ArgumentPromotion.cpp revision ac0b6ae358944ae8b2b5a11dc08f52c3ed89f2da
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 RegisterPass<ArgPromotion> X("argpromotion", 75 "Promote 'by reference' arguments to scalars"); 76} 77 78ModulePass *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::arg_iterator I = F->arg_begin(), E = F->arg_end(); 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/// IsAlwaysValidPointer - Return true if the specified pointer is always legal 149/// to load. 150static bool IsAlwaysValidPointer(Value *V) { 151 if (isa<AllocaInst>(V) || isa<GlobalVariable>(V)) return true; 152 if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(V)) 153 return IsAlwaysValidPointer(GEP->getOperand(0)); 154 if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) 155 if (CE->getOpcode() == Instruction::GetElementPtr) 156 return IsAlwaysValidPointer(CE->getOperand(0)); 157 158 return false; 159} 160 161/// AllCalleesPassInValidPointerForArgument - Return true if we can prove that 162/// all callees pass in a valid pointer for the specified function argument. 163static bool AllCalleesPassInValidPointerForArgument(Argument *Arg) { 164 Function *Callee = Arg->getParent(); 165 166 unsigned ArgNo = std::distance(Callee->arg_begin(), Function::arg_iterator(Arg)); 167 168 // Look at all call sites of the function. At this pointer we know we only 169 // have direct callees. 170 for (Value::use_iterator UI = Callee->use_begin(), E = Callee->use_end(); 171 UI != E; ++UI) { 172 CallSite CS = CallSite::get(*UI); 173 assert(CS.getInstruction() && "Should only have direct calls!"); 174 175 if (!IsAlwaysValidPointer(CS.getArgument(ArgNo))) 176 return false; 177 } 178 return true; 179} 180 181 182/// isSafeToPromoteArgument - As you might guess from the name of this method, 183/// it checks to see if it is both safe and useful to promote the argument. 184/// This method limits promotion of aggregates to only promote up to three 185/// elements of the aggregate in order to avoid exploding the number of 186/// arguments passed in. 187bool ArgPromotion::isSafeToPromoteArgument(Argument *Arg) const { 188 // We can only promote this argument if all of the uses are loads, or are GEP 189 // instructions (with constant indices) that are subsequently loaded. 190 bool HasLoadInEntryBlock = false; 191 BasicBlock *EntryBlock = Arg->getParent()->begin(); 192 std::vector<LoadInst*> Loads; 193 std::vector<std::vector<ConstantInt*> > GEPIndices; 194 for (Value::use_iterator UI = Arg->use_begin(), E = Arg->use_end(); 195 UI != E; ++UI) 196 if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) { 197 if (LI->isVolatile()) return false; // Don't hack volatile loads 198 Loads.push_back(LI); 199 HasLoadInEntryBlock |= LI->getParent() == EntryBlock; 200 } else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(*UI)) { 201 if (GEP->use_empty()) { 202 // Dead GEP's cause trouble later. Just remove them if we run into 203 // them. 204 getAnalysis<AliasAnalysis>().deleteValue(GEP); 205 GEP->getParent()->getInstList().erase(GEP); 206 return isSafeToPromoteArgument(Arg); 207 } 208 // Ensure that all of the indices are constants. 209 std::vector<ConstantInt*> Operands; 210 for (unsigned i = 1, e = GEP->getNumOperands(); i != e; ++i) 211 if (ConstantInt *C = dyn_cast<ConstantInt>(GEP->getOperand(i))) 212 Operands.push_back(C); 213 else 214 return false; // Not a constant operand GEP! 215 216 // Ensure that the only users of the GEP are load instructions. 217 for (Value::use_iterator UI = GEP->use_begin(), E = GEP->use_end(); 218 UI != E; ++UI) 219 if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) { 220 if (LI->isVolatile()) return false; // Don't hack volatile loads 221 Loads.push_back(LI); 222 HasLoadInEntryBlock |= LI->getParent() == EntryBlock; 223 } else { 224 return false; 225 } 226 227 // See if there is already a GEP with these indices. If not, check to 228 // make sure that we aren't promoting too many elements. If so, nothing 229 // to do. 230 if (std::find(GEPIndices.begin(), GEPIndices.end(), Operands) == 231 GEPIndices.end()) { 232 if (GEPIndices.size() == 3) { 233 DOUT << "argpromotion disable promoting argument '" 234 << Arg->getName() << "' because it would require adding more " 235 << "than 3 arguments to the function.\n"; 236 // We limit aggregate promotion to only promoting up to three elements 237 // of the aggregate. 238 return false; 239 } 240 GEPIndices.push_back(Operands); 241 } 242 } else { 243 return false; // Not a load or a GEP. 244 } 245 246 if (Loads.empty()) return true; // No users, this is a dead argument. 247 248 // If we decide that we want to promote this argument, the value is going to 249 // be unconditionally loaded in all callees. This is only safe to do if the 250 // pointer was going to be unconditionally loaded anyway (i.e. there is a load 251 // of the pointer in the entry block of the function) or if we can prove that 252 // all pointers passed in are always to legal locations (for example, no null 253 // pointers are passed in, no pointers to free'd memory, etc). 254 if (!HasLoadInEntryBlock && !AllCalleesPassInValidPointerForArgument(Arg)) 255 return false; // Cannot prove that this is safe!! 256 257 // Okay, now we know that the argument is only used by load instructions and 258 // it is safe to unconditionally load the pointer. Use alias analysis to 259 // check to see if the pointer is guaranteed to not be modified from entry of 260 // the function to each of the load instructions. 261 262 // Because there could be several/many load instructions, remember which 263 // blocks we know to be transparent to the load. 264 std::set<BasicBlock*> TranspBlocks; 265 266 AliasAnalysis &AA = getAnalysis<AliasAnalysis>(); 267 TargetData &TD = getAnalysis<TargetData>(); 268 269 for (unsigned i = 0, e = Loads.size(); i != e; ++i) { 270 // Check to see if the load is invalidated from the start of the block to 271 // the load itself. 272 LoadInst *Load = Loads[i]; 273 BasicBlock *BB = Load->getParent(); 274 275 const PointerType *LoadTy = 276 cast<PointerType>(Load->getOperand(0)->getType()); 277 unsigned LoadSize = (unsigned)TD.getTypeSize(LoadTy->getElementType()); 278 279 if (AA.canInstructionRangeModify(BB->front(), *Load, Arg, LoadSize)) 280 return false; // Pointer is invalidated! 281 282 // Now check every path from the entry block to the load for transparency. 283 // To do this, we perform a depth first search on the inverse CFG from the 284 // loading block. 285 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) 286 for (idf_ext_iterator<BasicBlock*> I = idf_ext_begin(*PI, TranspBlocks), 287 E = idf_ext_end(*PI, TranspBlocks); I != E; ++I) 288 if (AA.canBasicBlockModify(**I, Arg, LoadSize)) 289 return false; 290 } 291 292 // If the path from the entry of the function to each load is free of 293 // instructions that potentially invalidate the load, we can make the 294 // transformation! 295 return true; 296} 297 298namespace { 299 /// GEPIdxComparator - Provide a strong ordering for GEP indices. All Value* 300 /// elements are instances of ConstantInt. 301 /// 302 struct GEPIdxComparator { 303 bool operator()(const std::vector<Value*> &LHS, 304 const std::vector<Value*> &RHS) const { 305 unsigned idx = 0; 306 for (; idx < LHS.size() && idx < RHS.size(); ++idx) { 307 if (LHS[idx] != RHS[idx]) { 308 return cast<ConstantInt>(LHS[idx])->getZExtValue() < 309 cast<ConstantInt>(RHS[idx])->getZExtValue(); 310 } 311 } 312 313 // Return less than if we ran out of stuff in LHS and we didn't run out of 314 // stuff in RHS. 315 return idx == LHS.size() && idx != RHS.size(); 316 } 317 }; 318} 319 320 321/// DoPromotion - This method actually performs the promotion of the specified 322/// arguments, and returns the new function. At this point, we know that it's 323/// safe to do so. 324Function *ArgPromotion::DoPromotion(Function *F, 325 std::vector<Argument*> &Args2Prom) { 326 std::set<Argument*> ArgsToPromote(Args2Prom.begin(), Args2Prom.end()); 327 328 // Start by computing a new prototype for the function, which is the same as 329 // the old function, but has modified arguments. 330 const FunctionType *FTy = F->getFunctionType(); 331 std::vector<const Type*> Params; 332 333 typedef std::set<std::vector<Value*>, GEPIdxComparator> ScalarizeTable; 334 335 // ScalarizedElements - If we are promoting a pointer that has elements 336 // accessed out of it, keep track of which elements are accessed so that we 337 // can add one argument for each. 338 // 339 // Arguments that are directly loaded will have a zero element value here, to 340 // handle cases where there are both a direct load and GEP accesses. 341 // 342 std::map<Argument*, ScalarizeTable> ScalarizedElements; 343 344 // OriginalLoads - Keep track of a representative load instruction from the 345 // original function so that we can tell the alias analysis implementation 346 // what the new GEP/Load instructions we are inserting look like. 347 std::map<std::vector<Value*>, LoadInst*> OriginalLoads; 348 349 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E; ++I) 350 if (!ArgsToPromote.count(I)) { 351 Params.push_back(I->getType()); 352 } else if (I->use_empty()) { 353 ++NumArgumentsDead; 354 } else { 355 // Okay, this is being promoted. Check to see if there are any GEP uses 356 // of the argument. 357 ScalarizeTable &ArgIndices = ScalarizedElements[I]; 358 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E; 359 ++UI) { 360 Instruction *User = cast<Instruction>(*UI); 361 assert(isa<LoadInst>(User) || isa<GetElementPtrInst>(User)); 362 std::vector<Value*> Indices(User->op_begin()+1, User->op_end()); 363 ArgIndices.insert(Indices); 364 LoadInst *OrigLoad; 365 if (LoadInst *L = dyn_cast<LoadInst>(User)) 366 OrigLoad = L; 367 else 368 OrigLoad = cast<LoadInst>(User->use_back()); 369 OriginalLoads[Indices] = OrigLoad; 370 } 371 372 // Add a parameter to the function for each element passed in. 373 for (ScalarizeTable::iterator SI = ArgIndices.begin(), 374 E = ArgIndices.end(); SI != E; ++SI) 375 Params.push_back(GetElementPtrInst::getIndexedType(I->getType(), *SI)); 376 377 if (ArgIndices.size() == 1 && ArgIndices.begin()->empty()) 378 ++NumArgumentsPromoted; 379 else 380 ++NumAggregatesPromoted; 381 } 382 383 const Type *RetTy = FTy->getReturnType(); 384 385 // Work around LLVM bug PR56: the CWriter cannot emit varargs functions which 386 // have zero fixed arguments. 387 bool ExtraArgHack = false; 388 if (Params.empty() && FTy->isVarArg()) { 389 ExtraArgHack = true; 390 Params.push_back(Type::IntTy); 391 } 392 FunctionType *NFTy = FunctionType::get(RetTy, Params, FTy->isVarArg()); 393 394 // Create the new function body and insert it into the module... 395 Function *NF = new Function(NFTy, F->getLinkage(), F->getName()); 396 NF->setCallingConv(F->getCallingConv()); 397 F->getParent()->getFunctionList().insert(F, NF); 398 399 // Get the alias analysis information that we need to update to reflect our 400 // changes. 401 AliasAnalysis &AA = getAnalysis<AliasAnalysis>(); 402 403 // Loop over all of the callers of the function, transforming the call sites 404 // to pass in the loaded pointers. 405 // 406 std::vector<Value*> Args; 407 while (!F->use_empty()) { 408 CallSite CS = CallSite::get(F->use_back()); 409 Instruction *Call = CS.getInstruction(); 410 411 // Loop over the operands, inserting GEP and loads in the caller as 412 // appropriate. 413 CallSite::arg_iterator AI = CS.arg_begin(); 414 for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); 415 I != E; ++I, ++AI) 416 if (!ArgsToPromote.count(I)) 417 Args.push_back(*AI); // Unmodified argument 418 else if (!I->use_empty()) { 419 // Non-dead argument: insert GEPs and loads as appropriate. 420 ScalarizeTable &ArgIndices = ScalarizedElements[I]; 421 for (ScalarizeTable::iterator SI = ArgIndices.begin(), 422 E = ArgIndices.end(); SI != E; ++SI) { 423 Value *V = *AI; 424 LoadInst *OrigLoad = OriginalLoads[*SI]; 425 if (!SI->empty()) { 426 V = new GetElementPtrInst(V, *SI, V->getName()+".idx", Call); 427 AA.copyValue(OrigLoad->getOperand(0), V); 428 } 429 Args.push_back(new LoadInst(V, V->getName()+".val", Call)); 430 AA.copyValue(OrigLoad, Args.back()); 431 } 432 } 433 434 if (ExtraArgHack) 435 Args.push_back(Constant::getNullValue(Type::IntTy)); 436 437 // Push any varargs arguments on the list 438 for (; AI != CS.arg_end(); ++AI) 439 Args.push_back(*AI); 440 441 Instruction *New; 442 if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) { 443 New = new InvokeInst(NF, II->getNormalDest(), II->getUnwindDest(), 444 Args, "", Call); 445 cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv()); 446 } else { 447 New = new CallInst(NF, Args, "", Call); 448 cast<CallInst>(New)->setCallingConv(CS.getCallingConv()); 449 if (cast<CallInst>(Call)->isTailCall()) 450 cast<CallInst>(New)->setTailCall(); 451 } 452 Args.clear(); 453 454 // Update the alias analysis implementation to know that we are replacing 455 // the old call with a new one. 456 AA.replaceWithNewValue(Call, New); 457 458 if (!Call->use_empty()) { 459 Call->replaceAllUsesWith(New); 460 std::string Name = Call->getName(); 461 Call->setName(""); 462 New->setName(Name); 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(), I2 = NF->arg_begin(); 479 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->setName(I->getName()); 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 += "."+itostr((int64_t)CI->getZExtValue()); 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::IntTy), 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