[LLVMdev] m_Not Pattern Question

[Bill Wendling]

I have a question about the pattern matching stuff that's used in the  
Instruction Combiner. If I have code like this:

     if (match(B, m_Select(m_Value(), m_ConstantInt(0),  
m_ConstantInt(-1)))) {
       if (match(C, m_Not(m_Value(B))))
         return SelectInst::Create(cast<User>(B)->getOperand(0), D, A);

and we match, the program fails during the "SelectInst::Create" phase.  
The problem is because B is now a Constant with "-2" as its value, so  
it has 0 operands. It appears that the m_Not() match is changing the  
value of B during this call.

Is this intentional? This happens in the "bind_ty" template in  
PatternMatch.h:

template<typename Class>
struct bind_ty {
   Class *&VR;
   bind_ty(Class *&V) : VR(V) {}

   template<typename ITy>
   bool match(ITy *V) {
     if (Class *CV = dyn_cast<Class>(V)) {
       VR = CV;
       return true;
     }
     return false;
   }
};

Why are we taking a reference to a pointer here? The assignment in the  
"match" method over writes the value of "B", which seems weird and  
wrong to me.

-bw