FunctionAttrs.cpp revision a14b31ab8d5ca36fdd0cb4569b728c37e97478e7
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/ADT/SmallSet.h" 30#include "llvm/ADT/Statistic.h" 31#include "llvm/ADT/UniqueVector.h" 32#include "llvm/Support/InstIterator.h" 33using namespace llvm; 34 35STATISTIC(NumReadNone, "Number of functions marked readnone"); 36STATISTIC(NumReadOnly, "Number of functions marked readonly"); 37STATISTIC(NumNoCapture, "Number of arguments marked nocapture"); 38STATISTIC(NumNoAlias, "Number of function returns marked noalias"); 39 40namespace { 41 struct FunctionAttrs : public CallGraphSCCPass { 42 static char ID; // Pass identification, replacement for typeid 43 FunctionAttrs() : CallGraphSCCPass(ID), AA(0) { 44 initializeFunctionAttrsPass(*PassRegistry::getPassRegistry()); 45 } 46 47 // runOnSCC - Analyze the SCC, performing the transformation if possible. 48 bool runOnSCC(CallGraphSCC &SCC); 49 50 // AddReadAttrs - Deduce readonly/readnone attributes for the SCC. 51 bool AddReadAttrs(const CallGraphSCC &SCC); 52 53 // AddNoCaptureAttrs - Deduce nocapture attributes for the SCC. 54 bool AddNoCaptureAttrs(const CallGraphSCC &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 CallGraphSCC &SCC); 62 63 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 64 AU.setPreservesCFG(); 65 AU.addRequired<AliasAnalysis>(); 66 CallGraphSCCPass::getAnalysisUsage(AU); 67 } 68 69 private: 70 AliasAnalysis *AA; 71 }; 72} 73 74char FunctionAttrs::ID = 0; 75INITIALIZE_PASS_BEGIN(FunctionAttrs, "functionattrs", 76 "Deduce function attributes", false, false) 77INITIALIZE_AG_DEPENDENCY(CallGraph) 78INITIALIZE_PASS_END(FunctionAttrs, "functionattrs", 79 "Deduce function attributes", false, false) 80 81Pass *llvm::createFunctionAttrsPass() { return new FunctionAttrs(); } 82 83 84/// AddReadAttrs - Deduce readonly/readnone attributes for the SCC. 85bool FunctionAttrs::AddReadAttrs(const CallGraphSCC &SCC) { 86 SmallPtrSet<Function*, 8> SCCNodes; 87 88 // Fill SCCNodes with the elements of the SCC. Used for quickly 89 // looking up whether a given CallGraphNode is in this SCC. 90 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) 91 SCCNodes.insert((*I)->getFunction()); 92 93 // Check if any of the functions in the SCC read or write memory. If they 94 // write memory then they can't be marked readnone or readonly. 95 bool ReadsMemory = false; 96 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) { 97 Function *F = (*I)->getFunction(); 98 99 if (F == 0) 100 // External node - may write memory. Just give up. 101 return false; 102 103 if (F->doesNotAccessMemory()) 104 // Already perfect! 105 continue; 106 107 // Definitions with weak linkage may be overridden at linktime with 108 // something that writes memory, so treat them like declarations. 109 if (F->isDeclaration() || F->mayBeOverridden()) { 110 if (!F->onlyReadsMemory()) 111 // May write memory. Just give up. 112 return false; 113 114 ReadsMemory = true; 115 continue; 116 } 117 118 // Scan the function body for instructions that may read or write memory. 119 for (inst_iterator II = inst_begin(F), E = inst_end(F); II != E; ++II) { 120 Instruction *I = &*II; 121 122 // Some instructions can be ignored even if they read or write memory. 123 // Detect these now, skipping to the next instruction if one is found. 124 CallSite CS(cast<Value>(I)); 125 if (CS) { 126 // Ignore calls to functions in the same SCC. 127 if (CS.getCalledFunction() && SCCNodes.count(CS.getCalledFunction())) 128 continue; 129 switch (AA->getModRefBehavior(CS)) { 130 case AliasAnalysis::DoesNotAccessMemory: 131 // Ignore calls that don't access memory. 132 continue; 133 case AliasAnalysis::OnlyReadsMemory: 134 // Handle calls that only read from memory. 135 ReadsMemory = true; 136 continue; 137 case AliasAnalysis::AccessesArguments: 138 // Check whether all pointer arguments point to local memory, and 139 // ignore calls that only access local memory. 140 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end(); 141 CI != CE; ++CI) { 142 Value *Arg = *CI; 143 if (Arg->getType()->isPointerTy() && 144 !AA->pointsToConstantMemory(Arg, /*OrLocal=*/true)) 145 // Writes memory. Just give up. 146 return false; 147 } 148 // Only reads and writes local memory. 149 continue; 150 default: 151 // Otherwise, be conservative. 152 break; 153 } 154 } else if (LoadInst *LI = dyn_cast<LoadInst>(I)) { 155 // Ignore non-volatile loads from local memory. 156 if (!LI->isVolatile() && 157 AA->pointsToConstantMemory(LI->getPointerOperand(), 158 /*OrLocal=*/true)) 159 continue; 160 } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) { 161 // Ignore non-volatile stores to local memory. 162 if (!SI->isVolatile() && 163 AA->pointsToConstantMemory(SI->getPointerOperand(), 164 /*OrLocal=*/true)) 165 continue; 166 } 167 168 // Any remaining instructions need to be taken seriously! Check if they 169 // read or write memory. 170 if (I->mayWriteToMemory()) 171 // Writes memory. Just give up. 172 return false; 173 174 // If this instruction may read memory, remember that. 175 ReadsMemory |= I->mayReadFromMemory(); 176 } 177 } 178 179 // Success! Functions in this SCC do not access memory, or only read memory. 180 // Give them the appropriate attribute. 181 bool MadeChange = false; 182 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) { 183 Function *F = (*I)->getFunction(); 184 185 if (F->doesNotAccessMemory()) 186 // Already perfect! 187 continue; 188 189 if (F->onlyReadsMemory() && ReadsMemory) 190 // No change. 191 continue; 192 193 MadeChange = true; 194 195 // Clear out any existing attributes. 196 F->removeAttribute(~0, Attribute::ReadOnly | Attribute::ReadNone); 197 198 // Add in the new attribute. 199 F->addAttribute(~0, ReadsMemory? Attribute::ReadOnly : Attribute::ReadNone); 200 201 if (ReadsMemory) 202 ++NumReadOnly; 203 else 204 ++NumReadNone; 205 } 206 207 return MadeChange; 208} 209 210/// AddNoCaptureAttrs - Deduce nocapture attributes for the SCC. 211bool FunctionAttrs::AddNoCaptureAttrs(const CallGraphSCC &SCC) { 212 bool Changed = false; 213 214 // Check each function in turn, determining which pointer arguments are not 215 // captured. 216 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) { 217 Function *F = (*I)->getFunction(); 218 219 if (F == 0) 220 // External node - skip it; 221 continue; 222 223 // Definitions with weak linkage may be overridden at linktime with 224 // something that writes memory, so treat them like declarations. 225 if (F->isDeclaration() || F->mayBeOverridden()) 226 continue; 227 228 for (Function::arg_iterator A = F->arg_begin(), E = F->arg_end(); A!=E; ++A) 229 if (A->getType()->isPointerTy() && !A->hasNoCaptureAttr() && 230 !PointerMayBeCaptured(A, true, /*StoreCaptures=*/false)) { 231 A->addAttr(Attribute::NoCapture); 232 ++NumNoCapture; 233 Changed = true; 234 } 235 } 236 237 return Changed; 238} 239 240/// IsFunctionMallocLike - A function is malloc-like if it returns either null 241/// or a pointer that doesn't alias any other pointer visible to the caller. 242bool FunctionAttrs::IsFunctionMallocLike(Function *F, 243 SmallPtrSet<Function*, 8> &SCCNodes) const { 244 UniqueVector<Value *> FlowsToReturn; 245 for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) 246 if (ReturnInst *Ret = dyn_cast<ReturnInst>(I->getTerminator())) 247 FlowsToReturn.insert(Ret->getReturnValue()); 248 249 for (unsigned i = 0; i != FlowsToReturn.size(); ++i) { 250 Value *RetVal = FlowsToReturn[i+1]; // UniqueVector[0] is reserved. 251 252 if (Constant *C = dyn_cast<Constant>(RetVal)) { 253 if (!C->isNullValue() && !isa<UndefValue>(C)) 254 return false; 255 256 continue; 257 } 258 259 if (isa<Argument>(RetVal)) 260 return false; 261 262 if (Instruction *RVI = dyn_cast<Instruction>(RetVal)) 263 switch (RVI->getOpcode()) { 264 // Extend the analysis by looking upwards. 265 case Instruction::BitCast: 266 case Instruction::GetElementPtr: 267 FlowsToReturn.insert(RVI->getOperand(0)); 268 continue; 269 case Instruction::Select: { 270 SelectInst *SI = cast<SelectInst>(RVI); 271 FlowsToReturn.insert(SI->getTrueValue()); 272 FlowsToReturn.insert(SI->getFalseValue()); 273 continue; 274 } 275 case Instruction::PHI: { 276 PHINode *PN = cast<PHINode>(RVI); 277 for (int i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 278 FlowsToReturn.insert(PN->getIncomingValue(i)); 279 continue; 280 } 281 282 // Check whether the pointer came from an allocation. 283 case Instruction::Alloca: 284 break; 285 case Instruction::Call: 286 case Instruction::Invoke: { 287 CallSite CS(RVI); 288 if (CS.paramHasAttr(0, Attribute::NoAlias)) 289 break; 290 if (CS.getCalledFunction() && 291 SCCNodes.count(CS.getCalledFunction())) 292 break; 293 } // fall-through 294 default: 295 return false; // Did not come from an allocation. 296 } 297 298 if (PointerMayBeCaptured(RetVal, false, /*StoreCaptures=*/false)) 299 return false; 300 } 301 302 return true; 303} 304 305/// AddNoAliasAttrs - Deduce noalias attributes for the SCC. 306bool FunctionAttrs::AddNoAliasAttrs(const CallGraphSCC &SCC) { 307 SmallPtrSet<Function*, 8> SCCNodes; 308 309 // Fill SCCNodes with the elements of the SCC. Used for quickly 310 // looking up whether a given CallGraphNode is in this SCC. 311 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) 312 SCCNodes.insert((*I)->getFunction()); 313 314 // Check each function in turn, determining which functions return noalias 315 // pointers. 316 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) { 317 Function *F = (*I)->getFunction(); 318 319 if (F == 0) 320 // External node - skip it; 321 return false; 322 323 // Already noalias. 324 if (F->doesNotAlias(0)) 325 continue; 326 327 // Definitions with weak linkage may be overridden at linktime, so 328 // treat them like declarations. 329 if (F->isDeclaration() || F->mayBeOverridden()) 330 return false; 331 332 // We annotate noalias return values, which are only applicable to 333 // pointer types. 334 if (!F->getReturnType()->isPointerTy()) 335 continue; 336 337 if (!IsFunctionMallocLike(F, SCCNodes)) 338 return false; 339 } 340 341 bool MadeChange = false; 342 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) { 343 Function *F = (*I)->getFunction(); 344 if (F->doesNotAlias(0) || !F->getReturnType()->isPointerTy()) 345 continue; 346 347 F->setDoesNotAlias(0); 348 ++NumNoAlias; 349 MadeChange = true; 350 } 351 352 return MadeChange; 353} 354 355bool FunctionAttrs::runOnSCC(CallGraphSCC &SCC) { 356 AA = &getAnalysis<AliasAnalysis>(); 357 358 bool Changed = AddReadAttrs(SCC); 359 Changed |= AddNoCaptureAttrs(SCC); 360 Changed |= AddNoAliasAttrs(SCC); 361 return Changed; 362} 363