llvm dev - Nov 2008 - [LLVMdev] m_Not Pattern Question

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

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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>

On Nov 9, 2008, at 7:51 AM, Nick Lewycky wrote:
> Bill Wendling wrote:
>> 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 ...
>
Ah! Okay. Thanks for the explanation.

-bw