ArgumentPromotion.cpp revision a0e0129e3f03fae1a275537a00e426a31d5eb649
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 private: 67 bool PromoteArguments(CallGraphNode *CGN); 68 bool isSafeToPromoteArgument(Argument *Arg) const; 69 Function *DoPromotion(Function *F, std::vector<Argument*> &ArgsToPromote); 70 }; 71 72 RegisterPass<ArgPromotion> X("argpromotion", 73 "Promote 'by reference' arguments to scalars"); 74} 75 76Pass *llvm::createArgumentPromotionPass() { 77 return new ArgPromotion(); 78} 79 80bool ArgPromotion::runOnSCC(const std::vector<CallGraphNode *> &SCC) { 81 bool Changed = false, LocalChange; 82 83 do { // Iterate until we stop promoting from this SCC. 84 LocalChange = false; 85 // Attempt to promote arguments from all functions in this SCC. 86 for (unsigned i = 0, e = SCC.size(); i != e; ++i) 87 LocalChange |= PromoteArguments(SCC[i]); 88 Changed |= LocalChange; // Remember that we changed something. 89 } while (LocalChange); 90 91 return Changed; 92} 93 94/// PromoteArguments - This method checks the specified function to see if there 95/// are any promotable arguments and if it is safe to promote the function (for 96/// example, all callers are direct). If safe to promote some arguments, it 97/// calls the DoPromotion method. 98/// 99bool ArgPromotion::PromoteArguments(CallGraphNode *CGN) { 100 Function *F = CGN->getFunction(); 101 102 // Make sure that it is local to this module. 103 if (!F || !F->hasInternalLinkage()) return false; 104 105 // First check: see if there are any pointer arguments! If not, quick exit. 106 std::vector<Argument*> PointerArgs; 107 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E; ++I) 108 if (isa<PointerType>(I->getType())) 109 PointerArgs.push_back(I); 110 if (PointerArgs.empty()) return false; 111 112 // Second check: make sure that all callers are direct callers. We can't 113 // transform functions that have indirect callers. 114 for (Value::use_iterator UI = F->use_begin(), E = F->use_end(); 115 UI != E; ++UI) { 116 CallSite CS = CallSite::get(*UI); 117 if (!CS.getInstruction()) // "Taking the address" of the function 118 return false; 119 120 // Ensure that this call site is CALLING the function, not passing it as 121 // an argument. 122 for (CallSite::arg_iterator AI = CS.arg_begin(), E = CS.arg_end(); 123 AI != E; ++AI) 124 if (*AI == F) return false; // Passing the function address in! 125 } 126 127 // Check to see which arguments are promotable. If an argument is not 128 // promotable, remove it from the PointerArgs vector. 129 for (unsigned i = 0; i != PointerArgs.size(); ++i) 130 if (!isSafeToPromoteArgument(PointerArgs[i])) { 131 std::swap(PointerArgs[i--], PointerArgs.back()); 132 PointerArgs.pop_back(); 133 } 134 135 // No promotable pointer arguments. 136 if (PointerArgs.empty()) return false; 137 138 // Okay, promote all of the arguments are rewrite the callees! 139 Function *NewF = DoPromotion(F, PointerArgs); 140 141 // Update the call graph to know that the old function is gone. 142 getAnalysis<CallGraph>().changeFunction(F, NewF); 143 return true; 144} 145 146/// IsAlwaysValidPointer - Return true if the specified pointer is always legal 147/// to load. 148static bool IsAlwaysValidPointer(Value *V) { 149 if (isa<AllocaInst>(V) || isa<GlobalVariable>(V)) return true; 150 if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(V)) 151 return IsAlwaysValidPointer(GEP->getOperand(0)); 152 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) 153 if (CE->getOpcode() == Instruction::GetElementPtr) 154 return IsAlwaysValidPointer(CE->getOperand(0)); 155 156 return false; 157} 158 159/// AllCalleesPassInValidPointerForArgument - Return true if we can prove that 160/// all callees pass in a valid pointer for the specified function argument. 161static bool AllCalleesPassInValidPointerForArgument(Argument *Arg) { 162 Function *Callee = Arg->getParent(); 163 164 unsigned ArgNo = std::distance(Callee->arg_begin(), 165 Function::arg_iterator(Arg)); 166 167 // Look at all call sites of the function. At this pointer we know we only 168 // have direct callees. 169 for (Value::use_iterator UI = Callee->use_begin(), E = Callee->use_end(); 170 UI != E; ++UI) { 171 CallSite CS = CallSite::get(*UI); 172 assert(CS.getInstruction() && "Should only have direct calls!"); 173 174 if (!IsAlwaysValidPointer(CS.getArgument(ArgNo))) 175 return false; 176 } 177 return true; 178} 179 180 181/// isSafeToPromoteArgument - As you might guess from the name of this method, 182/// it checks to see if it is both safe and useful to promote the argument. 183/// This method limits promotion of aggregates to only promote up to three 184/// elements of the aggregate in order to avoid exploding the number of 185/// arguments passed in. 186bool ArgPromotion::isSafeToPromoteArgument(Argument *Arg) const { 187 // We can only promote this argument if all of the uses are loads, or are GEP 188 // instructions (with constant indices) that are subsequently loaded. 189 bool HasLoadInEntryBlock = false; 190 BasicBlock *EntryBlock = Arg->getParent()->begin(); 191 std::vector<LoadInst*> Loads; 192 std::vector<std::vector<ConstantInt*> > GEPIndices; 193 for (Value::use_iterator UI = Arg->use_begin(), E = Arg->use_end(); 194 UI != E; ++UI) 195 if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) { 196 if (LI->isVolatile()) return false; // Don't hack volatile loads 197 Loads.push_back(LI); 198 HasLoadInEntryBlock |= LI->getParent() == EntryBlock; 199 } else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(*UI)) { 200 if (GEP->use_empty()) { 201 // Dead GEP's cause trouble later. Just remove them if we run into 202 // them. 203 getAnalysis<AliasAnalysis>().deleteValue(GEP); 204 GEP->getParent()->getInstList().erase(GEP); 205 return isSafeToPromoteArgument(Arg); 206 } 207 // Ensure that all of the indices are constants. 208 std::vector<ConstantInt*> Operands; 209 for (unsigned i = 1, e = GEP->getNumOperands(); i != e; ++i) 210 if (ConstantInt *C = dyn_cast<ConstantInt>(GEP->getOperand(i))) 211 Operands.push_back(C); 212 else 213 return false; // Not a constant operand GEP! 214 215 // Ensure that the only users of the GEP are load instructions. 216 for (Value::use_iterator UI = GEP->use_begin(), E = GEP->use_end(); 217 UI != E; ++UI) 218 if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) { 219 if (LI->isVolatile()) return false; // Don't hack volatile loads 220 Loads.push_back(LI); 221 HasLoadInEntryBlock |= LI->getParent() == EntryBlock; 222 } else { 223 return false; 224 } 225 226 // See if there is already a GEP with these indices. If not, check to 227 // make sure that we aren't promoting too many elements. If so, nothing 228 // to do. 229 if (std::find(GEPIndices.begin(), GEPIndices.end(), Operands) == 230 GEPIndices.end()) { 231 if (GEPIndices.size() == 3) { 232 DOUT << "argpromotion disable promoting argument '" 233 << Arg->getName() << "' because it would require adding more " 234 << "than 3 arguments to the function.\n"; 235 // We limit aggregate promotion to only promoting up to three elements 236 // of the aggregate. 237 return false; 238 } 239 GEPIndices.push_back(Operands); 240 } 241 } else { 242 return false; // Not a load or a GEP. 243 } 244 245 if (Loads.empty()) return true; // No users, this is a dead argument. 246 247 // If we decide that we want to promote this argument, the value is going to 248 // be unconditionally loaded in all callees. This is only safe to do if the 249 // pointer was going to be unconditionally loaded anyway (i.e. there is a load 250 // of the pointer in the entry block of the function) or if we can prove that 251 // all pointers passed in are always to legal locations (for example, no null 252 // pointers are passed in, no pointers to free'd memory, etc). 253 if (!HasLoadInEntryBlock && !AllCalleesPassInValidPointerForArgument(Arg)) 254 return false; // Cannot prove that this is safe!! 255 256 // Okay, now we know that the argument is only used by load instructions and 257 // it is safe to unconditionally load the pointer. Use alias analysis to 258 // check to see if the pointer is guaranteed to not be modified from entry of 259 // the function to each of the load instructions. 260 261 // Because there could be several/many load instructions, remember which 262 // blocks we know to be transparent to the load. 263 std::set<BasicBlock*> TranspBlocks; 264 265 AliasAnalysis &AA = getAnalysis<AliasAnalysis>(); 266 TargetData &TD = getAnalysis<TargetData>(); 267 268 for (unsigned i = 0, e = Loads.size(); i != e; ++i) { 269 // Check to see if the load is invalidated from the start of the block to 270 // the load itself. 271 LoadInst *Load = Loads[i]; 272 BasicBlock *BB = Load->getParent(); 273 274 const PointerType *LoadTy = 275 cast<PointerType>(Load->getOperand(0)->getType()); 276 unsigned LoadSize = (unsigned)TD.getTypeSize(LoadTy->getElementType()); 277 278 if (AA.canInstructionRangeModify(BB->front(), *Load, Arg, LoadSize)) 279 return false; // Pointer is invalidated! 280 281 // Now check every path from the entry block to the load for transparency. 282 // To do this, we perform a depth first search on the inverse CFG from the 283 // loading block. 284 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) 285 for (idf_ext_iterator<BasicBlock*> I = idf_ext_begin(*PI, TranspBlocks), 286 E = idf_ext_end(*PI, TranspBlocks); I != E; ++I) 287 if (AA.canBasicBlockModify(**I, Arg, LoadSize)) 288 return false; 289 } 290 291 // If the path from the entry of the function to each load is free of 292 // instructions that potentially invalidate the load, we can make the 293 // transformation! 294 return true; 295} 296 297namespace { 298 /// GEPIdxComparator - Provide a strong ordering for GEP indices. All Value* 299 /// elements are instances of ConstantInt. 300 /// 301 struct GEPIdxComparator { 302 bool operator()(const std::vector<Value*> &LHS, 303 const std::vector<Value*> &RHS) const { 304 unsigned idx = 0; 305 for (; idx < LHS.size() && idx < RHS.size(); ++idx) { 306 if (LHS[idx] != RHS[idx]) { 307 return cast<ConstantInt>(LHS[idx])->getZExtValue() < 308 cast<ConstantInt>(RHS[idx])->getZExtValue(); 309 } 310 } 311 312 // Return less than if we ran out of stuff in LHS and we didn't run out of 313 // stuff in RHS. 314 return idx == LHS.size() && idx != RHS.size(); 315 } 316 }; 317} 318 319 320/// DoPromotion - This method actually performs the promotion of the specified 321/// arguments, and returns the new function. At this point, we know that it's 322/// safe to do so. 323Function *ArgPromotion::DoPromotion(Function *F, 324 std::vector<Argument*> &Args2Prom) { 325 std::set<Argument*> ArgsToPromote(Args2Prom.begin(), Args2Prom.end()); 326 327 // Start by computing a new prototype for the function, which is the same as 328 // the old function, but has modified arguments. 329 const FunctionType *FTy = F->getFunctionType(); 330 std::vector<const Type*> Params; 331 332 typedef std::set<std::vector<Value*>, GEPIdxComparator> ScalarizeTable; 333 334 // ScalarizedElements - If we are promoting a pointer that has elements 335 // accessed out of it, keep track of which elements are accessed so that we 336 // can add one argument for each. 337 // 338 // Arguments that are directly loaded will have a zero element value here, to 339 // handle cases where there are both a direct load and GEP accesses. 340 // 341 std::map<Argument*, ScalarizeTable> ScalarizedElements; 342 343 // OriginalLoads - Keep track of a representative load instruction from the 344 // original function so that we can tell the alias analysis implementation 345 // what the new GEP/Load instructions we are inserting look like. 346 std::map<std::vector<Value*>, LoadInst*> OriginalLoads; 347 348 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E; ++I) 349 if (!ArgsToPromote.count(I)) { 350 Params.push_back(I->getType()); 351 } else if (I->use_empty()) { 352 ++NumArgumentsDead; 353 } else { 354 // Okay, this is being promoted. Check to see if there are any GEP uses 355 // of the argument. 356 ScalarizeTable &ArgIndices = ScalarizedElements[I]; 357 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E; 358 ++UI) { 359 Instruction *User = cast<Instruction>(*UI); 360 assert(isa<LoadInst>(User) || isa<GetElementPtrInst>(User)); 361 std::vector<Value*> Indices(User->op_begin()+1, User->op_end()); 362 ArgIndices.insert(Indices); 363 LoadInst *OrigLoad; 364 if (LoadInst *L = dyn_cast<LoadInst>(User)) 365 OrigLoad = L; 366 else 367 OrigLoad = cast<LoadInst>(User->use_back()); 368 OriginalLoads[Indices] = OrigLoad; 369 } 370 371 // Add a parameter to the function for each element passed in. 372 for (ScalarizeTable::iterator SI = ArgIndices.begin(), 373 E = ArgIndices.end(); SI != E; ++SI) 374 Params.push_back(GetElementPtrInst::getIndexedType(I->getType(), 375 &(*SI)[0], 376 SI->size())); 377 378 if (ArgIndices.size() == 1 && ArgIndices.begin()->empty()) 379 ++NumArgumentsPromoted; 380 else 381 ++NumAggregatesPromoted; 382 } 383 384 const Type *RetTy = FTy->getReturnType(); 385 386 // Work around LLVM bug PR56: the CWriter cannot emit varargs functions which 387 // have zero fixed arguments. 388 bool ExtraArgHack = false; 389 if (Params.empty() && FTy->isVarArg()) { 390 ExtraArgHack = true; 391 Params.push_back(Type::Int32Ty); 392 } 393 FunctionType *NFTy = FunctionType::get(RetTy, Params, FTy->isVarArg()); 394 395 // Create the new function body and insert it into the module... 396 Function *NF = new Function(NFTy, F->getLinkage(), F->getName()); 397 NF->setCallingConv(F->getCallingConv()); 398 F->getParent()->getFunctionList().insert(F, NF); 399 400 // Get the alias analysis information that we need to update to reflect our 401 // changes. 402 AliasAnalysis &AA = getAnalysis<AliasAnalysis>(); 403 404 // Loop over all of the callers of the function, transforming the call sites 405 // to pass in the loaded pointers. 406 // 407 std::vector<Value*> Args; 408 while (!F->use_empty()) { 409 CallSite CS = CallSite::get(F->use_back()); 410 Instruction *Call = CS.getInstruction(); 411 412 // Loop over the operands, inserting GEP and loads in the caller as 413 // appropriate. 414 CallSite::arg_iterator AI = CS.arg_begin(); 415 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); 416 I != E; ++I, ++AI) 417 if (!ArgsToPromote.count(I)) 418 Args.push_back(*AI); // Unmodified argument 419 else if (!I->use_empty()) { 420 // Non-dead argument: insert GEPs and loads as appropriate. 421 ScalarizeTable &ArgIndices = ScalarizedElements[I]; 422 for (ScalarizeTable::iterator SI = ArgIndices.begin(), 423 E = ArgIndices.end(); SI != E; ++SI) { 424 Value *V = *AI; 425 LoadInst *OrigLoad = OriginalLoads[*SI]; 426 if (!SI->empty()) { 427 V = new GetElementPtrInst(V, &(*SI)[0], SI->size(), 428 V->getName()+".idx", Call); 429 AA.copyValue(OrigLoad->getOperand(0), V); 430 } 431 Args.push_back(new LoadInst(V, V->getName()+".val", Call)); 432 AA.copyValue(OrigLoad, Args.back()); 433 } 434 } 435 436 if (ExtraArgHack) 437 Args.push_back(Constant::getNullValue(Type::Int32Ty)); 438 439 // Push any varargs arguments on the list 440 for (; AI != CS.arg_end(); ++AI) 441 Args.push_back(*AI); 442 443 Instruction *New; 444 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) { 445 New = new InvokeInst(NF, II->getNormalDest(), II->getUnwindDest(), 446 &Args[0], Args.size(), "", Call); 447 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv()); 448 } else { 449 New = new CallInst(NF, &Args[0], Args.size(), "", Call); 450 cast<CallInst>(New)->setCallingConv(CS.getCallingConv()); 451 if (cast<CallInst>(Call)->isTailCall()) 452 cast<CallInst>(New)->setTailCall(); 453 } 454 Args.clear(); 455 456 // Update the alias analysis implementation to know that we are replacing 457 // the old call with a new one. 458 AA.replaceWithNewValue(Call, New); 459 460 if (!Call->use_empty()) { 461 Call->replaceAllUsesWith(New); 462 New->takeName(Call); 463 } 464 465 // Finally, remove the old call from the program, reducing the use-count of 466 // F. 467 Call->getParent()->getInstList().erase(Call); 468 } 469 470 // Since we have now created the new function, splice the body of the old 471 // function right into the new function, leaving the old rotting hulk of the 472 // function empty. 473 NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList()); 474 475 // Loop over the argument list, transfering uses of the old arguments over to 476 // the new arguments, also transfering over the names as well. 477 // 478 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(), 479 I2 = NF->arg_begin(); I != E; ++I) 480 if (!ArgsToPromote.count(I)) { 481 // If this is an unmodified argument, move the name and users over to the 482 // new version. 483 I->replaceAllUsesWith(I2); 484 I2->takeName(I); 485 AA.replaceWithNewValue(I, I2); 486 ++I2; 487 } else if (I->use_empty()) { 488 AA.deleteValue(I); 489 } else { 490 // Otherwise, if we promoted this argument, then all users are load 491 // instructions, and all loads should be using the new argument that we 492 // added. 493 ScalarizeTable &ArgIndices = ScalarizedElements[I]; 494 495 while (!I->use_empty()) { 496 if (LoadInst *LI = dyn_cast<LoadInst>(I->use_back())) { 497 assert(ArgIndices.begin()->empty() && 498 "Load element should sort to front!"); 499 I2->setName(I->getName()+".val"); 500 LI->replaceAllUsesWith(I2); 501 AA.replaceWithNewValue(LI, I2); 502 LI->getParent()->getInstList().erase(LI); 503 DOUT << "*** Promoted load of argument '" << I->getName() 504 << "' in function '" << F->getName() << "'\n"; 505 } else { 506 GetElementPtrInst *GEP = cast<GetElementPtrInst>(I->use_back()); 507 std::vector<Value*> Operands(GEP->op_begin()+1, GEP->op_end()); 508 509 Function::arg_iterator TheArg = I2; 510 for (ScalarizeTable::iterator It = ArgIndices.begin(); 511 *It != Operands; ++It, ++TheArg) { 512 assert(It != ArgIndices.end() && "GEP not handled??"); 513 } 514 515 std::string NewName = I->getName(); 516 for (unsigned i = 0, e = Operands.size(); i != e; ++i) 517 if (ConstantInt *CI = dyn_cast<ConstantInt>(Operands[i])) 518 NewName += "." + CI->getValue().toString(10); 519 else 520 NewName += ".x"; 521 TheArg->setName(NewName+".val"); 522 523 DOUT << "*** Promoted agg argument '" << TheArg->getName() 524 << "' of function '" << F->getName() << "'\n"; 525 526 // All of the uses must be load instructions. Replace them all with 527 // the argument specified by ArgNo. 528 while (!GEP->use_empty()) { 529 LoadInst *L = cast<LoadInst>(GEP->use_back()); 530 L->replaceAllUsesWith(TheArg); 531 AA.replaceWithNewValue(L, TheArg); 532 L->getParent()->getInstList().erase(L); 533 } 534 AA.deleteValue(GEP); 535 GEP->getParent()->getInstList().erase(GEP); 536 } 537 } 538 539 // Increment I2 past all of the arguments added for this promoted pointer. 540 for (unsigned i = 0, e = ArgIndices.size(); i != e; ++i) 541 ++I2; 542 } 543 544 // Notify the alias analysis implementation that we inserted a new argument. 545 if (ExtraArgHack) 546 AA.copyValue(Constant::getNullValue(Type::Int32Ty), NF->arg_begin()); 547 548 549 // Tell the alias analysis that the old function is about to disappear. 550 AA.replaceWithNewValue(F, NF); 551 552 // Now that the old function is dead, delete it. 553 F->getParent()->getFunctionList().erase(F); 554 return NF; 555} 556