ArgumentPromotion.cpp revision f5afcabff887c2e9023f0c69c44f1de15b5c4347
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 <set> 48using namespace llvm; 49 50namespace { 51 Statistic<> NumArgumentsPromoted("argpromotion", 52 "Number of pointer arguments promoted"); 53 Statistic<> NumAggregatesPromoted("argpromotion", 54 "Number of aggregate arguments promoted"); 55 Statistic<> NumArgumentsDead("argpromotion", 56 "Number of dead pointer args eliminated"); 57 58 /// ArgPromotion - The 'by reference' to 'by value' argument promotion pass. 59 /// 60 struct ArgPromotion : public CallGraphSCCPass { 61 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 62 AU.addRequired<AliasAnalysis>(); 63 AU.addRequired<TargetData>(); 64 CallGraphSCCPass::getAnalysisUsage(AU); 65 } 66 67 virtual bool runOnSCC(const std::vector<CallGraphNode *> &SCC); 68 private: 69 bool PromoteArguments(CallGraphNode *CGN); 70 bool isSafeToPromoteArgument(Argument *Arg) const; 71 Function *DoPromotion(Function *F, std::vector<Argument*> &ArgsToPromote); 72 }; 73 74 RegisterOpt<ArgPromotion> X("argpromotion", 75 "Promote 'by reference' arguments to scalars"); 76} 77 78Pass *llvm::createArgumentPromotionPass() { 79 return new ArgPromotion(); 80} 81 82bool ArgPromotion::runOnSCC(const std::vector<CallGraphNode *> &SCC) { 83 bool Changed = false, LocalChange; 84 85 do { // Iterate until we stop promoting from this SCC. 86 LocalChange = false; 87 // Attempt to promote arguments from all functions in this SCC. 88 for (unsigned i = 0, e = SCC.size(); i != e; ++i) 89 LocalChange |= PromoteArguments(SCC[i]); 90 Changed |= LocalChange; // Remember that we changed something. 91 } while (LocalChange); 92 93 return Changed; 94} 95 96/// PromoteArguments - This method checks the specified function to see if there 97/// are any promotable arguments and if it is safe to promote the function (for 98/// example, all callers are direct). If safe to promote some arguments, it 99/// calls the DoPromotion method. 100/// 101bool ArgPromotion::PromoteArguments(CallGraphNode *CGN) { 102 Function *F = CGN->getFunction(); 103 104 // Make sure that it is local to this module. 105 if (!F || !F->hasInternalLinkage()) return false; 106 107 // First check: see if there are any pointer arguments! If not, quick exit. 108 std::vector<Argument*> PointerArgs; 109 for (Function::aiterator I = F->abegin(), E = F->aend(); I != E; ++I) 110 if (isa<PointerType>(I->getType())) 111 PointerArgs.push_back(I); 112 if (PointerArgs.empty()) return false; 113 114 // Second check: make sure that all callers are direct callers. We can't 115 // transform functions that have indirect callers. 116 for (Value::use_iterator UI = F->use_begin(), E = F->use_end(); 117 UI != E; ++UI) { 118 CallSite CS = CallSite::get(*UI); 119 if (!CS.getInstruction()) // "Taking the address" of the function 120 return false; 121 122 // Ensure that this call site is CALLING the function, not passing it as 123 // an argument. 124 for (CallSite::arg_iterator AI = CS.arg_begin(), E = CS.arg_end(); 125 AI != E; ++AI) 126 if (*AI == F) return false; // Passing the function address in! 127 } 128 129 // Check to see which arguments are promotable. If an argument is not 130 // promotable, remove it from the PointerArgs vector. 131 for (unsigned i = 0; i != PointerArgs.size(); ++i) 132 if (!isSafeToPromoteArgument(PointerArgs[i])) { 133 std::swap(PointerArgs[i--], PointerArgs.back()); 134 PointerArgs.pop_back(); 135 } 136 137 // No promotable pointer arguments. 138 if (PointerArgs.empty()) return false; 139 140 // Okay, promote all of the arguments are rewrite the callees! 141 Function *NewF = DoPromotion(F, PointerArgs); 142 143 // Update the call graph to know that the old function is gone. 144 getAnalysis<CallGraph>().changeFunction(F, NewF); 145 return true; 146} 147 148 149/// isSafeToPromoteArgument - As you might guess from the name of this method, 150/// it checks to see if it is both safe and useful to promote the argument. 151/// This method limits promotion of aggregates to only promote up to three 152/// elements of the aggregate in order to avoid exploding the number of 153/// arguments passed in. 154bool ArgPromotion::isSafeToPromoteArgument(Argument *Arg) const { 155 // We can only promote this argument if all of the uses are loads, or are GEP 156 // instructions (with constant indices) that are subsequently loaded. 157 std::vector<LoadInst*> Loads; 158 std::vector<std::vector<ConstantInt*> > GEPIndices; 159 for (Value::use_iterator UI = Arg->use_begin(), E = Arg->use_end(); 160 UI != E; ++UI) 161 if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) { 162 if (LI->isVolatile()) return false; // Don't hack volatile loads 163 Loads.push_back(LI); 164 } else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(*UI)) { 165 if (GEP->use_empty()) { 166 // Dead GEP's cause trouble later. Just remove them if we run into 167 // them. 168 getAnalysis<AliasAnalysis>().deleteValue(GEP); 169 GEP->getParent()->getInstList().erase(GEP); 170 return isSafeToPromoteArgument(Arg); 171 } 172 // Ensure that all of the indices are constants. 173 std::vector<ConstantInt*> Operands; 174 for (unsigned i = 1, e = GEP->getNumOperands(); i != e; ++i) 175 if (ConstantInt *C = dyn_cast<ConstantInt>(GEP->getOperand(i))) 176 Operands.push_back(C); 177 else 178 return false; // Not a constant operand GEP! 179 180 // Ensure that the only users of the GEP are load instructions. 181 for (Value::use_iterator UI = GEP->use_begin(), E = GEP->use_end(); 182 UI != E; ++UI) 183 if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) { 184 if (LI->isVolatile()) return false; // Don't hack volatile loads 185 Loads.push_back(LI); 186 } else { 187 return false; 188 } 189 190 // See if there is already a GEP with these indices. If not, check to 191 // make sure that we aren't promoting too many elements. If so, nothing 192 // to do. 193 if (std::find(GEPIndices.begin(), GEPIndices.end(), Operands) == 194 GEPIndices.end()) { 195 if (GEPIndices.size() == 3) { 196 DEBUG(std::cerr << "argpromotion disable promoting argument '" 197 << Arg->getName() << "' because it would require adding more " 198 << "than 3 arguments to the function.\n"); 199 // We limit aggregate promotion to only promoting up to three elements 200 // of the aggregate. 201 return false; 202 } 203 GEPIndices.push_back(Operands); 204 } 205 } else { 206 return false; // Not a load or a GEP. 207 } 208 209 if (Loads.empty()) return true; // No users, this is a dead argument. 210 211 // Okay, now we know that the argument is only used by load instructions. Use 212 // alias analysis to check to see if the pointer is guaranteed to not be 213 // modified from entry of the function to each of the load instructions. 214 Function &F = *Arg->getParent(); 215 216 // Because there could be several/many load instructions, remember which 217 // blocks we know to be transparent to the load. 218 std::set<BasicBlock*> TranspBlocks; 219 220 AliasAnalysis &AA = getAnalysis<AliasAnalysis>(); 221 TargetData &TD = getAnalysis<TargetData>(); 222 223 for (unsigned i = 0, e = Loads.size(); i != e; ++i) { 224 // Check to see if the load is invalidated from the start of the block to 225 // the load itself. 226 LoadInst *Load = Loads[i]; 227 BasicBlock *BB = Load->getParent(); 228 229 const PointerType *LoadTy = 230 cast<PointerType>(Load->getOperand(0)->getType()); 231 unsigned LoadSize = TD.getTypeSize(LoadTy->getElementType()); 232 233 if (AA.canInstructionRangeModify(BB->front(), *Load, Arg, LoadSize)) 234 return false; // Pointer is invalidated! 235 236 // Now check every path from the entry block to the load for transparency. 237 // To do this, we perform a depth first search on the inverse CFG from the 238 // loading block. 239 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) 240 for (idf_ext_iterator<BasicBlock*> I = idf_ext_begin(*PI, TranspBlocks), 241 E = idf_ext_end(*PI, TranspBlocks); I != E; ++I) 242 if (AA.canBasicBlockModify(**I, Arg, LoadSize)) 243 return false; 244 } 245 246 // If the path from the entry of the function to each load is free of 247 // instructions that potentially invalidate the load, we can make the 248 // transformation! 249 return true; 250} 251 252namespace { 253 /// GEPIdxComparator - Provide a strong ordering for GEP indices. All Value* 254 /// elements are instances of ConstantInt. 255 /// 256 struct GEPIdxComparator { 257 bool operator()(const std::vector<Value*> &LHS, 258 const std::vector<Value*> &RHS) const { 259 unsigned idx = 0; 260 for (; idx < LHS.size() && idx < RHS.size(); ++idx) { 261 if (LHS[idx] != RHS[idx]) { 262 return cast<ConstantInt>(LHS[idx])->getRawValue() < 263 cast<ConstantInt>(RHS[idx])->getRawValue(); 264 } 265 } 266 267 // Return less than if we ran out of stuff in LHS and we didn't run out of 268 // stuff in RHS. 269 return idx == LHS.size() && idx != RHS.size(); 270 } 271 }; 272} 273 274 275/// DoPromotion - This method actually performs the promotion of the specified 276/// arguments, and returns the new function. At this point, we know that it's 277/// safe to do so. 278Function *ArgPromotion::DoPromotion(Function *F, 279 std::vector<Argument*> &Args2Prom) { 280 std::set<Argument*> ArgsToPromote(Args2Prom.begin(), Args2Prom.end()); 281 282 // Start by computing a new prototype for the function, which is the same as 283 // the old function, but has modified arguments. 284 const FunctionType *FTy = F->getFunctionType(); 285 std::vector<const Type*> Params; 286 287 typedef std::set<std::vector<Value*>, GEPIdxComparator> ScalarizeTable; 288 289 // ScalarizedElements - If we are promoting a pointer that has elements 290 // accessed out of it, keep track of which elements are accessed so that we 291 // can add one argument for each. 292 // 293 // Arguments that are directly loaded will have a zero element value here, to 294 // handle cases where there are both a direct load and GEP accesses. 295 // 296 std::map<Argument*, ScalarizeTable> ScalarizedElements; 297 298 // OriginalLoads - Keep track of a representative load instruction from the 299 // original function so that we can tell the alias analysis implementation 300 // what the new GEP/Load instructions we are inserting look like. 301 std::map<std::vector<Value*>, LoadInst*> OriginalLoads; 302 303 for (Function::aiterator I = F->abegin(), E = F->aend(); I != E; ++I) 304 if (!ArgsToPromote.count(I)) { 305 Params.push_back(I->getType()); 306 } else if (I->use_empty()) { 307 ++NumArgumentsDead; 308 } else { 309 // Okay, this is being promoted. Check to see if there are any GEP uses 310 // of the argument. 311 ScalarizeTable &ArgIndices = ScalarizedElements[I]; 312 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E; 313 ++UI) { 314 Instruction *User = cast<Instruction>(*UI); 315 assert(isa<LoadInst>(User) || isa<GetElementPtrInst>(User)); 316 std::vector<Value*> Indices(User->op_begin()+1, User->op_end()); 317 ArgIndices.insert(Indices); 318 LoadInst *OrigLoad; 319 if (LoadInst *L = dyn_cast<LoadInst>(User)) 320 OrigLoad = L; 321 else 322 OrigLoad = cast<LoadInst>(User->use_back()); 323 OriginalLoads[Indices] = OrigLoad; 324 } 325 326 // Add a parameter to the function for each element passed in. 327 for (ScalarizeTable::iterator SI = ArgIndices.begin(), 328 E = ArgIndices.end(); SI != E; ++SI) 329 Params.push_back(GetElementPtrInst::getIndexedType(I->getType(), *SI)); 330 331 if (ArgIndices.size() == 1 && ArgIndices.begin()->empty()) 332 ++NumArgumentsPromoted; 333 else 334 ++NumAggregatesPromoted; 335 } 336 337 const Type *RetTy = FTy->getReturnType(); 338 339 // Work around LLVM bug PR56: the CWriter cannot emit varargs functions which 340 // have zero fixed arguments. 341 bool ExtraArgHack = false; 342 if (Params.empty() && FTy->isVarArg()) { 343 ExtraArgHack = true; 344 Params.push_back(Type::IntTy); 345 } 346 FunctionType *NFTy = FunctionType::get(RetTy, Params, FTy->isVarArg()); 347 348 // Create the new function body and insert it into the module... 349 Function *NF = new Function(NFTy, F->getLinkage(), F->getName()); 350 F->getParent()->getFunctionList().insert(F, NF); 351 352 // Get the alias analysis information that we need to update to reflect our 353 // changes. 354 AliasAnalysis &AA = getAnalysis<AliasAnalysis>(); 355 356 // Loop over all of the callers of the function, transforming the call sites 357 // to pass in the loaded pointers. 358 // 359 std::vector<Value*> Args; 360 while (!F->use_empty()) { 361 CallSite CS = CallSite::get(F->use_back()); 362 Instruction *Call = CS.getInstruction(); 363 364 // Loop over the operands, inserting GEP and loads in the caller as 365 // appropriate. 366 CallSite::arg_iterator AI = CS.arg_begin(); 367 for (Function::aiterator I = F->abegin(), E = F->aend(); I != E; ++I, ++AI) 368 if (!ArgsToPromote.count(I)) 369 Args.push_back(*AI); // Unmodified argument 370 else if (!I->use_empty()) { 371 // Non-dead argument: insert GEPs and loads as appropriate. 372 ScalarizeTable &ArgIndices = ScalarizedElements[I]; 373 for (ScalarizeTable::iterator SI = ArgIndices.begin(), 374 E = ArgIndices.end(); SI != E; ++SI) { 375 Value *V = *AI; 376 LoadInst *OrigLoad = OriginalLoads[*SI]; 377 if (!SI->empty()) { 378 V = new GetElementPtrInst(V, *SI, V->getName()+".idx", Call); 379 AA.copyValue(OrigLoad->getOperand(0), V); 380 } 381 Args.push_back(new LoadInst(V, V->getName()+".val", Call)); 382 AA.copyValue(OrigLoad, Args.back()); 383 } 384 } 385 386 if (ExtraArgHack) 387 Args.push_back(Constant::getNullValue(Type::IntTy)); 388 389 // Push any varargs arguments on the list 390 for (; AI != CS.arg_end(); ++AI) 391 Args.push_back(*AI); 392 393 Instruction *New; 394 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) { 395 New = new InvokeInst(NF, II->getNormalDest(), II->getUnwindDest(), 396 Args, "", Call); 397 } else { 398 New = new CallInst(NF, Args, "", Call); 399 } 400 Args.clear(); 401 402 // Update the alias analysis implementation to know that we are replacing 403 // the old call with a new one. 404 AA.replaceWithNewValue(Call, New); 405 406 if (!Call->use_empty()) { 407 Call->replaceAllUsesWith(New); 408 std::string Name = Call->getName(); 409 Call->setName(""); 410 New->setName(Name); 411 } 412 413 // Finally, remove the old call from the program, reducing the use-count of 414 // F. 415 Call->getParent()->getInstList().erase(Call); 416 } 417 418 // Since we have now created the new function, splice the body of the old 419 // function right into the new function, leaving the old rotting hulk of the 420 // function empty. 421 NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList()); 422 423 // Loop over the argument list, transfering uses of the old arguments over to 424 // the new arguments, also transfering over the names as well. 425 // 426 for (Function::aiterator I = F->abegin(), E = F->aend(), I2 = NF->abegin(); 427 I != E; ++I) 428 if (!ArgsToPromote.count(I)) { 429 // If this is an unmodified argument, move the name and users over to the 430 // new version. 431 I->replaceAllUsesWith(I2); 432 I2->setName(I->getName()); 433 AA.replaceWithNewValue(I, I2); 434 ++I2; 435 } else if (I->use_empty()) { 436 AA.deleteValue(I); 437 } else { 438 // Otherwise, if we promoted this argument, then all users are load 439 // instructions, and all loads should be using the new argument that we 440 // added. 441 ScalarizeTable &ArgIndices = ScalarizedElements[I]; 442 443 while (!I->use_empty()) { 444 if (LoadInst *LI = dyn_cast<LoadInst>(I->use_back())) { 445 assert(ArgIndices.begin()->empty() && 446 "Load element should sort to front!"); 447 I2->setName(I->getName()+".val"); 448 LI->replaceAllUsesWith(I2); 449 AA.replaceWithNewValue(LI, I2); 450 LI->getParent()->getInstList().erase(LI); 451 DEBUG(std::cerr << "*** Promoted load of argument '" << I->getName() 452 << "' in function '" << F->getName() << "'\n"); 453 } else { 454 GetElementPtrInst *GEP = cast<GetElementPtrInst>(I->use_back()); 455 std::vector<Value*> Operands(GEP->op_begin()+1, GEP->op_end()); 456 457 unsigned ArgNo = 0; 458 Function::aiterator TheArg = I2; 459 for (ScalarizeTable::iterator It = ArgIndices.begin(); 460 *It != Operands; ++It, ++TheArg) { 461 assert(It != ArgIndices.end() && "GEP not handled??"); 462 } 463 464 std::string NewName = I->getName(); 465 for (unsigned i = 0, e = Operands.size(); i != e; ++i) 466 if (ConstantInt *CI = dyn_cast<ConstantInt>(Operands[i])) 467 NewName += "."+itostr((int64_t)CI->getRawValue()); 468 else 469 NewName += ".x"; 470 TheArg->setName(NewName+".val"); 471 472 DEBUG(std::cerr << "*** Promoted agg argument '" << TheArg->getName() 473 << "' of function '" << F->getName() << "'\n"); 474 475 // All of the uses must be load instructions. Replace them all with 476 // the argument specified by ArgNo. 477 while (!GEP->use_empty()) { 478 LoadInst *L = cast<LoadInst>(GEP->use_back()); 479 L->replaceAllUsesWith(TheArg); 480 AA.replaceWithNewValue(L, TheArg); 481 L->getParent()->getInstList().erase(L); 482 } 483 AA.deleteValue(GEP); 484 GEP->getParent()->getInstList().erase(GEP); 485 } 486 } 487 488 // Increment I2 past all of the arguments added for this promoted pointer. 489 for (unsigned i = 0, e = ArgIndices.size(); i != e; ++i) 490 ++I2; 491 } 492 493 // Notify the alias analysis implementation that we inserted a new argument. 494 if (ExtraArgHack) 495 AA.copyValue(Constant::getNullValue(Type::IntTy), NF->abegin()); 496 497 498 // Tell the alias analysis that the old function is about to disappear. 499 AA.replaceWithNewValue(F, NF); 500 501 // Now that the old function is dead, delete it. 502 F->getParent()->getFunctionList().erase(F); 503 return NF; 504} 505