src/llvm/lib/Transforms/IPO/FunctionAttrs.cpp

   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/LLVMContext.h"
  27 #include "llvm/Analysis/AliasAnalysis.h"
  28 #include "llvm/Analysis/CallGraph.h"
  29 #include "llvm/Analysis/CaptureTracking.h"
  30 #include "llvm/ADT/SCCIterator.h"
  31 #include "llvm/ADT/SmallSet.h"
  32 #include "llvm/ADT/Statistic.h"
  33 #include "llvm/ADT/UniqueVector.h"
  34 #include "llvm/Support/InstIterator.h"
  35 using namespace llvm;
  36
  37 STATISTIC(NumReadNone, "Number of functions marked readnone");
  38 STATISTIC(NumReadOnly, "Number of functions marked readonly");
  39 STATISTIC(NumNoCapture, "Number of arguments marked nocapture");
  40 STATISTIC(NumNoAlias, "Number of function returns marked noalias");
  41
  42 namespace {
  43   struct FunctionAttrs : public CallGraphSCCPass {
  44     static char ID; // Pass identification, replacement for typeid
  45     FunctionAttrs() : CallGraphSCCPass(ID), AA(0) {
  46       initializeFunctionAttrsPass(*PassRegistry::getPassRegistry());
  47     }
  48
  49     // runOnSCC - Analyze the SCC, performing the transformation if possible.
  50     bool runOnSCC(CallGraphSCC &SCC);
  51
  52     // AddReadAttrs - Deduce readonly/readnone attributes for the SCC.
  53     bool AddReadAttrs(const CallGraphSCC &SCC);
  54
  55     // AddNoCaptureAttrs - Deduce nocapture attributes for the SCC.
  56     bool AddNoCaptureAttrs(const CallGraphSCC &SCC);
  57
  58     // IsFunctionMallocLike - Does this function allocate new memory?
  59     bool IsFunctionMallocLike(Function *F,
  60                               SmallPtrSet<Function*, 8> &) const;
  61
  62     // AddNoAliasAttrs - Deduce noalias attributes for the SCC.
  63     bool AddNoAliasAttrs(const CallGraphSCC &SCC);
  64
  65     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  66       AU.setPreservesCFG();
  67       AU.addRequired<AliasAnalysis>();
  68       CallGraphSCCPass::getAnalysisUsage(AU);
  69     }
  70
  71   private:
  72     AliasAnalysis *AA;
  73   };
  74 }
  75
  76 char FunctionAttrs::ID = 0;
  77 INITIALIZE_PASS_BEGIN(FunctionAttrs, "functionattrs",
  78                 "Deduce function attributes", false, false)
  79 INITIALIZE_AG_DEPENDENCY(CallGraph)
  80 INITIALIZE_PASS_END(FunctionAttrs, "functionattrs",
  81                 "Deduce function attributes", false, false)
  82
  83 Pass *llvm::createFunctionAttrsPass() { return new FunctionAttrs(); }
  84
  85
  86 /// AddReadAttrs - Deduce readonly/readnone attributes for the SCC.
  87 bool FunctionAttrs::AddReadAttrs(const CallGraphSCC &SCC) {
  88   SmallPtrSet<Function*, 8> SCCNodes;
  89
  90   // Fill SCCNodes with the elements of the SCC.  Used for quickly
  91   // looking up whether a given CallGraphNode is in this SCC.
  92   for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
  93     SCCNodes.insert((*I)->getFunction());
  94
  95   // Check if any of the functions in the SCC read or write memory.  If they
  96   // write memory then they can't be marked readnone or readonly.
  97   bool ReadsMemory = false;
  98   for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
  99     Function *F = (*I)->getFunction();
 100
 101     if (F == 0)
 102       // External node - may write memory.  Just give up.
 103       return false;
 104
 105     AliasAnalysis::ModRefBehavior MRB = AA->getModRefBehavior(F);
 106     if (MRB == AliasAnalysis::DoesNotAccessMemory)
 107       // Already perfect!
 108       continue;
 109
 110     // Definitions with weak linkage may be overridden at linktime with
 111     // something that writes memory, so treat them like declarations.
 112     if (F->isDeclaration() || F->mayBeOverridden()) {
 113       if (!AliasAnalysis::onlyReadsMemory(MRB))
 114         // May write memory.  Just give up.
 115         return false;
 116
 117       ReadsMemory = true;
 118       continue;
 119     }
 120
 121     // Scan the function body for instructions that may read or write memory.
 122     for (inst_iterator II = inst_begin(F), E = inst_end(F); II != E; ++II) {
 123       Instruction *I = &*II;
 124
 125       // Some instructions can be ignored even if they read or write memory.
 126       // Detect these now, skipping to the next instruction if one is found.
 127       CallSite CS(cast<Value>(I));
 128       if (CS) {
 129         // Ignore calls to functions in the same SCC.
 130         if (CS.getCalledFunction() && SCCNodes.count(CS.getCalledFunction()))
 131           continue;
 132         AliasAnalysis::ModRefBehavior MRB = AA->getModRefBehavior(CS);
 133         // If the call doesn't access arbitrary memory, we may be able to
 134         // figure out something.
 135         if (AliasAnalysis::onlyAccessesArgPointees(MRB)) {
 136           // If the call does access argument pointees, check each argument.
 137           if (AliasAnalysis::doesAccessArgPointees(MRB))
 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                 AliasAnalysis::Location Loc(Arg,
 145                                             AliasAnalysis::UnknownSize,
 146                                             I->getMetadata(LLVMContext::MD_tbaa));
 147                 if (!AA->pointsToConstantMemory(Loc, /*OrLocal=*/true)) {
 148                   if (MRB & AliasAnalysis::Mod)
 149                     // Writes non-local memory.  Give up.
 150                     return false;
 151                   if (MRB & AliasAnalysis::Ref)
 152                     // Ok, it reads non-local memory.
 153                     ReadsMemory = true;
 154                 }
 155               }
 156             }
 157           continue;
 158         }
 159         // The call could access any memory. If that includes writes, give up.
 160         if (MRB & AliasAnalysis::Mod)
 161           return false;
 162         // If it reads, note it.
 163         if (MRB & AliasAnalysis::Ref)
 164           ReadsMemory = true;
 165         continue;
 166       } else if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
 167         // Ignore non-volatile loads from local memory. (Atomic is okay here.)
 168         if (!LI->isVolatile()) {
 169           AliasAnalysis::Location Loc = AA->getLocation(LI);
 170           if (AA->pointsToConstantMemory(Loc, /*OrLocal=*/true))
 171             continue;
 172         }
 173       } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
 174         // Ignore non-volatile stores to local memory. (Atomic is okay here.)
 175         if (!SI->isVolatile()) {
 176           AliasAnalysis::Location Loc = AA->getLocation(SI);
 177           if (AA->pointsToConstantMemory(Loc, /*OrLocal=*/true))
 178             continue;
 179         }
 180       } else if (VAArgInst *VI = dyn_cast<VAArgInst>(I)) {
 181         // Ignore vaargs on local memory.
 182         AliasAnalysis::Location Loc = AA->getLocation(VI);
 183         if (AA->pointsToConstantMemory(Loc, /*OrLocal=*/true))
 184           continue;
 185       }
 186
 187       // Any remaining instructions need to be taken seriously!  Check if they
 188       // read or write memory.
 189       if (I->mayWriteToMemory())
 190         // Writes memory.  Just give up.
 191         return false;
 192
 193       // If this instruction may read memory, remember that.
 194       ReadsMemory |= I->mayReadFromMemory();
 195     }
 196   }
 197
 198   // Success!  Functions in this SCC do not access memory, or only read memory.
 199   // Give them the appropriate attribute.
 200   bool MadeChange = false;
 201   for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
 202     Function *F = (*I)->getFunction();
 203
 204     if (F->doesNotAccessMemory())
 205       // Already perfect!
 206       continue;
 207
 208     if (F->onlyReadsMemory() && ReadsMemory)
 209       // No change.
 210       continue;
 211
 212     MadeChange = true;
 213
 214     // Clear out any existing attributes.
 215     F->removeAttribute(~0, Attribute::ReadOnly | Attribute::ReadNone);
 216
 217     // Add in the new attribute.
 218     F->addAttribute(~0, ReadsMemory? Attribute::ReadOnly : Attribute::ReadNone);
 219
 220     if (ReadsMemory)
 221       ++NumReadOnly;
 222     else
 223       ++NumReadNone;
 224   }
 225
 226   return MadeChange;
 227 }
 228
 229 namespace {
 230   // For a given pointer Argument, this retains a list of Arguments of functions
 231   // in the same SCC that the pointer data flows into. We use this to build an
 232   // SCC of the arguments.
 233   struct ArgumentGraphNode {
 234     Argument *Definition;
 235     SmallVector<ArgumentGraphNode*, 4> Uses;
 236   };
 237
 238   class ArgumentGraph {
 239     // We store pointers to ArgumentGraphNode objects, so it's important that
 240     // that they not move around upon insert.
 241     typedef std::map<Argument*, ArgumentGraphNode> ArgumentMapTy;
 242
 243     ArgumentMapTy ArgumentMap;
 244
 245     // There is no root node for the argument graph, in fact:
 246     //   void f(int *x, int *y) { if (...) f(x, y); }
 247     // is an example where the graph is disconnected. The SCCIterator requires a
 248     // single entry point, so we maintain a fake ("synthetic") root node that
 249     // uses every node. Because the graph is directed and nothing points into
 250     // the root, it will not participate in any SCCs (except for its own).
 251     ArgumentGraphNode SyntheticRoot;
 252
 253   public:
 254     ArgumentGraph() { SyntheticRoot.Definition = 0; }
 255
 256     typedef SmallVectorImpl<ArgumentGraphNode*>::iterator iterator;
 257
 258     iterator begin() { return SyntheticRoot.Uses.begin(); }
 259     iterator end() { return SyntheticRoot.Uses.end(); }
 260     ArgumentGraphNode *getEntryNode() { return &SyntheticRoot; }
 261
 262     ArgumentGraphNode *operator[](Argument *A) {
 263       ArgumentGraphNode &Node = ArgumentMap[A];
 264       Node.Definition = A;
 265       SyntheticRoot.Uses.push_back(&Node);
 266       return &Node;
 267     }
 268   };
 269
 270   // This tracker checks whether callees are in the SCC, and if so it does not
 271   // consider that a capture, instead adding it to the "Uses" list and
 272   // continuing with the analysis.
 273   struct ArgumentUsesTracker : public CaptureTracker {
 274     ArgumentUsesTracker(const SmallPtrSet<Function*, 8> &SCCNodes)
 275       : Captured(false), SCCNodes(SCCNodes) {}
 276
 277     void tooManyUses() { Captured = true; }
 278
 279     bool shouldExplore(Use *U) { return true; }
 280
 281     bool captured(Use *U) {
 282       CallSite CS(U->getUser());
 283       if (!CS.getInstruction()) { Captured = true; return true; }
 284
 285       Function *F = CS.getCalledFunction();
 286       if (!F || !SCCNodes.count(F)) { Captured = true; return true; }
 287
 288       Function::arg_iterator AI = F->arg_begin(), AE = F->arg_end();
 289       for (CallSite::arg_iterator PI = CS.arg_begin(), PE = CS.arg_end();
 290            PI != PE; ++PI, ++AI) {
 291         if (AI == AE) {
 292           assert(F->isVarArg() && "More params than args in non-varargs call");
 293           Captured = true;
 294           return true;
 295         }
 296         if (PI == U) {
 297           Uses.push_back(AI);
 298           break;
 299         }
 300       }
 301       assert(!Uses.empty() && "Capturing call-site captured nothing?");
 302       return false;
 303     }
 304
 305     bool Captured;  // True only if certainly captured (used outside our SCC).
 306     SmallVector<Argument*, 4> Uses;  // Uses within our SCC.
 307
 308     const SmallPtrSet<Function*, 8> &SCCNodes;
 309   };
 310 }
 311
 312 namespace llvm {
 313   template<> struct GraphTraits<ArgumentGraphNode*> {
 314     typedef ArgumentGraphNode NodeType;
 315     typedef SmallVectorImpl<ArgumentGraphNode*>::iterator ChildIteratorType;
 316
 317     static inline NodeType *getEntryNode(NodeType *A) { return A; }
 318     static inline ChildIteratorType child_begin(NodeType *N) {
 319       return N->Uses.begin();
 320     }
 321     static inline ChildIteratorType child_end(NodeType *N) {
 322       return N->Uses.end();
 323     }
 324   };
 325   template<> struct GraphTraits<ArgumentGraph*>
 326     : public GraphTraits<ArgumentGraphNode*> {
 327     static NodeType *getEntryNode(ArgumentGraph *AG) {
 328       return AG->getEntryNode();
 329     }
 330     static ChildIteratorType nodes_begin(ArgumentGraph *AG) {
 331       return AG->begin();
 332     }
 333     static ChildIteratorType nodes_end(ArgumentGraph *AG) {
 334       return AG->end();
 335     }
 336   };
 337 }
 338
 339 /// AddNoCaptureAttrs - Deduce nocapture attributes for the SCC.
 340 bool FunctionAttrs::AddNoCaptureAttrs(const CallGraphSCC &SCC) {
 341   bool Changed = false;
 342
 343   SmallPtrSet<Function*, 8> SCCNodes;
 344
 345   // Fill SCCNodes with the elements of the SCC.  Used for quickly
 346   // looking up whether a given CallGraphNode is in this SCC.
 347   for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
 348     Function *F = (*I)->getFunction();
 349     if (F && !F->isDeclaration() && !F->mayBeOverridden())
 350       SCCNodes.insert(F);
 351   }
 352
 353   ArgumentGraph AG;
 354
 355   // Check each function in turn, determining which pointer arguments are not
 356   // captured.
 357   for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
 358     Function *F = (*I)->getFunction();
 359
 360     if (F == 0)
 361       // External node - only a problem for arguments that we pass to it.
 362       continue;
 363
 364     // Definitions with weak linkage may be overridden at linktime with
 365     // something that captures pointers, so treat them like declarations.
 366     if (F->isDeclaration() || F->mayBeOverridden())
 367       continue;
 368
 369     // Functions that are readonly (or readnone) and nounwind and don't return
 370     // a value can't capture arguments. Don't analyze them.
 371     if (F->onlyReadsMemory() && F->doesNotThrow() &&
 372         F->getReturnType()->isVoidTy()) {
 373       for (Function::arg_iterator A = F->arg_begin(), E = F->arg_end();
 374            A != E; ++A) {
 375         if (A->getType()->isPointerTy() && !A->hasNoCaptureAttr()) {
 376           A->addAttr(Attribute::NoCapture);
 377           ++NumNoCapture;
 378           Changed = true;
 379         }
 380       }
 381       continue;
 382     }
 383
 384     for (Function::arg_iterator A = F->arg_begin(), E = F->arg_end(); A!=E; ++A)
 385       if (A->getType()->isPointerTy() && !A->hasNoCaptureAttr()) {
 386         ArgumentUsesTracker Tracker(SCCNodes);
 387         PointerMayBeCaptured(A, &Tracker);
 388         if (!Tracker.Captured) {
 389           if (Tracker.Uses.empty()) {
 390             // If it's trivially not captured, mark it nocapture now.
 391             A->addAttr(Attribute::NoCapture);
 392             ++NumNoCapture;
 393             Changed = true;
 394           } else {
 395             // If it's not trivially captured and not trivially not captured,
 396             // then it must be calling into another function in our SCC. Save
 397             // its particulars for Argument-SCC analysis later.
 398             ArgumentGraphNode *Node = AG[A];
 399             for (SmallVectorImpl<Argument*>::iterator UI = Tracker.Uses.begin(),
 400                    UE = Tracker.Uses.end(); UI != UE; ++UI)
 401               Node->Uses.push_back(AG[*UI]);
 402           }
 403         }
 404         // Otherwise, it's captured. Don't bother doing SCC analysis on it.
 405       }
 406   }
 407
 408   // The graph we've collected is partial because we stopped scanning for
 409   // argument uses once we solved the argument trivially. These partial nodes
 410   // show up as ArgumentGraphNode objects with an empty Uses list, and for
 411   // these nodes the final decision about whether they capture has already been
 412   // made.  If the definition doesn't have a 'nocapture' attribute by now, it
 413   // captures.
 414
 415   for (scc_iterator<ArgumentGraph*> I = scc_begin(&AG), E = scc_end(&AG);
 416        I != E; ++I) {
 417     std::vector<ArgumentGraphNode*> &ArgumentSCC = *I;
 418     if (ArgumentSCC.size() == 1) {
 419       if (!ArgumentSCC[0]->Definition) continue;  // synthetic root node
 420
 421       // eg. "void f(int* x) { if (...) f(x); }"
 422       if (ArgumentSCC[0]->Uses.size() == 1 &&
 423           ArgumentSCC[0]->Uses[0] == ArgumentSCC[0]) {
 424         ArgumentSCC[0]->Definition->addAttr(Attribute::NoCapture);
 425         ++NumNoCapture;
 426         Changed = true;
 427       }
 428       continue;
 429     }
 430
 431     bool SCCCaptured = false;
 432     for (std::vector<ArgumentGraphNode*>::iterator I = ArgumentSCC.begin(),
 433            E = ArgumentSCC.end(); I != E && !SCCCaptured; ++I) {
 434       ArgumentGraphNode *Node = *I;
 435       if (Node->Uses.empty()) {
 436         if (!Node->Definition->hasNoCaptureAttr())
 437           SCCCaptured = true;
 438       }
 439     }
 440     if (SCCCaptured) continue;
 441
 442     SmallPtrSet<Argument*, 8> ArgumentSCCNodes;
 443     // Fill ArgumentSCCNodes with the elements of the ArgumentSCC.  Used for
 444     // quickly looking up whether a given Argument is in this ArgumentSCC.
 445     for (std::vector<ArgumentGraphNode*>::iterator I = ArgumentSCC.begin(),
 446            E = ArgumentSCC.end(); I != E; ++I) {
 447       ArgumentSCCNodes.insert((*I)->Definition);
 448     }
 449
 450     for (std::vector<ArgumentGraphNode*>::iterator I = ArgumentSCC.begin(),
 451            E = ArgumentSCC.end(); I != E && !SCCCaptured; ++I) {
 452       ArgumentGraphNode *N = *I;
 453       for (SmallVectorImpl<ArgumentGraphNode*>::iterator UI = N->Uses.begin(),
 454              UE = N->Uses.end(); UI != UE; ++UI) {
 455         Argument *A = (*UI)->Definition;
 456         if (A->hasNoCaptureAttr() || ArgumentSCCNodes.count(A))
 457           continue;
 458         SCCCaptured = true;
 459         break;
 460       }
 461     }
 462     if (SCCCaptured) continue;
 463
 464     for (unsigned i = 0, e = ArgumentSCC.size(); i != e; ++i) {
 465       Argument *A = ArgumentSCC[i]->Definition;
 466       A->addAttr(Attribute::NoCapture);
 467       ++NumNoCapture;
 468       Changed = true;
 469     }
 470   }
 471
 472   return Changed;
 473 }
 474
 475 /// IsFunctionMallocLike - A function is malloc-like if it returns either null
 476 /// or a pointer that doesn't alias any other pointer visible to the caller.
 477 bool FunctionAttrs::IsFunctionMallocLike(Function *F,
 478                               SmallPtrSet<Function*, 8> &SCCNodes) const {
 479   UniqueVector<Value *> FlowsToReturn;
 480   for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I)
 481     if (ReturnInst *Ret = dyn_cast<ReturnInst>(I->getTerminator()))
 482       FlowsToReturn.insert(Ret->getReturnValue());
 483
 484   for (unsigned i = 0; i != FlowsToReturn.size(); ++i) {
 485     Value *RetVal = FlowsToReturn[i+1];   // UniqueVector[0] is reserved.
 486
 487     if (Constant *C = dyn_cast<Constant>(RetVal)) {
 488       if (!C->isNullValue() && !isa<UndefValue>(C))
 489         return false;
 490
 491       continue;
 492     }
 493
 494     if (isa<Argument>(RetVal))
 495       return false;
 496
 497     if (Instruction *RVI = dyn_cast<Instruction>(RetVal))
 498       switch (RVI->getOpcode()) {
 499         // Extend the analysis by looking upwards.
 500         case Instruction::BitCast:
 501         case Instruction::GetElementPtr:
 502           FlowsToReturn.insert(RVI->getOperand(0));
 503           continue;
 504         case Instruction::Select: {
 505           SelectInst *SI = cast<SelectInst>(RVI);
 506           FlowsToReturn.insert(SI->getTrueValue());
 507           FlowsToReturn.insert(SI->getFalseValue());
 508           continue;
 509         }
 510         case Instruction::PHI: {
 511           PHINode *PN = cast<PHINode>(RVI);
 512           for (int i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
 513             FlowsToReturn.insert(PN->getIncomingValue(i));
 514           continue;
 515         }
 516
 517         // Check whether the pointer came from an allocation.
 518         case Instruction::Alloca:
 519           break;
 520         case Instruction::Call:
 521         case Instruction::Invoke: {
 522           CallSite CS(RVI);
 523           if (CS.paramHasAttr(0, Attribute::NoAlias))
 524             break;
 525           if (CS.getCalledFunction() &&
 526               SCCNodes.count(CS.getCalledFunction()))
 527             break;
 528         } // fall-through
 529         default:
 530           return false;  // Did not come from an allocation.
 531       }
 532
 533     if (PointerMayBeCaptured(RetVal, false, /*StoreCaptures=*/false))
 534       return false;
 535   }
 536
 537   return true;
 538 }
 539
 540 /// AddNoAliasAttrs - Deduce noalias attributes for the SCC.
 541 bool FunctionAttrs::AddNoAliasAttrs(const CallGraphSCC &SCC) {
 542   SmallPtrSet<Function*, 8> SCCNodes;
 543
 544   // Fill SCCNodes with the elements of the SCC.  Used for quickly
 545   // looking up whether a given CallGraphNode is in this SCC.
 546   for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I)
 547     SCCNodes.insert((*I)->getFunction());
 548
 549   // Check each function in turn, determining which functions return noalias
 550   // pointers.
 551   for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
 552     Function *F = (*I)->getFunction();
 553
 554     if (F == 0)
 555       // External node - skip it;
 556       return false;
 557
 558     // Already noalias.
 559     if (F->doesNotAlias(0))
 560       continue;
 561
 562     // Definitions with weak linkage may be overridden at linktime, so
 563     // treat them like declarations.
 564     if (F->isDeclaration() || F->mayBeOverridden())
 565       return false;
 566
 567     // We annotate noalias return values, which are only applicable to
 568     // pointer types.
 569     if (!F->getReturnType()->isPointerTy())
 570       continue;
 571
 572     if (!IsFunctionMallocLike(F, SCCNodes))
 573       return false;
 574   }
 575
 576   bool MadeChange = false;
 577   for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
 578     Function *F = (*I)->getFunction();
 579     if (F->doesNotAlias(0) || !F->getReturnType()->isPointerTy())
 580       continue;
 581
 582     F->setDoesNotAlias(0);
 583     ++NumNoAlias;
 584     MadeChange = true;
 585   }
 586
 587   return MadeChange;
 588 }
 589
 590 bool FunctionAttrs::runOnSCC(CallGraphSCC &SCC) {
 591   AA = &getAnalysis<AliasAnalysis>();
 592
 593   bool Changed = AddReadAttrs(SCC);
 594   Changed |= AddNoCaptureAttrs(SCC);
 595   Changed |= AddNoAliasAttrs(SCC);
 596   return Changed;
 597 }