FunctionAttrs.cpp revision 1010941954a44520d12037d8b6d81a4af89b57a6
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. It implements 13// this as a bottom-up traversal of the call-graph. 14// 15//===----------------------------------------------------------------------===// 16 17#define DEBUG_TYPE "functionattrs" 18#include "llvm/Transforms/IPO.h" 19#include "llvm/CallGraphSCCPass.h" 20#include "llvm/GlobalVariable.h" 21#include "llvm/Instructions.h" 22#include "llvm/Analysis/CallGraph.h" 23#include "llvm/ADT/SmallPtrSet.h" 24#include "llvm/ADT/Statistic.h" 25#include "llvm/Support/Compiler.h" 26#include "llvm/Support/InstIterator.h" 27using namespace llvm; 28 29STATISTIC(NumReadNone, "Number of functions marked readnone"); 30STATISTIC(NumReadOnly, "Number of functions marked readonly"); 31STATISTIC(NumNoCapture, "Number of arguments marked nocapture"); 32 33namespace { 34 struct VISIBILITY_HIDDEN FunctionAttrs : public CallGraphSCCPass { 35 static char ID; // Pass identification, replacement for typeid 36 FunctionAttrs() : CallGraphSCCPass(&ID) {} 37 38 // runOnSCC - Analyze the SCC, performing the transformation if possible. 39 bool runOnSCC(const std::vector<CallGraphNode *> &SCC); 40 41 // AddReadAttrs - Deduce readonly/readnone attributes for the SCC. 42 bool AddReadAttrs(const std::vector<CallGraphNode *> &SCC); 43 44 // AddNoCaptureAttrs - Deduce nocapture attributes for the SCC. 45 bool AddNoCaptureAttrs(const std::vector<CallGraphNode *> &SCC); 46 47 // isCaptured - Returns whether this pointer value is captured. 48 bool isCaptured(Function &F, Value *V); 49 50 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 51 AU.setPreservesCFG(); 52 CallGraphSCCPass::getAnalysisUsage(AU); 53 } 54 55 bool PointsToLocalMemory(Value *V); 56 }; 57} 58 59char FunctionAttrs::ID = 0; 60static RegisterPass<FunctionAttrs> 61X("functionattrs", "Deduce function attributes"); 62 63Pass *llvm::createFunctionAttrsPass() { return new FunctionAttrs(); } 64 65 66/// PointsToLocalMemory - Returns whether the given pointer value points to 67/// memory that is local to the function. Global constants are considered 68/// local to all functions. 69bool FunctionAttrs::PointsToLocalMemory(Value *V) { 70 V = V->getUnderlyingObject(); 71 // An alloca instruction defines local memory. 72 if (isa<AllocaInst>(V)) 73 return true; 74 // A global constant counts as local memory for our purposes. 75 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) 76 return GV->isConstant(); 77 // Could look through phi nodes and selects here, but it doesn't seem 78 // to be useful in practice. 79 return false; 80} 81 82/// AddReadAttrs - Deduce readonly/readnone attributes for the SCC. 83bool FunctionAttrs::AddReadAttrs(const std::vector<CallGraphNode *> &SCC) { 84 SmallPtrSet<CallGraphNode*, 8> SCCNodes; 85 CallGraph &CG = getAnalysis<CallGraph>(); 86 87 // Fill SCCNodes with the elements of the SCC. Used for quickly 88 // looking up whether a given CallGraphNode is in this SCC. 89 for (unsigned i = 0, e = SCC.size(); i != e; ++i) 90 SCCNodes.insert(SCC[i]); 91 92 // Check if any of the functions in the SCC read or write memory. If they 93 // write memory then they can't be marked readnone or readonly. 94 bool ReadsMemory = false; 95 for (unsigned i = 0, e = SCC.size(); i != e; ++i) { 96 Function *F = SCC[i]->getFunction(); 97 98 if (F == 0) 99 // External node - may write memory. Just give up. 100 return false; 101 102 if (F->doesNotAccessMemory()) 103 // Already perfect! 104 continue; 105 106 // Definitions with weak linkage may be overridden at linktime with 107 // something that writes memory, so treat them like declarations. 108 if (F->isDeclaration() || F->mayBeOverridden()) { 109 if (!F->onlyReadsMemory()) 110 // May write memory. Just give up. 111 return false; 112 113 ReadsMemory = true; 114 continue; 115 } 116 117 // Scan the function body for instructions that may read or write memory. 118 for (inst_iterator II = inst_begin(F), E = inst_end(F); II != E; ++II) { 119 Instruction *I = &*II; 120 121 // Some instructions can be ignored even if they read or write memory. 122 // Detect these now, skipping to the next instruction if one is found. 123 CallSite CS = CallSite::get(I); 124 if (CS.getInstruction()) { 125 // Ignore calls to functions in the same SCC. 126 if (SCCNodes.count(CG[CS.getCalledFunction()])) 127 continue; 128 } else if (LoadInst *LI = dyn_cast<LoadInst>(I)) { 129 // Ignore loads from local memory. 130 if (PointsToLocalMemory(LI->getPointerOperand())) 131 continue; 132 } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) { 133 // Ignore stores to local memory. 134 if (PointsToLocalMemory(SI->getPointerOperand())) 135 continue; 136 } 137 138 // Any remaining instructions need to be taken seriously! Check if they 139 // read or write memory. 140 if (I->mayWriteToMemory()) 141 // Writes memory. Just give up. 142 return false; 143 // If this instruction may read memory, remember that. 144 ReadsMemory |= I->mayReadFromMemory(); 145 } 146 } 147 148 // Success! Functions in this SCC do not access memory, or only read memory. 149 // Give them the appropriate attribute. 150 bool MadeChange = false; 151 for (unsigned i = 0, e = SCC.size(); i != e; ++i) { 152 Function *F = SCC[i]->getFunction(); 153 154 if (F->doesNotAccessMemory()) 155 // Already perfect! 156 continue; 157 158 if (F->onlyReadsMemory() && ReadsMemory) 159 // No change. 160 continue; 161 162 MadeChange = true; 163 164 // Clear out any existing attributes. 165 F->removeAttribute(~0, Attribute::ReadOnly | Attribute::ReadNone); 166 167 // Add in the new attribute. 168 F->addAttribute(~0, ReadsMemory? Attribute::ReadOnly : Attribute::ReadNone); 169 170 if (ReadsMemory) 171 NumReadOnly++; 172 else 173 NumReadNone++; 174 } 175 176 return MadeChange; 177} 178 179/// isCaptured - Returns whether this pointer value is captured. 180bool FunctionAttrs::isCaptured(Function &F, Value *V) { 181 SmallVector<Use*, 16> Worklist; 182 SmallPtrSet<Use*, 16> Visited; 183 184 for (Value::use_iterator UI = V->use_begin(), UE = V->use_end(); UI != UE; 185 ++UI) { 186 Use *U = &UI.getUse(); 187 Visited.insert(U); 188 Worklist.push_back(U); 189 } 190 191 while (!Worklist.empty()) { 192 Use *U = Worklist.pop_back_val(); 193 Instruction *I = cast<Instruction>(U->getUser()); 194 V = U->get(); 195 196 if (isa<LoadInst>(I)) { 197 // Loading a pointer does not cause it to escape. 198 continue; 199 } 200 201 if (isa<StoreInst>(I)) { 202 if (V == I->getOperand(0)) 203 // Stored the pointer - escapes. TODO: improve this. 204 return true; 205 // Storing to the pointee does not cause the pointer to escape. 206 continue; 207 } 208 209 CallSite CS = CallSite::get(I); 210 if (CS.getInstruction()) { 211 // Does not escape if only passed via 'nocapture' arguments. Note 212 // that calling a function pointer does not in itself cause that 213 // function pointer to escape. This is a subtle point considering 214 // that (for example) the callee might return its own address. It 215 // is analogous to saying that loading a value from a pointer does 216 // not cause the pointer to escape, even though the loaded value 217 // might be the pointer itself (think of self-referential objects). 218 CallSite::arg_iterator B = CS.arg_begin(), E = CS.arg_end(); 219 for (CallSite::arg_iterator A = B; A != E; ++A) 220 if (A->get() == V && !CS.paramHasAttr(A-B+1, Attribute::NoCapture)) 221 // The parameter is not marked 'nocapture' - escapes. 222 return true; 223 // Only passed via 'nocapture' arguments, or is the called function. 224 // Does not escape. 225 continue; 226 } 227 228 if (isa<BitCastInst>(I) || isa<GetElementPtrInst>(I) || 229 isa<PHINode>(I) || isa<SelectInst>(I)) { 230 // Type conversion, calculating an offset, or merging values. 231 // The original value does not escape via this if the new value doesn't. 232 // Note that in the case of a select instruction it is important that 233 // the value not be used as the condition, since otherwise one bit of 234 // information might escape. It cannot be the condition because it has 235 // the wrong type. 236 for (Instruction::use_iterator UI = I->use_begin(), UE = I->use_end(); 237 UI != UE; ++UI) { 238 Use *U = &UI.getUse(); 239 if (Visited.insert(U)) 240 Worklist.push_back(U); 241 } 242 continue; 243 } 244 245 // Something else - be conservative and say it escapes. 246 return true; 247 } 248 249 return false; 250} 251 252/// AddNoCaptureAttrs - Deduce nocapture attributes for the SCC. 253bool FunctionAttrs::AddNoCaptureAttrs(const std::vector<CallGraphNode *> &SCC) { 254 bool Changed = false; 255 256 // Check each function in turn, determining which pointer arguments are not 257 // captured. 258 for (unsigned i = 0, e = SCC.size(); i != e; ++i) { 259 Function *F = SCC[i]->getFunction(); 260 261 if (F == 0) 262 // External node - skip it; 263 continue; 264 265 // Definitions with weak linkage may be overridden at linktime with 266 // something that writes memory, so treat them like declarations. 267 if (F->isDeclaration() || F->mayBeOverridden()) 268 continue; 269 270 for (Function::arg_iterator A = F->arg_begin(), E = F->arg_end(); A!=E; ++A) 271 if (isa<PointerType>(A->getType()) && !A->hasNoCaptureAttr() && 272 !isCaptured(*F, A)) { 273 A->addAttr(Attribute::NoCapture); 274 NumNoCapture++; 275 Changed = true; 276 } 277 } 278 279 return Changed; 280} 281 282bool FunctionAttrs::runOnSCC(const std::vector<CallGraphNode *> &SCC) { 283 bool Changed = AddReadAttrs(SCC); 284 Changed |= AddNoCaptureAttrs(SCC); 285 return Changed; 286} 287