C++ UseVector::clear方法代码示例

//.........这里部分代码省略.........
          != F.getFunctionType()->getReturnType()) {
        // We don't support old style multiple return values.
        MarkLive(F);
        return;
      }

  if (!F.hasLocalLinkage() && (!ShouldHackArguments() || F.isIntrinsic())) {
    MarkLive(F);
    return;
  }

  DEBUG(dbgs() << "DAE - Inspecting callers for fn: " << F.getName() << "\n");
  // Keep track of the number of live retvals, so we can skip checks once all
  // of them turn out to be live.
  unsigned NumLiveRetVals = 0;
  // Loop all uses of the function.
  for (const Use &U : F.uses()) {
    // If the function is PASSED IN as an argument, its address has been
    // taken.
    ImmutableCallSite CS(U.getUser());
    if (!CS || !CS.isCallee(&U)) {
      MarkLive(F);
      return;
    }

    // If this use is anything other than a call site, the function is alive.
    const Instruction *TheCall = CS.getInstruction();
    if (!TheCall) {   // Not a direct call site?
      MarkLive(F);
      return;
    }

    // If we end up here, we are looking at a direct call to our function.

    // Now, check how our return value(s) is/are used in this caller. Don't
    // bother checking return values if all of them are live already.
    if (NumLiveRetVals == RetCount)
      continue;

    // Check all uses of the return value.
    for (const Use &U : TheCall->uses()) {
      if (ExtractValueInst *Ext = dyn_cast<ExtractValueInst>(U.getUser())) {
        // This use uses a part of our return value, survey the uses of
        // that part and store the results for this index only.
        unsigned Idx = *Ext->idx_begin();
        if (RetValLiveness[Idx] != Live) {
          RetValLiveness[Idx] = SurveyUses(Ext, MaybeLiveRetUses[Idx]);
          if (RetValLiveness[Idx] == Live)
            NumLiveRetVals++;
        }
      } else {
        // Used by something else than extractvalue. Survey, but assume that the
        // result applies to all sub-values.
        UseVector MaybeLiveAggregateUses;
        if (SurveyUse(&U, MaybeLiveAggregateUses) == Live) {
          NumLiveRetVals = RetCount;
          RetValLiveness.assign(RetCount, Live);
          break;
        } else {
          for (unsigned i = 0; i != RetCount; ++i) {
            if (RetValLiveness[i] != Live)
              MaybeLiveRetUses[i].append(MaybeLiveAggregateUses.begin(),
                                         MaybeLiveAggregateUses.end());
          }
        }
      }
    }
  }

  // Now we've inspected all callers, record the liveness of our return values.
  for (unsigned i = 0; i != RetCount; ++i)
    MarkValue(CreateRet(&F, i), RetValLiveness[i], MaybeLiveRetUses[i]);

  DEBUG(dbgs() << "DAE - Inspecting args for fn: " << F.getName() << "\n");

  // Now, check all of our arguments.
  unsigned i = 0;
  UseVector MaybeLiveArgUses;
  for (Function::const_arg_iterator AI = F.arg_begin(),
       E = F.arg_end(); AI != E; ++AI, ++i) {
    Liveness Result;
    if (F.getFunctionType()->isVarArg()) {
      // Variadic functions will already have a va_arg function expanded inside
      // them, making them potentially very sensitive to ABI changes resulting
      // from removing arguments entirely, so don't. For example AArch64 handles
      // register and stack HFAs very differently, and this is reflected in the
      // IR which has already been generated.
      Result = Live;
    } else {
      // See what the effect of this use is (recording any uses that cause
      // MaybeLive in MaybeLiveArgUses). 
      Result = SurveyUses(&*AI, MaybeLiveArgUses);
    }

    // Mark the result.
    MarkValue(CreateArg(&F, i), Result, MaybeLiveArgUses);
    // Clear the vector again for the next iteration.
    MaybeLiveArgUses.clear();
  }
}

//.........这里部分代码省略.........
  typedef SmallVector<UseVector, 5> RetUses;
  // These vectors map each return value to the uses that make it MaybeLive, so
  // we can add those to the Uses map if the return value really turns out to be
  // MaybeLive. Initialized to a list of RetCount empty lists.
  RetUses MaybeLiveRetUses(RetCount);

  for (Function::const_iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
    if (const ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator()))
      if (RI->getNumOperands() != 0 && RI->getOperand(0)->getType()
          != F.getFunctionType()->getReturnType()) {
        // We don't support old style multiple return values.
        MarkLive(F);
        return;
      }

  if (!F.hasLocalLinkage() && (!ShouldHackArguments() || F.isIntrinsic())) {
    MarkLive(F);
    return;
  }

  DEBUG(dbgs() << "DAE - Inspecting callers for fn: " << F.getName() << "\n");
  // Keep track of the number of live retvals, so we can skip checks once all
  // of them turn out to be live.
  unsigned NumLiveRetVals = 0;
  Type *STy = dyn_cast<StructType>(F.getReturnType());
  // Loop all uses of the function.
  for (Value::const_use_iterator I = F.use_begin(), E = F.use_end();
       I != E; ++I) {
    // If the function is PASSED IN as an argument, its address has been
    // taken.
    ImmutableCallSite CS(*I);
    if (!CS || !CS.isCallee(I)) {
      MarkLive(F);
      return;
    }

    // If this use is anything other than a call site, the function is alive.
    const Instruction *TheCall = CS.getInstruction();
    if (!TheCall) {   // Not a direct call site?
      MarkLive(F);
      return;
    }

    // If we end up here, we are looking at a direct call to our function.

    // Now, check how our return value(s) is/are used in this caller. Don't
    // bother checking return values if all of them are live already.
    if (NumLiveRetVals != RetCount) {
      if (STy) {
        // Check all uses of the return value.
        for (Value::const_use_iterator I = TheCall->use_begin(),
             E = TheCall->use_end(); I != E; ++I) {
          const ExtractValueInst *Ext = dyn_cast<ExtractValueInst>(*I);
          if (Ext && Ext->hasIndices()) {
            // This use uses a part of our return value, survey the uses of
            // that part and store the results for this index only.
            unsigned Idx = *Ext->idx_begin();
            if (RetValLiveness[Idx] != Live) {
              RetValLiveness[Idx] = SurveyUses(Ext, MaybeLiveRetUses[Idx]);
              if (RetValLiveness[Idx] == Live)
                NumLiveRetVals++;
            }
          } else {
            // Used by something else than extractvalue. Mark all return
            // values as live.
            for (unsigned i = 0; i != RetCount; ++i )
              RetValLiveness[i] = Live;
            NumLiveRetVals = RetCount;
            break;
          }
        }
      } else {
        // Single return value
        RetValLiveness[0] = SurveyUses(TheCall, MaybeLiveRetUses[0]);
        if (RetValLiveness[0] == Live)
          NumLiveRetVals = RetCount;
      }
    }
  }

  // Now we've inspected all callers, record the liveness of our return values.
  for (unsigned i = 0; i != RetCount; ++i)
    MarkValue(CreateRet(&F, i), RetValLiveness[i], MaybeLiveRetUses[i]);

  DEBUG(dbgs() << "DAE - Inspecting args for fn: " << F.getName() << "\n");

  // Now, check all of our arguments.
  unsigned i = 0;
  UseVector MaybeLiveArgUses;
  for (Function::const_arg_iterator AI = F.arg_begin(),
       E = F.arg_end(); AI != E; ++AI, ++i) {
    // See what the effect of this use is (recording any uses that cause
    // MaybeLive in MaybeLiveArgUses).
    Liveness Result = SurveyUses(AI, MaybeLiveArgUses);
    // Mark the result.
    MarkValue(CreateArg(&F, i), Result, MaybeLiveArgUses);
    // Clear the vector again for the next iteration.
    MaybeLiveArgUses.clear();
  }
}