C++ GlobalVariable::mayBeOverridden方法代码示例

/// analyzeDestructor - Given the heap.metadata argument to swift_allocObject,
/// take a look a the destructor and try to decide if it has side effects or any
/// other bad effects that can prevent it from being optimized.
static DtorKind analyzeDestructor(Value *P) {
  // If we have a null pointer for the metadata info, the dtor has no side
  // effects.  Actually, the final release would crash.  This is really only
  // useful for writing testcases.
  if (isa<ConstantPointerNull>(P->stripPointerCasts()))
    return DtorKind::NoSideEffects;

  // We have to have a known heap metadata value, reject dynamically computed
  // ones, or places
  GlobalVariable *GV = dyn_cast<GlobalVariable>(P->stripPointerCasts());
  if (GV == 0 || GV->mayBeOverridden()) return DtorKind::Unknown;

  ConstantStruct *CS = dyn_cast_or_null<ConstantStruct>(GV->getInitializer());
  if (CS == 0 || CS->getNumOperands() == 0) return DtorKind::Unknown;

  // FIXME: Would like to abstract the dtor slot (#0) out from this to somewhere
  // unified.
  enum { DTorSlotOfHeapMetadata = 0 };
  Function *DtorFn =dyn_cast<Function>(CS->getOperand(DTorSlotOfHeapMetadata));
  if (DtorFn == 0 || DtorFn->mayBeOverridden() || DtorFn->hasExternalLinkage())
    return DtorKind::Unknown;

  // Okay, we have a body, and we can trust it.  If the function is marked
  // readonly, then we know it can't have any interesting side effects, so we
  // don't need to analyze it at all.
  if (DtorFn->onlyReadsMemory())
    return DtorKind::NoSideEffects;

  // The first argument is the object being destroyed.
  assert(DtorFn->arg_size() == 1 && !DtorFn->isVarArg() &&
         "expected a single object argument to destructors");
  Value *ThisObject = &*DtorFn->arg_begin();

  // Scan the body of the function, looking for anything scary.
  for (BasicBlock &BB : *DtorFn) {
    for (Instruction &I : BB) {
      // Note that the destructor may not be in any particular canonical form.
      switch (classifyInstruction(I)) {
      // These instructions should not reach here based on the pass ordering.
      // i.e. LLVMARCOpt -> LLVMContractOpt.
      case RT_RetainN:
      case RT_UnknownRetainN:
      case RT_BridgeRetainN:
      case RT_ReleaseN:
      case RT_UnknownReleaseN:
      case RT_BridgeReleaseN:
        llvm_unreachable("These are only created by LLVMARCContract !");
      case RT_NoMemoryAccessed:
      case RT_AllocObject:
      case RT_FixLifetime:
      case RT_CheckUnowned:
        // Skip over random instructions that don't touch memory in the caller.
        continue;

      case RT_RetainUnowned:
      case RT_BridgeRetain:          // x = swift_bridgeRetain(y)
      case RT_Retain: {      // swift_retain(obj)

        // Ignore retains of the "self" object, no resurrection is possible.
        Value *ThisRetainedObject = cast<CallInst>(I).getArgOperand(0);
        if (ThisRetainedObject->stripPointerCasts() ==
            ThisObject->stripPointerCasts())
          continue;
        // Otherwise, we may be retaining something scary.
        break;
      }

      case RT_Release: {
        // If we get to a release that is provably to this object, then we can
        // ignore it.
        Value *ThisReleasedObject = cast<CallInst>(I).getArgOperand(0);

        if (ThisReleasedObject->stripPointerCasts() ==
            ThisObject->stripPointerCasts())
          continue;
        // Otherwise, we may be retaining something scary.
        break;
      }

      case RT_ObjCRelease:
      case RT_ObjCRetain:
      case RT_UnknownRetain:
      case RT_UnknownRelease:
      case RT_BridgeRelease:
        // Objective-C retain and release can have arbitrary side effects.
        break;

      case RT_Unknown:
        // Ignore all instructions with no side effects.
        if (!I.mayHaveSideEffects()) continue;

        // store, memcpy, memmove *to* the object can be dropped.
        if (StoreInst *SI = dyn_cast<StoreInst>(&I)) {
          if (SI->getPointerOperand()->stripInBoundsOffsets() == ThisObject)
            continue;
        }

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