FunctionAttrs.cpp revision 88e76757deb4872fc5d0db8592019fa73bfecfe7
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 addition, 13// it deduces which function arguments (of pointer type) do not escape, 14// and marks them nocapture. It implements this as a bottom-up traversal 15// of the call-graph. 16// 17//===----------------------------------------------------------------------===// 18 19#define DEBUG_TYPE "functionattrs" 20#include "llvm/Transforms/IPO.h" 21#include "llvm/CallGraphSCCPass.h" 22#include "llvm/GlobalVariable.h" 23#include "llvm/Instructions.h" 24#include "llvm/Analysis/CallGraph.h" 25#include "llvm/ADT/SmallPtrSet.h" 26#include "llvm/ADT/Statistic.h" 27#include "llvm/Support/Compiler.h" 28#include "llvm/Support/InstIterator.h" 29using namespace llvm; 30 31STATISTIC(NumReadNone, "Number of functions marked readnone"); 32STATISTIC(NumReadOnly, "Number of functions marked readonly"); 33STATISTIC(NumNoCapture, "Number of arguments marked nocapture"); 34 35namespace { 36 struct VISIBILITY_HIDDEN FunctionAttrs : public CallGraphSCCPass { 37 static char ID; // Pass identification, replacement for typeid 38 FunctionAttrs() : CallGraphSCCPass(&ID) {} 39 40 // runOnSCC - Analyze the SCC, performing the transformation if possible. 41 bool runOnSCC(const std::vector<CallGraphNode *> &SCC); 42 43 // AddReadAttrs - Deduce readonly/readnone attributes for the SCC. 44 bool AddReadAttrs(const std::vector<CallGraphNode *> &SCC); 45 46 // AddNoCaptureAttrs - Deduce nocapture attributes for the SCC. 47 bool AddNoCaptureAttrs(const std::vector<CallGraphNode *> &SCC); 48 49 // isCaptured - Returns true if this pointer value escapes. 50 bool isCaptured(Function &F, Value *V); 51 52 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 53 AU.setPreservesCFG(); 54 CallGraphSCCPass::getAnalysisUsage(AU); 55 } 56 57 bool PointsToLocalMemory(Value *V); 58 }; 59} 60 61char FunctionAttrs::ID = 0; 62static RegisterPass<FunctionAttrs> 63X("functionattrs", "Deduce function attributes"); 64 65Pass *llvm::createFunctionAttrsPass() { return new FunctionAttrs(); } 66 67 68/// PointsToLocalMemory - Returns whether the given pointer value points to 69/// memory that is local to the function. Global constants are considered 70/// local to all functions. 71bool FunctionAttrs::PointsToLocalMemory(Value *V) { 72 V = V->getUnderlyingObject(); 73 // An alloca instruction defines local memory. 74 if (isa<AllocaInst>(V)) 75 return true; 76 // A global constant counts as local memory for our purposes. 77 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(V)) 78 return GV->isConstant(); 79 // Could look through phi nodes and selects here, but it doesn't seem 80 // to be useful in practice. 81 return false; 82} 83 84/// AddReadAttrs - Deduce readonly/readnone attributes for the SCC. 85bool FunctionAttrs::AddReadAttrs(const std::vector<CallGraphNode *> &SCC) { 86 SmallPtrSet<CallGraphNode*, 8> SCCNodes; 87 CallGraph &CG = getAnalysis<CallGraph>(); 88 89 // Fill SCCNodes with the elements of the SCC. Used for quickly 90 // looking up whether a given CallGraphNode is in this SCC. 91 for (unsigned i = 0, e = SCC.size(); i != e; ++i) 92 SCCNodes.insert(SCC[i]); 93 94 // Check if any of the functions in the SCC read or write memory. If they 95 // write memory then they can't be marked readnone or readonly. 96 bool ReadsMemory = false; 97 for (unsigned i = 0, e = SCC.size(); i != e; ++i) { 98 Function *F = SCC[i]->getFunction(); 99 100 if (F == 0) 101 // External node - may write memory. Just give up. 102 return false; 103 104 if (F->doesNotAccessMemory()) 105 // Already perfect! 106 continue; 107 108 // Definitions with weak linkage may be overridden at linktime with 109 // something that writes memory, so treat them like declarations. 110 if (F->isDeclaration() || F->mayBeOverridden()) { 111 if (!F->onlyReadsMemory()) 112 // May write memory. Just give up. 113 return false; 114 115 ReadsMemory = true; 116 continue; 117 } 118 119 // Scan the function body for instructions that may read or write memory. 120 for (inst_iterator II = inst_begin(F), E = inst_end(F); II != E; ++II) { 121 Instruction *I = &*II; 122 123 // Some instructions can be ignored even if they read or write memory. 124 // Detect these now, skipping to the next instruction if one is found. 125 CallSite CS = CallSite::get(I); 126 if (CS.getInstruction()) { 127 // Ignore calls to functions in the same SCC. 128 if (SCCNodes.count(CG[CS.getCalledFunction()])) 129 continue; 130 } else if (LoadInst *LI = dyn_cast<LoadInst>(I)) { 131 // Ignore loads from local memory. 132 if (PointsToLocalMemory(LI->getPointerOperand())) 133 continue; 134 } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) { 135 // Ignore stores to local memory. 136 if (PointsToLocalMemory(SI->getPointerOperand())) 137 continue; 138 } 139 140 // Any remaining instructions need to be taken seriously! Check if they 141 // read or write memory. 142 if (I->mayWriteToMemory()) 143 // Writes memory. Just give up. 144 return false; 145 // If this instruction may read memory, remember that. 146 ReadsMemory |= I->mayReadFromMemory(); 147 } 148 } 149 150 // Success! Functions in this SCC do not access memory, or only read memory. 151 // Give them the appropriate attribute. 152 bool MadeChange = false; 153 for (unsigned i = 0, e = SCC.size(); i != e; ++i) { 154 Function *F = SCC[i]->getFunction(); 155 156 if (F->doesNotAccessMemory()) 157 // Already perfect! 158 continue; 159 160 if (F->onlyReadsMemory() && ReadsMemory) 161 // No change. 162 continue; 163 164 MadeChange = true; 165 166 // Clear out any existing attributes. 167 F->removeAttribute(~0, Attribute::ReadOnly | Attribute::ReadNone); 168 169 // Add in the new attribute. 170 F->addAttribute(~0, ReadsMemory? Attribute::ReadOnly : Attribute::ReadNone); 171 172 if (ReadsMemory) 173 NumReadOnly++; 174 else 175 NumReadNone++; 176 } 177 178 return MadeChange; 179} 180 181/// isCaptured - Returns whether this pointer value is captured. 182bool FunctionAttrs::isCaptured(Function &F, Value *V) { 183 SmallVector<Use*, 16> Worklist; 184 SmallPtrSet<Use*, 16> Visited; 185 186 for (Value::use_iterator UI = V->use_begin(), UE = V->use_end(); UI != UE; 187 ++UI) { 188 Use *U = &UI.getUse(); 189 Visited.insert(U); 190 Worklist.push_back(U); 191 } 192 193 while (!Worklist.empty()) { 194 Use *U = Worklist.pop_back_val(); 195 Instruction *I = cast<Instruction>(U->getUser()); 196 V = U->get(); 197 198 if (isa<LoadInst>(I)) { 199 // Loading a pointer does not cause it to escape. 200 continue; 201 } 202 203 if (isa<StoreInst>(I)) { 204 if (V == I->getOperand(0)) 205 // Stored the pointer - escapes. TODO: improve this. 206 return true; 207 // Storing to the pointee does not cause the pointer to escape. 208 continue; 209 } 210 211 CallSite CS = CallSite::get(I); 212 if (CS.getInstruction()) { 213 // Does not escape if only passed via 'nocapture' arguments. Note 214 // that calling a function pointer does not in itself cause that 215 // function pointer to escape. This is a subtle point considering 216 // that (for example) the callee might return its own address. It 217 // is analogous to saying that loading a value from a pointer does 218 // not cause the pointer to escape, even though the loaded value 219 // might be the pointer itself (think of self-referential objects). 220 CallSite::arg_iterator B = CS.arg_begin(), E = CS.arg_end(); 221 for (CallSite::arg_iterator A = B; A != E; ++A) 222 if (A->get() == V && !CS.paramHasAttr(A-B+1, Attribute::NoCapture)) 223 // The parameter is not marked 'nocapture' - escapes. 224 return true; 225 // Only passed via 'nocapture' arguments, or is the called function. 226 // Does not escape. 227 continue; 228 } 229 230 if (isa<BitCastInst>(I) || isa<GetElementPtrInst>(I) || 231 isa<PHINode>(I) || isa<SelectInst>(I)) { 232 // Type conversion, calculating an offset, or merging values. 233 // The original value does not escape via this if the new value doesn't. 234 // Note that in the case of a select instruction it is important that 235 // the value not be used as the condition, since otherwise one bit of 236 // information might escape. It cannot be the condition because it has 237 // the wrong type. 238 for (Instruction::use_iterator UI = I->use_begin(), UE = I->use_end(); 239 UI != UE; ++UI) { 240 Use *U = &UI.getUse(); 241 if (Visited.insert(U)) 242 Worklist.push_back(U); 243 } 244 continue; 245 } 246 247 // Something else - be conservative and say it escapes. 248 return true; 249 } 250 251 return false; 252} 253 254/// AddNoCaptureAttrs - Deduce nocapture attributes for the SCC. 255bool FunctionAttrs::AddNoCaptureAttrs(const std::vector<CallGraphNode *> &SCC) { 256 bool Changed = false; 257 258 // Check each function in turn, determining which pointer arguments are not 259 // captured. 260 for (unsigned i = 0, e = SCC.size(); i != e; ++i) { 261 Function *F = SCC[i]->getFunction(); 262 263 if (F == 0) 264 // External node - skip it; 265 continue; 266 267 // Definitions with weak linkage may be overridden at linktime with 268 // something that writes memory, so treat them like declarations. 269 if (F->isDeclaration() || F->mayBeOverridden()) 270 continue; 271 272 for (Function::arg_iterator A = F->arg_begin(), E = F->arg_end(); A!=E; ++A) 273 if (isa<PointerType>(A->getType()) && !A->hasNoCaptureAttr() && 274 !isCaptured(*F, A)) { 275 A->addAttr(Attribute::NoCapture); 276 NumNoCapture++; 277 Changed = true; 278 } 279 } 280 281 return Changed; 282} 283 284bool FunctionAttrs::runOnSCC(const std::vector<CallGraphNode *> &SCC) { 285 bool Changed = AddReadAttrs(SCC); 286 Changed |= AddNoCaptureAttrs(SCC); 287 return Changed; 288} 289