FunctionAttrs.cpp revision 90c579de5a383cee278acc3f7e7b9d0a656e6a35
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/MemoryBuiltins.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(CallGraphSCC &SCC); 48 49 // AddReadAttrs - Deduce readonly/readnone attributes for the SCC. 50 bool AddReadAttrs(const CallGraphSCC &SCC); 51 52 // AddNoCaptureAttrs - Deduce nocapture attributes for the SCC. 53 bool AddNoCaptureAttrs(const CallGraphSCC &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 CallGraphSCC &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; 72INITIALIZE_PASS(FunctionAttrs, "functionattrs", 73 "Deduce function attributes", false, false); 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 SmallVector<Value*, 16> Worklist; 83 unsigned MaxLookup = 8; 84 85 Worklist.push_back(V); 86 87 do { 88 V = Worklist.pop_back_val()->getUnderlyingObject(); 89 90 // An alloca instruction defines local memory. 91 if (isa<AllocaInst>(V)) 92 continue; 93 94 // A global constant counts as local memory for our purposes. 95 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) { 96 if (!GV->isConstant()) 97 return false; 98 continue; 99 } 100 101 // If both select values point to local memory, then so does the select. 102 if (SelectInst *SI = dyn_cast<SelectInst>(V)) { 103 Worklist.push_back(SI->getTrueValue()); 104 Worklist.push_back(SI->getFalseValue()); 105 continue; 106 } 107 108 // If all values incoming to a phi node point to local memory, then so does 109 // the phi. 110 if (PHINode *PN = dyn_cast<PHINode>(V)) { 111 // Don't bother inspecting phi nodes with many operands. 112 if (PN->getNumIncomingValues() > MaxLookup) 113 return false; 114 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 115 Worklist.push_back(PN->getIncomingValue(i)); 116 continue; 117 } 118 119 return false; 120 } while (!Worklist.empty() && --MaxLookup); 121 122 return Worklist.empty(); 123} 124 125/// AddReadAttrs - Deduce readonly/readnone attributes for the SCC. 126bool FunctionAttrs::AddReadAttrs(const CallGraphSCC &SCC) { 127 SmallPtrSet<Function*, 8> SCCNodes; 128 129 // Fill SCCNodes with the elements of the SCC. Used for quickly 130 // looking up whether a given CallGraphNode is in this SCC. 131 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) 132 SCCNodes.insert((*I)->getFunction()); 133 134 // Check if any of the functions in the SCC read or write memory. If they 135 // write memory then they can't be marked readnone or readonly. 136 bool ReadsMemory = false; 137 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) { 138 Function *F = (*I)->getFunction(); 139 140 if (F == 0) 141 // External node - may write memory. Just give up. 142 return false; 143 144 if (F->doesNotAccessMemory()) 145 // Already perfect! 146 continue; 147 148 // Definitions with weak linkage may be overridden at linktime with 149 // something that writes memory, so treat them like declarations. 150 if (F->isDeclaration() || F->mayBeOverridden()) { 151 if (!F->onlyReadsMemory()) 152 // May write memory. Just give up. 153 return false; 154 155 ReadsMemory = true; 156 continue; 157 } 158 159 // Scan the function body for instructions that may read or write memory. 160 for (inst_iterator II = inst_begin(F), E = inst_end(F); II != E; ++II) { 161 Instruction *I = &*II; 162 163 // Some instructions can be ignored even if they read or write memory. 164 // Detect these now, skipping to the next instruction if one is found. 165 CallSite CS(cast<Value>(I)); 166 if (CS && CS.getCalledFunction()) { 167 // Ignore calls to functions in the same SCC. 168 if (SCCNodes.count(CS.getCalledFunction())) 169 continue; 170 // Ignore intrinsics that only access local memory. 171 if (unsigned id = CS.getCalledFunction()->getIntrinsicID()) 172 if (AliasAnalysis::getIntrinsicModRefBehavior(id) == 173 AliasAnalysis::AccessesArguments) { 174 // Check that all pointer arguments point to local memory. 175 for (CallSite::arg_iterator CI = CS.arg_begin(), CE = CS.arg_end(); 176 CI != CE; ++CI) { 177 Value *Arg = *CI; 178 if (Arg->getType()->isPointerTy() && !PointsToLocalMemory(Arg)) 179 // Writes memory. Just give up. 180 return false; 181 } 182 // Only reads and writes local memory. 183 continue; 184 } 185 } else if (LoadInst *LI = dyn_cast<LoadInst>(I)) { 186 // Ignore loads from local memory. 187 if (PointsToLocalMemory(LI->getPointerOperand())) 188 continue; 189 } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) { 190 // Ignore stores to local memory. 191 if (PointsToLocalMemory(SI->getPointerOperand())) 192 continue; 193 } 194 195 // Any remaining instructions need to be taken seriously! Check if they 196 // read or write memory. 197 if (I->mayWriteToMemory()) 198 // Writes memory. Just give up. 199 return false; 200 201 if (isMalloc(I)) 202 // malloc claims not to write memory! PR3754. 203 return false; 204 205 // If this instruction may read memory, remember that. 206 ReadsMemory |= I->mayReadFromMemory(); 207 } 208 } 209 210 // Success! Functions in this SCC do not access memory, or only read memory. 211 // Give them the appropriate attribute. 212 bool MadeChange = false; 213 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) { 214 Function *F = (*I)->getFunction(); 215 216 if (F->doesNotAccessMemory()) 217 // Already perfect! 218 continue; 219 220 if (F->onlyReadsMemory() && ReadsMemory) 221 // No change. 222 continue; 223 224 MadeChange = true; 225 226 // Clear out any existing attributes. 227 F->removeAttribute(~0, Attribute::ReadOnly | Attribute::ReadNone); 228 229 // Add in the new attribute. 230 F->addAttribute(~0, ReadsMemory? Attribute::ReadOnly : Attribute::ReadNone); 231 232 if (ReadsMemory) 233 ++NumReadOnly; 234 else 235 ++NumReadNone; 236 } 237 238 return MadeChange; 239} 240 241/// AddNoCaptureAttrs - Deduce nocapture attributes for the SCC. 242bool FunctionAttrs::AddNoCaptureAttrs(const CallGraphSCC &SCC) { 243 bool Changed = false; 244 245 // Check each function in turn, determining which pointer arguments are not 246 // captured. 247 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) { 248 Function *F = (*I)->getFunction(); 249 250 if (F == 0) 251 // External node - skip it; 252 continue; 253 254 // Definitions with weak linkage may be overridden at linktime with 255 // something that writes memory, so treat them like declarations. 256 if (F->isDeclaration() || F->mayBeOverridden()) 257 continue; 258 259 for (Function::arg_iterator A = F->arg_begin(), E = F->arg_end(); A!=E; ++A) 260 if (A->getType()->isPointerTy() && !A->hasNoCaptureAttr() && 261 !PointerMayBeCaptured(A, true, /*StoreCaptures=*/false)) { 262 A->addAttr(Attribute::NoCapture); 263 ++NumNoCapture; 264 Changed = true; 265 } 266 } 267 268 return Changed; 269} 270 271/// IsFunctionMallocLike - A function is malloc-like if it returns either null 272/// or a pointer that doesn't alias any other pointer visible to the caller. 273bool FunctionAttrs::IsFunctionMallocLike(Function *F, 274 SmallPtrSet<Function*, 8> &SCCNodes) const { 275 UniqueVector<Value *> FlowsToReturn; 276 for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) 277 if (ReturnInst *Ret = dyn_cast<ReturnInst>(I->getTerminator())) 278 FlowsToReturn.insert(Ret->getReturnValue()); 279 280 for (unsigned i = 0; i != FlowsToReturn.size(); ++i) { 281 Value *RetVal = FlowsToReturn[i+1]; // UniqueVector[0] is reserved. 282 283 if (Constant *C = dyn_cast<Constant>(RetVal)) { 284 if (!C->isNullValue() && !isa<UndefValue>(C)) 285 return false; 286 287 continue; 288 } 289 290 if (isa<Argument>(RetVal)) 291 return false; 292 293 if (Instruction *RVI = dyn_cast<Instruction>(RetVal)) 294 switch (RVI->getOpcode()) { 295 // Extend the analysis by looking upwards. 296 case Instruction::BitCast: 297 case Instruction::GetElementPtr: 298 FlowsToReturn.insert(RVI->getOperand(0)); 299 continue; 300 case Instruction::Select: { 301 SelectInst *SI = cast<SelectInst>(RVI); 302 FlowsToReturn.insert(SI->getTrueValue()); 303 FlowsToReturn.insert(SI->getFalseValue()); 304 continue; 305 } 306 case Instruction::PHI: { 307 PHINode *PN = cast<PHINode>(RVI); 308 for (int i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 309 FlowsToReturn.insert(PN->getIncomingValue(i)); 310 continue; 311 } 312 313 // Check whether the pointer came from an allocation. 314 case Instruction::Alloca: 315 break; 316 case Instruction::Call: 317 case Instruction::Invoke: { 318 CallSite CS(RVI); 319 if (CS.paramHasAttr(0, Attribute::NoAlias)) 320 break; 321 if (CS.getCalledFunction() && 322 SCCNodes.count(CS.getCalledFunction())) 323 break; 324 } // fall-through 325 default: 326 return false; // Did not come from an allocation. 327 } 328 329 if (PointerMayBeCaptured(RetVal, false, /*StoreCaptures=*/false)) 330 return false; 331 } 332 333 return true; 334} 335 336/// AddNoAliasAttrs - Deduce noalias attributes for the SCC. 337bool FunctionAttrs::AddNoAliasAttrs(const CallGraphSCC &SCC) { 338 SmallPtrSet<Function*, 8> SCCNodes; 339 340 // Fill SCCNodes with the elements of the SCC. Used for quickly 341 // looking up whether a given CallGraphNode is in this SCC. 342 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) 343 SCCNodes.insert((*I)->getFunction()); 344 345 // Check each function in turn, determining which functions return noalias 346 // pointers. 347 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) { 348 Function *F = (*I)->getFunction(); 349 350 if (F == 0) 351 // External node - skip it; 352 return false; 353 354 // Already noalias. 355 if (F->doesNotAlias(0)) 356 continue; 357 358 // Definitions with weak linkage may be overridden at linktime, so 359 // treat them like declarations. 360 if (F->isDeclaration() || F->mayBeOverridden()) 361 return false; 362 363 // We annotate noalias return values, which are only applicable to 364 // pointer types. 365 if (!F->getReturnType()->isPointerTy()) 366 continue; 367 368 if (!IsFunctionMallocLike(F, SCCNodes)) 369 return false; 370 } 371 372 bool MadeChange = false; 373 for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) { 374 Function *F = (*I)->getFunction(); 375 if (F->doesNotAlias(0) || !F->getReturnType()->isPointerTy()) 376 continue; 377 378 F->setDoesNotAlias(0); 379 ++NumNoAlias; 380 MadeChange = true; 381 } 382 383 return MadeChange; 384} 385 386bool FunctionAttrs::runOnSCC(CallGraphSCC &SCC) { 387 bool Changed = AddReadAttrs(SCC); 388 Changed |= AddNoCaptureAttrs(SCC); 389 Changed |= AddNoAliasAttrs(SCC); 390 return Changed; 391} 392