FunctionAttrs.cpp revision 9cc582afe2902cd8695c8d55f87534fc681372cb
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/Compiler.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 VISIBILITY_HIDDEN 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(const 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<CallGraphNode*, 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<CallGraphNode*, 8> SCCNodes; 97 CallGraph &CG = getAnalysis<CallGraph>(); 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]); 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()) { 137 // Ignore calls to functions in the same SCC. 138 if (SCCNodes.count(CG[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 // Ignore dbg info intrinsics. 151 if (isa<DbgInfoIntrinsic>(I)) 152 continue; 153 154 // Any remaining instructions need to be taken seriously! Check if they 155 // read or write memory. 156 if (I->mayWriteToMemory()) 157 // Writes memory. Just give up. 158 return false; 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<CallGraphNode*, 8> &SCCNodes) const { 229 CallGraph &CG = getAnalysis<CallGraph>(); 230 231 UniqueVector<Value *> FlowsToReturn; 232 for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) 233 if (ReturnInst *Ret = dyn_cast<ReturnInst>(I->getTerminator())) 234 FlowsToReturn.insert(Ret->getReturnValue()); 235 236 for (unsigned i = 0; i != FlowsToReturn.size(); ++i) { 237 Value *RetVal = FlowsToReturn[i+1]; // UniqueVector[0] is reserved. 238 239 if (Constant *C = dyn_cast<Constant>(RetVal)) { 240 if (!C->isNullValue() && !isa<UndefValue>(C)) 241 return false; 242 243 continue; 244 } 245 246 if (isa<Argument>(RetVal)) 247 return false; 248 249 if (Instruction *RVI = dyn_cast<Instruction>(RetVal)) 250 switch (RVI->getOpcode()) { 251 // Extend the analysis by looking upwards. 252 case Instruction::GetElementPtr: 253 case Instruction::BitCast: 254 FlowsToReturn.insert(RVI->getOperand(0)); 255 continue; 256 case Instruction::Select: { 257 SelectInst *SI = cast<SelectInst>(RVI); 258 FlowsToReturn.insert(SI->getTrueValue()); 259 FlowsToReturn.insert(SI->getFalseValue()); 260 } continue; 261 case Instruction::PHI: { 262 PHINode *PN = cast<PHINode>(RVI); 263 for (int i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 264 FlowsToReturn.insert(PN->getIncomingValue(i)); 265 } continue; 266 267 // Check whether the pointer came from an allocation. 268 case Instruction::Alloca: 269 case Instruction::Malloc: 270 break; 271 case Instruction::Call: 272 case Instruction::Invoke: { 273 CallSite CS(RVI); 274 if (CS.paramHasAttr(0, Attribute::NoAlias)) 275 break; 276 if (CS.getCalledFunction() && 277 SCCNodes.count(CG[CS.getCalledFunction()])) 278 break; 279 } // fall-through 280 default: 281 return false; // Did not come from an allocation. 282 } 283 284 if (PointerMayBeCaptured(RetVal, false)) 285 return false; 286 } 287 288 return true; 289} 290 291/// AddNoAliasAttrs - Deduce noalias attributes for the SCC. 292bool FunctionAttrs::AddNoAliasAttrs(const std::vector<CallGraphNode *> &SCC) { 293 SmallPtrSet<CallGraphNode*, 8> SCCNodes; 294 295 // Fill SCCNodes with the elements of the SCC. Used for quickly 296 // looking up whether a given CallGraphNode is in this SCC. 297 for (unsigned i = 0, e = SCC.size(); i != e; ++i) 298 SCCNodes.insert(SCC[i]); 299 300 // Check each function in turn, determining which functions return noalias 301 // pointers. 302 for (unsigned i = 0, e = SCC.size(); i != e; ++i) { 303 Function *F = SCC[i]->getFunction(); 304 305 if (F == 0) 306 // External node - skip it; 307 return false; 308 309 // Already noalias. 310 if (F->doesNotAlias(0)) 311 continue; 312 313 // Definitions with weak linkage may be overridden at linktime, so 314 // treat them like declarations. 315 if (F->isDeclaration() || F->mayBeOverridden()) 316 return false; 317 318 // We annotate noalias return values, which are only applicable to 319 // pointer types. 320 if (!isa<PointerType>(F->getReturnType())) 321 continue; 322 323 if (!IsFunctionMallocLike(F, SCCNodes)) 324 return false; 325 } 326 327 bool MadeChange = false; 328 for (unsigned i = 0, e = SCC.size(); i != e; ++i) { 329 Function *F = SCC[i]->getFunction(); 330 if (F->doesNotAlias(0) || !isa<PointerType>(F->getReturnType())) 331 continue; 332 333 F->setDoesNotAlias(0); 334 ++NumNoAlias; 335 MadeChange = true; 336 } 337 338 return MadeChange; 339} 340 341bool FunctionAttrs::runOnSCC(const std::vector<CallGraphNode *> &SCC) { 342 bool Changed = AddReadAttrs(SCC); 343 Changed |= AddNoCaptureAttrs(SCC); 344 Changed |= AddNoAliasAttrs(SCC); 345 return Changed; 346} 347