 171 // Look at all call sites of the function. At this pointer we know we only 172 // have direct callees. 173 for (Value::use_iterator UI = Callee->use_begin(), E = Callee->use_end(); 174 UI != E; ++UI) { 175 CallSite CS = CallSite::get(*UI); 176 assert(CS.getInstruction() && "Should only have direct calls!"); 177 178 if (!IsAlwaysValidPointer(CS.getArgument(ArgNo))) 179 return false; 180 } 181 return true; 182} 183 184 185/// isSafeToPromoteArgument - As you might guess from the name of this method, 186/// it checks to see if it is both safe and useful to promote the argument. 187/// This method limits promotion of aggregates to only promote up to three 188/// elements of the aggregate in order to avoid exploding the number of 189/// arguments passed in. 190bool ArgPromotion::isSafeToPromoteArgument(Argument *Arg) const { 191 // We can only promote this argument if all of the uses are loads, or are GEP 192 // instructions (with constant indices) that are subsequently loaded. 193 bool HasLoadInEntryBlock = false; 194 BasicBlock *EntryBlock = Arg->getParent()->begin(); 195 std::vector<LoadInst*> Loads; 196 std::vector<std::vector<ConstantInt*> > GEPIndices; 197 for (Value::use_iterator UI = Arg->use_begin(), E = Arg->use_end(); 198 UI != E; ++UI) 199 if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) { 200 if (LI->isVolatile()) return false; // Don't hack volatile loads 201 Loads.push_back(LI); 202 HasLoadInEntryBlock |= LI->getParent() == EntryBlock; 203 } else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(*UI)) { 204 if (GEP->use_empty()) { 205 // Dead GEP's cause trouble later. Just remove them if we run into 206 // them. 207 getAnalysis<AliasAnalysis>().deleteValue(GEP); 208 GEP->getParent()->getInstList().erase(GEP); 209 return isSafeToPromoteArgument(Arg); 210 } 211 // Ensure that all of the indices are constants. 212 std::vector<ConstantInt*> Operands; 213 for (unsigned i = 1, e = GEP->getNumOperands(); i != e; ++i) 214 if (ConstantInt *C = dyn_cast<ConstantInt>(GEP->getOperand(i))) 215 Operands.push_back(C); 216 else 217 return false; // Not a constant operand GEP! 218 219 // Ensure that the only users of the GEP are load instructions. 220 for (Value::use_iterator UI = GEP->use_begin(), E = GEP->use_end(); 221 UI != E; ++UI) 222 if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) { 223 if (LI->isVolatile()) return false; // Don't hack volatile loads 224 Loads.push_back(LI); 225 HasLoadInEntryBlock |= LI->getParent() == EntryBlock; 226 } else { 227 return false; 228 } 229 230 // See if there is already a GEP with these indices. If not, check to 231 // make sure that we aren't promoting too many elements. If so, nothing 232 // to do. 233 if (std::find(GEPIndices.begin(), GEPIndices.end(), Operands) == 234 GEPIndices.end()) { 235 if (GEPIndices.size() == 3) { 236 DOUT << "argpromotion disable promoting argument '" 237 << Arg->getName() << "' because it would require adding more " 238 << "than 3 arguments to the function.\n"; 239 // We limit aggregate promotion to only promoting up to three elements 240 // of the aggregate. 241 return false; 242 } 243 GEPIndices.push_back(Operands); 244 } 245 } else { 246 return false; // Not a load or a GEP. 247 } 248 249 if (Loads.empty()) return true; // No users, this is a dead argument. 250 251 // If we decide that we want to promote this argument, the value is going to 252 // be unconditionally loaded in all callees. This is only safe to do if the 253 // pointer was going to be unconditionally loaded anyway (i.e. there is a load 254 // of the pointer in the entry block of the function) or if we can prove that 255 // all pointers passed in are always to legal locations (for example, no null 256 // pointers are passed in, no pointers to free'd memory, etc). 257 if (!HasLoadInEntryBlock && !AllCalleesPassInValidPointerForArgument(Arg)) 258 return false; // Cannot prove that this is safe!! 259 260 // Okay, now we know that the argument is only used by load instructions and 261 // it is safe to unconditionally load the pointer. Use alias analysis to 262 // check to see if the pointer is guaranteed to not be modified from entry of 263 // the function to each of the load instructions. 264 265 // Because there could be several/many load instructions, remember which 266 // blocks we know to be transparent to the load. 267 std::set<BasicBlock*> TranspBlocks; 268 269 AliasAnalysis &AA = getAnalysis<AliasAnalysis>(); 270 TargetData &TD = getAnalysis<TargetData>(); 271 272 for (unsigned i = 0, e = Loads.size(); i != e; ++i) { 273 // Check to see if the load is invalidated from the start of the block to 274 // the load itself. 275 LoadInst *Load = Loads[i]; 276 BasicBlock *BB = Load->getParent(); 277 278 const PointerType *LoadTy = 279 cast<PointerType>(Load->getOperand(0)->getType()); 280 unsigned LoadSize = (unsigned)TD.getTypeSize(LoadTy->getElementType()); 281 282 if (AA.canInstructionRangeModify(BB->front(), *Load, Arg, LoadSize)) 283 return false; // Pointer is invalidated! 284 285 // Now check every path from the entry block to the load for transparency. 286 // To do this, we perform a depth first search on the inverse CFG from the 287 // loading block. 288 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) 289 for (idf_ext_iterator<BasicBlock*> I = idf_ext_begin(*PI, TranspBlocks), 290 E = idf_ext_end(*PI, TranspBlocks); I != E; ++I) 291 if (AA.canBasicBlockModify(**I, Arg, LoadSize)) 292 return false; 293 } 294 295 // If the path from the entry of the function to each load is free of 296 // instructions that potentially invalidate the load, we can make the 297 // transformation! 298 return true; 299} 300 301namespace { 302 /// GEPIdxComparator - Provide a strong ordering for GEP indices. All Value* 303 /// elements are instances of ConstantInt. 304 /// 305 struct GEPIdxComparator { 306 bool operator()(const std::vector<Value*> &LHS, 307 const std::vector<Value*> &RHS) const { 308 unsigned idx = 0; 309 for (; idx < LHS.size() && idx < RHS.size(); ++idx) { 310 if (LHS[idx] != RHS[idx]) { 311 return cast<ConstantInt>(LHS[idx])->getZExtValue() < 312 cast<ConstantInt>(RHS[idx])->getZExtValue(); 313 } 314 } 315 316 // Return less than if we ran out of stuff in LHS and we didn't run out of 317 // stuff in RHS. 318 return idx == LHS.size() && idx != RHS.size(); 319 } 320 }; 321} 322 323 324/// DoPromotion - This method actually performs the promotion of the specified 325/// arguments, and returns the new function. At this point, we know that it's 326/// safe to do so. 327Function *ArgPromotion::DoPromotion(Function *F, 328 std::vector<Argument*> &Args2Prom) { 329 std::set<Argument*> ArgsToPromote(Args2Prom.begin(), Args2Prom.end()); 330 331 // Start by computing a new prototype for the function, which is the same as 332 // the old function, but has modified arguments. 333 const FunctionType *FTy = F->getFunctionType(); 334 std::vector<const Type*> Params; 335 336 typedef std::set<std::vector<Value*>, GEPIdxComparator> ScalarizeTable; 337 338 // ScalarizedElements - If we are promoting a pointer that has elements 339 // accessed out of it, keep track of which elements are accessed so that we 340 // can add one argument for each. 341 // 342 // Arguments that are directly loaded will have a zero element value here, to 343 // handle cases where there are both a direct load and GEP accesses. 344 // 345 std::map<Argument*, ScalarizeTable> ScalarizedElements; 346 347 // OriginalLoads - Keep track of a representative load instruction from the 348 // original function so that we can tell the alias analysis implementation 349 // what the new GEP/Load instructions we are inserting look like. 350 std::map<std::vector<Value*>, LoadInst*> OriginalLoads; 351 352 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E; ++I) 353 if (!ArgsToPromote.count(I)) { 354 Params.push_back(I->getType()); 355 } else if (I->use_empty()) { 356 ++NumArgumentsDead; 357 } else { 358 // Okay, this is being promoted. Check to see if there are any GEP uses 359 // of the argument. 360 ScalarizeTable &ArgIndices = ScalarizedElements[I]; 361 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E; 362 ++UI) { 363 Instruction *User = cast<Instruction>(*UI); 364 assert(isa<LoadInst>(User) || isa<GetElementPtrInst>(User)); 365 std::vector<Value*> Indices(User->op_begin()+1, User->op_end()); 366 ArgIndices.insert(Indices); 367 LoadInst *OrigLoad; 368 if (LoadInst *L = dyn_cast<LoadInst>(User)) 369 OrigLoad = L; 370 else 371 OrigLoad = cast<LoadInst>(User->use_back()); 372 OriginalLoads[Indices] = OrigLoad; 373 } 374 375 // Add a parameter to the function for each element passed in. 376 for (ScalarizeTable::iterator SI = ArgIndices.begin(), 377 E = ArgIndices.end(); SI != E; ++SI) 378 Params.push_back(GetElementPtrInst::getIndexedType(I->getType(), 379 &(*SI)[0], 380 SI->size())); 381 382 if (ArgIndices.size() == 1 && ArgIndices.begin()->empty()) 383 ++NumArgumentsPromoted; 384 else 385 ++NumAggregatesPromoted; 386 } 387 388 const Type *RetTy = FTy->getReturnType(); 389 390 // Work around LLVM bug PR56: the CWriter cannot emit varargs functions which 391 // have zero fixed arguments. 392 bool ExtraArgHack = false; 393 if (Params.empty() && FTy->isVarArg()) { 394 ExtraArgHack = true; 395 Params.push_back(Type::Int32Ty); 396 } 397 FunctionType *NFTy = FunctionType::get(RetTy, Params, FTy->isVarArg()); 398 399 // Create the new function body and insert it into the module... 400 Function *NF = new Function(NFTy, F->getLinkage(), F->getName()); 401 NF->setCallingConv(F->getCallingConv()); 402 F->getParent()->getFunctionList().insert(F, NF); 403 404 // Get the alias analysis information that we need to update to reflect our 405 // changes. 406 AliasAnalysis &AA = getAnalysis<AliasAnalysis>(); 407 408 // Loop over all of the callers of the function, transforming the call sites 409 // to pass in the loaded pointers. 410 // 411 std::vector<Value*> Args; 412 while (!F->use_empty()) { 413 CallSite CS = CallSite::get(F->use_back()); 414 Instruction *Call = CS.getInstruction(); 415 416 // Loop over the operands, inserting GEP and loads in the caller as 417 // appropriate. 418 CallSite::arg_iterator AI = CS.arg_begin(); 419 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); 420 I != E; ++I, ++AI) 421 if (!ArgsToPromote.count(I)) 422 Args.push_back(*AI); // Unmodified argument 423 else if (!I->use_empty()) { 424 // Non-dead argument: insert GEPs and loads as appropriate. 425 ScalarizeTable &ArgIndices = ScalarizedElements[I]; 426 for (ScalarizeTable::iterator SI = ArgIndices.begin(), 427 E = ArgIndices.end(); SI != E; ++SI) { 428 Value *V = *AI; 429 LoadInst *OrigLoad = OriginalLoads[*SI]; 430 if (!SI->empty()) { 431 V = new GetElementPtrInst(V, &(*SI)[0], SI->size(), 432 V->getName()+".idx", Call); 433 AA.copyValue(OrigLoad->getOperand(0), V); 434 } 435 Args.push_back(new LoadInst(V, V->getName()+".val", Call)); 436 AA.copyValue(OrigLoad, Args.back()); 437 } 438 } 439 440 if (ExtraArgHack) 441 Args.push_back(Constant::getNullValue(Type::Int32Ty)); 442 443 // Push any varargs arguments on the list 444 for (; AI != CS.arg_end(); ++AI) 445 Args.push_back(*AI); 446 447 Instruction *New; 448 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) { 449 New = new InvokeInst(NF, II->getNormalDest(), II->getUnwindDest(), 450 &Args[0], Args.size(), "", Call); 451 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv()); 452 } else { 453 New = new CallInst(NF, &Args[0], Args.size(), "", Call); 454 cast<CallInst>(New)->setCallingConv(CS.getCallingConv()); 455 if (cast<CallInst>(Call)->isTailCall()) 456 cast<CallInst>(New)->setTailCall(); 457 } 458 Args.clear(); 459 460 // Update the alias analysis implementation to know that we are replacing 461 // the old call with a new one. 462 AA.replaceWithNewValue(Call, New); 463 464 if (!Call->use_empty()) { 465 Call->replaceAllUsesWith(New); 466 New->takeName(Call); 467 } 468 469 // Finally, remove the old call from the program, reducing the use-count of 470 // F. 471 Call->getParent()->getInstList().erase(Call); 472 } 473 474 // Since we have now created the new function, splice the body of the old 475 // function right into the new function, leaving the old rotting hulk of the 476 // function empty. 477 NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList()); 478 479 // Loop over the argument list, transfering uses of the old arguments over to 480 // the new arguments, also transfering over the names as well. 481 // 482 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(), 483 I2 = NF->arg_begin(); I != E; ++I) 484 if (!ArgsToPromote.count(I)) { 485 // If this is an unmodified argument, move the name and users over to the 486 // new version. 487 I->replaceAllUsesWith(I2); 488 I2->takeName(I); 489 AA.replaceWithNewValue(I, I2); 490 ++I2; 491 } else if (I->use_empty()) { 492 AA.deleteValue(I); 493 } else { 494 // Otherwise, if we promoted this argument, then all users are load 495 // instructions, and all loads should be using the new argument that we 496 // added. 497 ScalarizeTable &ArgIndices = ScalarizedElements[I]; 498 499 while (!I->use_empty()) { 500 if (LoadInst *LI = dyn_cast<LoadInst>(I->use_back())) { 501 assert(ArgIndices.begin()->empty() && 502 "Load element should sort to front!"); 503 I2->setName(I->getName()+".val"); 504 LI->replaceAllUsesWith(I2); 505 AA.replaceWithNewValue(LI, I2); 506 LI->getParent()->getInstList().erase(LI); 507 DOUT << "*** Promoted load of argument '" << I->getName() 508 << "' in function '" << F->getName() << "'\n"; 509 } else { 510 GetElementPtrInst *GEP = cast<GetElementPtrInst>(I->use_back()); 511 std::vector<Value*> Operands(GEP->op_begin()+1, GEP->op_end()); 512 513 Function::arg_iterator TheArg = I2; 514 for (ScalarizeTable::iterator It = ArgIndices.begin(); 515 *It != Operands; ++It, ++TheArg) { 516 assert(It != ArgIndices.end() && "GEP not handled??"); 517 } 518 519 std::string NewName = I->getName(); 520 for (unsigned i = 0, e = Operands.size(); i != e; ++i) 521 if (ConstantInt *CI = dyn_cast<ConstantInt>(Operands[i])) 522 NewName += "." + CI->getValue().toString(10); 523 else 524 NewName += ".x"; 525 TheArg->setName(NewName+".val"); 526 527 DOUT << "*** Promoted agg argument '" << TheArg->getName() 528 << "' of function '" << F->getName() << "'\n"; 529 530 // All of the uses must be load instructions. Replace them all with 531 // the argument specified by ArgNo. 532 while (!GEP->use_empty()) { 533 LoadInst *L = cast<LoadInst>(GEP->use_back()); 534 L->replaceAllUsesWith(TheArg); 535 AA.replaceWithNewValue(L, TheArg); 536 L->getParent()->getInstList().erase(L); 537 } 538 AA.deleteValue(GEP); 539 GEP->getParent()->getInstList().erase(GEP); 540 } 541 } 542 543 // Increment I2 past all of the arguments added for this promoted pointer. 544 for (unsigned i = 0, e = ArgIndices.size(); i != e; ++i) 545 ++I2; 546 } 547 548 // Notify the alias analysis implementation that we inserted a new argument. 549 if (ExtraArgHack) 550 AA.copyValue(Constant::getNullValue(Type::Int32Ty), NF->arg_begin()); 551 552 553 // Tell the alias analysis that the old function is about to disappear. 554 AA.replaceWithNewValue(F, NF); 555 556 // Now that the old function is dead, delete it. 557 F->getParent()->getFunctionList().erase(F); 558 return NF; 559} 560