llvm dev - Jul 2007 - [LLVMdev] VirtRegMap GLIBCXX assert

The following code in VirtRegMap.cpp is asserting in the C++ library
because back() is called on an empty container:

  /// addLastUse - Add the last use information of all stack slots whose
  /// values are available in the specific register.
  void addLastUse(unsigned PhysReg, MachineInstr *Use) {
    std::multimap<unsigned, int>::iterator I      
PhysRegsAvailable.lower_bound(PhysReg);
    while (I != PhysRegsAvailable.end() && I->first == PhysReg) {
      int Slot = I->second;
      I++;

      std::map<int, SSInfo>::iterator II = SpillSlotsAvailable.find(Slot);
      assert(II != SpillSlotsAvailable.end() && "Slot not
available!");
      unsigned Val = II->second.first;
      assert((Val >> 1) == PhysReg && "Bidirectional map
mismatch!");
      // This can be true if there are multiple uses of the same register.
      if (II->second.second.back() != Use)  <========== assert here
        II->second.second.push_back(Use);
    }
  }

This happens when llvm-gcc tries to build libgcc.

An obvious fix is to change the line to:

      if (II->second.second.empty() || II->second.second.back() != Use)

Is this a proper fix?  Is there some assumption that II->second.second should
NOT be empty at this point?  In other words, is this a simple oversight bug or 
is there some deeper flaw in the program logic that gets exposed here?

                                                   -Dave

You are in luck. This function will be eliminated shortly. :-)

Evan

On Jul 10, 2007, at 11:57 AM, David A. Greene wrote:
> The following code in VirtRegMap.cpp is asserting in the C++ library
> because back() is called on an empty container:
>
>   /// addLastUse - Add the last use information of all stack slots  
> whose
>   /// values are available in the specific register.
>   void addLastUse(unsigned PhysReg, MachineInstr *Use) {
>     std::multimap<unsigned, int>::iterator I >      
PhysRegsAvailable.lower_bound(PhysReg);
>     while (I != PhysRegsAvailable.end() && I->first == PhysReg)
{
>       int Slot = I->second;
>       I++;
>
>       std::map<int, SSInfo>::iterator II = SpillSlotsAvailable.find 
> (Slot);
>       assert(II != SpillSlotsAvailable.end() && "Slot not  
> available!");
>       unsigned Val = II->second.first;
>       assert((Val >> 1) == PhysReg && "Bidirectional map
mismatch!");
>       // This can be true if there are multiple uses of the same  
> register.
>       if (II->second.second.back() != Use)  <========== assert here
>         II->second.second.push_back(Use);
>     }
>   }
>
> This happens when llvm-gcc tries to build libgcc.
>
> An obvious fix is to change the line to:
>
>       if (II->second.second.empty() || II->second.second.back() !=  
> Use)
>
> Is this a proper fix?  Is there some assumption that II- 
> >second.second should
> NOT be empty at this point?  In other words, is this a simple  
> oversight bug or
> is there some deeper flaw in the program logic that gets exposed here?
>
>                                                    -Dave
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