[LLVMdev] m_Not Pattern Question

[Nick Lewycky]

Bill Wendling wrote:
> 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.

This is deliberate. m_Foo(V) means "match any Foo and put its result 
into V." The way it's generally used is to declare empty variables, 
match against them, and test the equality:

   Value *A, *B, *C, *D;
   if (match(Expr, m_Or(m_And(A, B), m_And(C, D))) {
     if (B == D) {
       ... do optimization ...

Nick

> -bw
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