FunctionAttrs.cpp revision 83d63919bd990ce00f62e18114504b9e4a5cb35e
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/Compiler.h" 34#include "llvm/Support/InstIterator.h" 35using namespace llvm; 36 37STATISTIC(NumReadNone, "Number of functions marked readnone"); 38STATISTIC(NumReadOnly, "Number of functions marked readonly"); 39STATISTIC(NumNoCapture, "Number of arguments marked nocapture"); 40STATISTIC(NumNoAlias, "Number of function returns marked noalias"); 41 42namespace { 43 struct VISIBILITY_HIDDEN FunctionAttrs : public CallGraphSCCPass { 44 static char ID; // Pass identification, replacement for typeid 45 FunctionAttrs() : CallGraphSCCPass(&ID) {} 46 47 // runOnSCC - Analyze the SCC, performing the transformation if possible. 48 bool runOnSCC(std::vector<CallGraphNode *> &SCC); 49 50 // AddReadAttrs - Deduce readonly/readnone attributes for the SCC. 51 bool AddReadAttrs(const std::vector<CallGraphNode *> &SCC); 52 53 // AddNoCaptureAttrs - Deduce nocapture attributes for the SCC. 54 bool AddNoCaptureAttrs(const std::vector<CallGraphNode *> &SCC); 55 56 // IsFunctionMallocLike - Does this function allocate new memory? 57 bool IsFunctionMallocLike(Function *F, 58 SmallPtrSet<Function*, 8> &) const; 59 60 // AddNoAliasAttrs - Deduce noalias attributes for the SCC. 61 bool AddNoAliasAttrs(const std::vector<CallGraphNode *> &SCC); 62 63 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 64 AU.setPreservesCFG(); 65 CallGraphSCCPass::getAnalysisUsage(AU); 66 } 67 68 bool PointsToLocalMemory(Value *V); 69 }; 70} 71 72char FunctionAttrs::ID = 0; 73static RegisterPass<FunctionAttrs> 74X("functionattrs", "Deduce function attributes"); 75 76Pass *llvm::createFunctionAttrsPass() { return new FunctionAttrs(); } 77 78 79/// PointsToLocalMemory - Returns whether the given pointer value points to 80/// memory that is local to the function. Global constants are considered 81/// local to all functions. 82bool FunctionAttrs::PointsToLocalMemory(Value *V) { 83 V = V->getUnderlyingObject(); 84 // An alloca instruction defines local memory. 85 if (isa<AllocaInst>(V)) 86 return true; 87 // A global constant counts as local memory for our purposes. 88 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) 89 return GV->isConstant(); 90 // Could look through phi nodes and selects here, but it doesn't seem 91 // to be useful in practice. 92 return false; 93} 94 95/// AddReadAttrs - Deduce readonly/readnone attributes for the SCC. 96bool FunctionAttrs::AddReadAttrs(const std::vector<CallGraphNode *> &SCC) { 97 SmallPtrSet<Function*, 8> SCCNodes; 98 99 // Fill SCCNodes with the elements of the SCC. Used for quickly 100 // looking up whether a given CallGraphNode is in this SCC. 101 for (unsigned i = 0, e = SCC.size(); i != e; ++i) 102 SCCNodes.insert(SCC[i]->getFunction()); 103 104 // Check if any of the functions in the SCC read or write memory. If they 105 // write memory then they can't be marked readnone or readonly. 106 bool ReadsMemory = false; 107 for (unsigned i = 0, e = SCC.size(); i != e; ++i) { 108 Function *F = SCC[i]->getFunction(); 109 110 if (F == 0) 111 // External node - may write memory. Just give up. 112 return false; 113 114 if (F->doesNotAccessMemory()) 115 // Already perfect! 116 continue; 117 118 // Definitions with weak linkage may be overridden at linktime with 119 // something that writes memory, so treat them like declarations. 120 if (F->isDeclaration() || F->mayBeOverridden()) { 121 if (!F->onlyReadsMemory()) 122 // May write memory. Just give up. 123 return false; 124 125 ReadsMemory = true; 126 continue; 127 } 128 129 // Scan the function body for instructions that may read or write memory. 130 for (inst_iterator II = inst_begin(F), E = inst_end(F); II != E; ++II) { 131 Instruction *I = &*II; 132 133 // Some instructions can be ignored even if they read or write memory. 134 // Detect these now, skipping to the next instruction if one is found. 135 CallSite CS = CallSite::get(I); 136 if (CS.getInstruction() && CS.getCalledFunction()) { 137 // Ignore calls to functions in the same SCC. 138 if (SCCNodes.count(CS.getCalledFunction())) 139 continue; 140 } else if (LoadInst *LI = dyn_cast<LoadInst>(I)) { 141 // Ignore loads from local memory. 142 if (PointsToLocalMemory(LI->getPointerOperand())) 143 continue; 144 } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) { 145 // Ignore stores to local memory. 146 if (PointsToLocalMemory(SI->getPointerOperand())) 147 continue; 148 } 149 150 // Any remaining instructions need to be taken seriously! Check if they 151 // read or write memory. 152 if (I->mayWriteToMemory()) 153 // Writes memory. Just give up. 154 return false; 155 156 if (isa<MallocInst>(I) || isMalloc(I)) 157 // malloc claims not to write memory! PR3754. 158 return false; 159 160 // If this instruction may read memory, remember that. 161 ReadsMemory |= I->mayReadFromMemory(); 162 } 163 } 164 165 // Success! Functions in this SCC do not access memory, or only read memory. 166 // Give them the appropriate attribute. 167 bool MadeChange = false; 168 for (unsigned i = 0, e = SCC.size(); i != e; ++i) { 169 Function *F = SCC[i]->getFunction(); 170 171 if (F->doesNotAccessMemory()) 172 // Already perfect! 173 continue; 174 175 if (F->onlyReadsMemory() && ReadsMemory) 176 // No change. 177 continue; 178 179 MadeChange = true; 180 181 // Clear out any existing attributes. 182 F->removeAttribute(~0, Attribute::ReadOnly | Attribute::ReadNone); 183 184 // Add in the new attribute. 185 F->addAttribute(~0, ReadsMemory? Attribute::ReadOnly : Attribute::ReadNone); 186 187 if (ReadsMemory) 188 ++NumReadOnly; 189 else 190 ++NumReadNone; 191 } 192 193 return MadeChange; 194} 195 196/// AddNoCaptureAttrs - Deduce nocapture attributes for the SCC. 197bool FunctionAttrs::AddNoCaptureAttrs(const std::vector<CallGraphNode *> &SCC) { 198 bool Changed = false; 199 200 // Check each function in turn, determining which pointer arguments are not 201 // captured. 202 for (unsigned i = 0, e = SCC.size(); i != e; ++i) { 203 Function *F = SCC[i]->getFunction(); 204 205 if (F == 0) 206 // External node - skip it; 207 continue; 208 209 // Definitions with weak linkage may be overridden at linktime with 210 // something that writes memory, so treat them like declarations. 211 if (F->isDeclaration() || F->mayBeOverridden()) 212 continue; 213 214 for (Function::arg_iterator A = F->arg_begin(), E = F->arg_end(); A!=E; ++A) 215 if (isa<PointerType>(A->getType()) && !A->hasNoCaptureAttr() && 216 !PointerMayBeCaptured(A, true)) { 217 A->addAttr(Attribute::NoCapture); 218 ++NumNoCapture; 219 Changed = true; 220 } 221 } 222 223 return Changed; 224} 225 226/// IsFunctionMallocLike - A function is malloc-like if it returns either null 227/// or a pointer that doesn't alias any other pointer visible to the caller. 228bool FunctionAttrs::IsFunctionMallocLike(Function *F, 229 SmallPtrSet<Function*, 8> &SCCNodes) const { 230 UniqueVector<Value *> FlowsToReturn; 231 for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) 232 if (ReturnInst *Ret = dyn_cast<ReturnInst>(I->getTerminator())) 233 FlowsToReturn.insert(Ret->getReturnValue()); 234 235 for (unsigned i = 0; i != FlowsToReturn.size(); ++i) { 236 Value *RetVal = FlowsToReturn[i+1]; // UniqueVector[0] is reserved. 237 238 if (Constant *C = dyn_cast<Constant>(RetVal)) { 239 if (!C->isNullValue() && !isa<UndefValue>(C)) 240 return false; 241 242 continue; 243 } 244 245 if (isa<Argument>(RetVal)) 246 return false; 247 248 if (Instruction *RVI = dyn_cast<Instruction>(RetVal)) 249 switch (RVI->getOpcode()) { 250 // Extend the analysis by looking upwards. 251 case Instruction::BitCast: 252 if (isMalloc(RVI)) 253 break; 254 // fall through 255 case Instruction::GetElementPtr: 256 FlowsToReturn.insert(RVI->getOperand(0)); 257 continue; 258 case Instruction::Select: { 259 SelectInst *SI = cast<SelectInst>(RVI); 260 FlowsToReturn.insert(SI->getTrueValue()); 261 FlowsToReturn.insert(SI->getFalseValue()); 262 } continue; 263 case Instruction::PHI: { 264 PHINode *PN = cast<PHINode>(RVI); 265 for (int i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 266 FlowsToReturn.insert(PN->getIncomingValue(i)); 267 } continue; 268 269 // Check whether the pointer came from an allocation. 270 case Instruction::Alloca: 271 case Instruction::Malloc: 272 break; 273 case Instruction::Call: 274 if (isMalloc(RVI)) 275 break; 276 case Instruction::Invoke: { 277 CallSite CS(RVI); 278 if (CS.paramHasAttr(0, Attribute::NoAlias)) 279 break; 280 if (CS.getCalledFunction() && 281 SCCNodes.count(CS.getCalledFunction())) 282 break; 283 } // fall-through 284 default: 285 return false; // Did not come from an allocation. 286 } 287 288 if (PointerMayBeCaptured(RetVal, false)) 289 return false; 290 } 291 292 return true; 293} 294 295/// AddNoAliasAttrs - Deduce noalias attributes for the SCC. 296bool FunctionAttrs::AddNoAliasAttrs(const std::vector<CallGraphNode *> &SCC) { 297 SmallPtrSet<Function*, 8> SCCNodes; 298 299 // Fill SCCNodes with the elements of the SCC. Used for quickly 300 // looking up whether a given CallGraphNode is in this SCC. 301 for (unsigned i = 0, e = SCC.size(); i != e; ++i) 302 SCCNodes.insert(SCC[i]->getFunction()); 303 304 // Check each function in turn, determining which functions return noalias 305 // pointers. 306 for (unsigned i = 0, e = SCC.size(); i != e; ++i) { 307 Function *F = SCC[i]->getFunction(); 308 309 if (F == 0) 310 // External node - skip it; 311 return false; 312 313 // Already noalias. 314 if (F->doesNotAlias(0)) 315 continue; 316 317 // Definitions with weak linkage may be overridden at linktime, so 318 // treat them like declarations. 319 if (F->isDeclaration() || F->mayBeOverridden()) 320 return false; 321 322 // We annotate noalias return values, which are only applicable to 323 // pointer types. 324 if (!isa<PointerType>(F->getReturnType())) 325 continue; 326 327 if (!IsFunctionMallocLike(F, SCCNodes)) 328 return false; 329 } 330 331 bool MadeChange = false; 332 for (unsigned i = 0, e = SCC.size(); i != e; ++i) { 333 Function *F = SCC[i]->getFunction(); 334 if (F->doesNotAlias(0) || !isa<PointerType>(F->getReturnType())) 335 continue; 336 337 F->setDoesNotAlias(0); 338 ++NumNoAlias; 339 MadeChange = true; 340 } 341 342 return MadeChange; 343} 344 345bool FunctionAttrs::runOnSCC(std::vector<CallGraphNode *> &SCC) { 346 bool Changed = AddReadAttrs(SCC); 347 Changed |= AddNoCaptureAttrs(SCC); 348 Changed |= AddNoAliasAttrs(SCC); 349 return Changed; 350} 351