FunctionAttrs.cpp revision f5a86f45e75ec744c203270ffa03659eb0a220c1
1//===- FunctionAttrs.cpp - Pass which marks functions readnone or readonly ===// 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 file implements a simple interprocedural pass which walks the 11// call-graph, looking for functions which do not access or only read 12// non-local memory, and marking them readnone/readonly. In addition, 13// it marks function arguments (of pointer type) 'nocapture' if a call 14// to the function does not create any copies of the pointer value that 15// outlive the call. This more or less means that the pointer is only 16// dereferenced, and not returned from the function or stored in a global. 17// This pass is implemented as a bottom-up traversal of the call-graph. 18// 19//===----------------------------------------------------------------------===// 20 21#define DEBUG_TYPE "functionattrs" 22#include "llvm/Transforms/IPO.h" 23#include "llvm/CallGraphSCCPass.h" 24#include "llvm/GlobalVariable.h" 25#include "llvm/IntrinsicInst.h" 26#include "llvm/Analysis/AliasAnalysis.h" 27#include "llvm/Analysis/CallGraph.h" 28#include "llvm/Analysis/CaptureTracking.h" 29#include "llvm/Analysis/MallocHelper.h" 30#include "llvm/ADT/SmallSet.h" 31#include "llvm/ADT/Statistic.h" 32#include "llvm/ADT/UniqueVector.h" 33#include "llvm/Support/InstIterator.h" 34using namespace llvm; 35 36STATISTIC(NumReadNone, "Number of functions marked readnone"); 37STATISTIC(NumReadOnly, "Number of functions marked readonly"); 38STATISTIC(NumNoCapture, "Number of arguments marked nocapture"); 39STATISTIC(NumNoAlias, "Number of function returns marked noalias"); 40 41namespace { 42 struct FunctionAttrs : public CallGraphSCCPass { 43 static char ID; // Pass identification, replacement for typeid 44 FunctionAttrs() : CallGraphSCCPass(&ID) {} 45 46 // runOnSCC - Analyze the SCC, performing the transformation if possible. 47 bool runOnSCC(std::vector<CallGraphNode *> &SCC); 48 49 // AddReadAttrs - Deduce readonly/readnone attributes for the SCC. 50 bool AddReadAttrs(const std::vector<CallGraphNode *> &SCC); 51 52 // AddNoCaptureAttrs - Deduce nocapture attributes for the SCC. 53 bool AddNoCaptureAttrs(const std::vector<CallGraphNode *> &SCC); 54 55 // IsFunctionMallocLike - Does this function allocate new memory? 56 bool IsFunctionMallocLike(Function *F, 57 SmallPtrSet<Function*, 8> &) const; 58 59 // AddNoAliasAttrs - Deduce noalias attributes for the SCC. 60 bool AddNoAliasAttrs(const std::vector<CallGraphNode *> &SCC); 61 62 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 63 AU.setPreservesCFG(); 64 CallGraphSCCPass::getAnalysisUsage(AU); 65 } 66 67 bool PointsToLocalMemory(Value *V); 68 }; 69} 70 71char FunctionAttrs::ID = 0; 72static RegisterPass<FunctionAttrs> 73X("functionattrs", "Deduce function attributes"); 74 75Pass *llvm::createFunctionAttrsPass() { return new FunctionAttrs(); } 76 77 78/// PointsToLocalMemory - Returns whether the given pointer value points to 79/// memory that is local to the function. Global constants are considered 80/// local to all functions. 81bool FunctionAttrs::PointsToLocalMemory(Value *V) { 82 V = V->getUnderlyingObject(); 83 // An alloca instruction defines local memory. 84 if (isa<AllocaInst>(V)) 85 return true; 86 // A global constant counts as local memory for our purposes. 87 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) 88 return GV->isConstant(); 89 // Could look through phi nodes and selects here, but it doesn't seem 90 // to be useful in practice. 91 return false; 92} 93 94/// AddReadAttrs - Deduce readonly/readnone attributes for the SCC. 95bool FunctionAttrs::AddReadAttrs(const std::vector<CallGraphNode *> &SCC) { 96 SmallPtrSet<Function*, 8> SCCNodes; 97 98 // Fill SCCNodes with the elements of the SCC. Used for quickly 99 // looking up whether a given CallGraphNode is in this SCC. 100 for (unsigned i = 0, e = SCC.size(); i != e; ++i) 101 SCCNodes.insert(SCC[i]->getFunction()); 102 103 // Check if any of the functions in the SCC read or write memory. If they 104 // write memory then they can't be marked readnone or readonly. 105 bool ReadsMemory = false; 106 for (unsigned i = 0, e = SCC.size(); i != e; ++i) { 107 Function *F = SCC[i]->getFunction(); 108 109 if (F == 0) 110 // External node - may write memory. Just give up. 111 return false; 112 113 if (F->doesNotAccessMemory()) 114 // Already perfect! 115 continue; 116 117 // Definitions with weak linkage may be overridden at linktime with 118 // something that writes memory, so treat them like declarations. 119 if (F->isDeclaration() || F->mayBeOverridden()) { 120 if (!F->onlyReadsMemory()) 121 // May write memory. Just give up. 122 return false; 123 124 ReadsMemory = true; 125 continue; 126 } 127 128 // Scan the function body for instructions that may read or write memory. 129 for (inst_iterator II = inst_begin(F), E = inst_end(F); II != E; ++II) { 130 Instruction *I = &*II; 131 132 // Some instructions can be ignored even if they read or write memory. 133 // Detect these now, skipping to the next instruction if one is found. 134 CallSite CS = CallSite::get(I); 135 if (CS.getInstruction() && CS.getCalledFunction()) { 136 // Ignore calls to functions in the same SCC. 137 if (SCCNodes.count(CS.getCalledFunction())) 138 continue; 139 } else if (LoadInst *LI = dyn_cast<LoadInst>(I)) { 140 // Ignore loads from local memory. 141 if (PointsToLocalMemory(LI->getPointerOperand())) 142 continue; 143 } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) { 144 // Ignore stores to local memory. 145 if (PointsToLocalMemory(SI->getPointerOperand())) 146 continue; 147 } 148 149 // Any remaining instructions need to be taken seriously! Check if they 150 // read or write memory. 151 if (I->mayWriteToMemory()) 152 // Writes memory. Just give up. 153 return false; 154 155 if (isMalloc(I)) 156 // malloc claims not to write memory! PR3754. 157 return false; 158 159 // If this instruction may read memory, remember that. 160 ReadsMemory |= I->mayReadFromMemory(); 161 } 162 } 163 164 // Success! Functions in this SCC do not access memory, or only read memory. 165 // Give them the appropriate attribute. 166 bool MadeChange = false; 167 for (unsigned i = 0, e = SCC.size(); i != e; ++i) { 168 Function *F = SCC[i]->getFunction(); 169 170 if (F->doesNotAccessMemory()) 171 // Already perfect! 172 continue; 173 174 if (F->onlyReadsMemory() && ReadsMemory) 175 // No change. 176 continue; 177 178 MadeChange = true; 179 180 // Clear out any existing attributes. 181 F->removeAttribute(~0, Attribute::ReadOnly | Attribute::ReadNone); 182 183 // Add in the new attribute. 184 F->addAttribute(~0, ReadsMemory? Attribute::ReadOnly : Attribute::ReadNone); 185 186 if (ReadsMemory) 187 ++NumReadOnly; 188 else 189 ++NumReadNone; 190 } 191 192 return MadeChange; 193} 194 195/// AddNoCaptureAttrs - Deduce nocapture attributes for the SCC. 196bool FunctionAttrs::AddNoCaptureAttrs(const std::vector<CallGraphNode *> &SCC) { 197 bool Changed = false; 198 199 // Check each function in turn, determining which pointer arguments are not 200 // captured. 201 for (unsigned i = 0, e = SCC.size(); i != e; ++i) { 202 Function *F = SCC[i]->getFunction(); 203 204 if (F == 0) 205 // External node - skip it; 206 continue; 207 208 // Definitions with weak linkage may be overridden at linktime with 209 // something that writes memory, so treat them like declarations. 210 if (F->isDeclaration() || F->mayBeOverridden()) 211 continue; 212 213 for (Function::arg_iterator A = F->arg_begin(), E = F->arg_end(); A!=E; ++A) 214 if (isa<PointerType>(A->getType()) && !A->hasNoCaptureAttr() && 215 !PointerMayBeCaptured(A, true)) { 216 A->addAttr(Attribute::NoCapture); 217 ++NumNoCapture; 218 Changed = true; 219 } 220 } 221 222 return Changed; 223} 224 225/// IsFunctionMallocLike - A function is malloc-like if it returns either null 226/// or a pointer that doesn't alias any other pointer visible to the caller. 227bool FunctionAttrs::IsFunctionMallocLike(Function *F, 228 SmallPtrSet<Function*, 8> &SCCNodes) const { 229 UniqueVector<Value *> FlowsToReturn; 230 for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) 231 if (ReturnInst *Ret = dyn_cast<ReturnInst>(I->getTerminator())) 232 FlowsToReturn.insert(Ret->getReturnValue()); 233 234 for (unsigned i = 0; i != FlowsToReturn.size(); ++i) { 235 Value *RetVal = FlowsToReturn[i+1]; // UniqueVector[0] is reserved. 236 237 if (Constant *C = dyn_cast<Constant>(RetVal)) { 238 if (!C->isNullValue() && !isa<UndefValue>(C)) 239 return false; 240 241 continue; 242 } 243 244 if (isa<Argument>(RetVal)) 245 return false; 246 247 if (Instruction *RVI = dyn_cast<Instruction>(RetVal)) 248 switch (RVI->getOpcode()) { 249 // Extend the analysis by looking upwards. 250 case Instruction::BitCast: 251 case Instruction::GetElementPtr: 252 FlowsToReturn.insert(RVI->getOperand(0)); 253 continue; 254 case Instruction::Select: { 255 SelectInst *SI = cast<SelectInst>(RVI); 256 FlowsToReturn.insert(SI->getTrueValue()); 257 FlowsToReturn.insert(SI->getFalseValue()); 258 continue; 259 } 260 case Instruction::PHI: { 261 PHINode *PN = cast<PHINode>(RVI); 262 for (int i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 263 FlowsToReturn.insert(PN->getIncomingValue(i)); 264 continue; 265 } 266 267 // Check whether the pointer came from an allocation. 268 case Instruction::Alloca: 269 break; 270 case Instruction::Call: 271 case Instruction::Invoke: { 272 CallSite CS(RVI); 273 if (CS.paramHasAttr(0, Attribute::NoAlias)) 274 break; 275 if (CS.getCalledFunction() && 276 SCCNodes.count(CS.getCalledFunction())) 277 break; 278 } // fall-through 279 default: 280 return false; // Did not come from an allocation. 281 } 282 283 if (PointerMayBeCaptured(RetVal, false)) 284 return false; 285 } 286 287 return true; 288} 289 290/// AddNoAliasAttrs - Deduce noalias attributes for the SCC. 291bool FunctionAttrs::AddNoAliasAttrs(const std::vector<CallGraphNode *> &SCC) { 292 SmallPtrSet<Function*, 8> SCCNodes; 293 294 // Fill SCCNodes with the elements of the SCC. Used for quickly 295 // looking up whether a given CallGraphNode is in this SCC. 296 for (unsigned i = 0, e = SCC.size(); i != e; ++i) 297 SCCNodes.insert(SCC[i]->getFunction()); 298 299 // Check each function in turn, determining which functions return noalias 300 // pointers. 301 for (unsigned i = 0, e = SCC.size(); i != e; ++i) { 302 Function *F = SCC[i]->getFunction(); 303 304 if (F == 0) 305 // External node - skip it; 306 return false; 307 308 // Already noalias. 309 if (F->doesNotAlias(0)) 310 continue; 311 312 // Definitions with weak linkage may be overridden at linktime, so 313 // treat them like declarations. 314 if (F->isDeclaration() || F->mayBeOverridden()) 315 return false; 316 317 // We annotate noalias return values, which are only applicable to 318 // pointer types. 319 if (!isa<PointerType>(F->getReturnType())) 320 continue; 321 322 if (!IsFunctionMallocLike(F, SCCNodes)) 323 return false; 324 } 325 326 bool MadeChange = false; 327 for (unsigned i = 0, e = SCC.size(); i != e; ++i) { 328 Function *F = SCC[i]->getFunction(); 329 if (F->doesNotAlias(0) || !isa<PointerType>(F->getReturnType())) 330 continue; 331 332 F->setDoesNotAlias(0); 333 ++NumNoAlias; 334 MadeChange = true; 335 } 336 337 return MadeChange; 338} 339 340bool FunctionAttrs::runOnSCC(std::vector<CallGraphNode *> &SCC) { 341 bool Changed = AddReadAttrs(SCC); 342 Changed |= AddNoCaptureAttrs(SCC); 343 Changed |= AddNoAliasAttrs(SCC); 344 return Changed; 345} 346