[LLVMdev] Casting.h illness

[Chris Lattner]

On Jul 21, 2010, at 12:49 PM, Eugene Toder wrote:
> Is this a recommended approach/good style/good idea to use
> dyn_cast_or_null<X>(I) instead of dyn_cast_or_null<X>(*I)? (And other
> is and cast functions).

We generally don't want auto-dereference.  There is some special magic for unwrapping Uses etc though.  Is this what you're in contact with Gabor?

-Chris

> 
> Eugene
> 
> On Wed, Jul 21, 2010 at 12:42 PM, Gabor Greif <gabor at mac.com> wrote:
>> Hi all,
>> 
>> at the moment I am trying to fix an unnecessary iterator dereferencing
>> in Casting.h and am uncovering several ugly things.
>> 
>> For example, look at this code:
>> 
>> 
>>> // dyn_cast_or_null<X> - Functionally identical to dyn_cast, except that a null
>>> // value is accepted.
>>> //
>>> template <class X, class Y>
>>> inline typename cast_retty<X, Y>::ret_type dyn_cast_or_null(const Y &Val) {
>>>   return (Val && isa<X>(Val)) ? cast<X, Y>(Val) : 0;
>>> }
>> 
>> When Y is a pointer type all works perfectly, but when it is a (structured)
>> iterator we encounter several problems:
>> 
>> 1) Val is cast to bool on the LHS of the &&-operator. This is semantically
>>   wrong, as we want to dereference (local jargon: simplify) Val to a
>>   pointer first and compare that. Also Y may have a conversion to bool,
>>   which may or may not coincide with comparison of the dereferenced pointer
>>   against NULL. In this case no compile error will occur, but we may get
>>   some strange behaviour.
>> 
>> 2) When Val is an iterator, it will be simplified at least two times, namely
>>   in isa<> and in cast<>.
>> 
>> My suggestion is to simplify Val to a pointer first and then do the
>> comparison against NULL, the instance check and the cast.
>> 
>> I tried this (using getSimplifiedValue), but got further problems and had to
>> add a specialization
>> 
>> +template<typename From> struct simplify_type<From*const> {
>> +  typedef From* SimpleType;
>> +  static SimpleType &getSimplifiedValue(From*const &Val) {
>> +    return const_cast<From*&>(Val);
>> +  }
>> +};
>> 
>> Is there a simpler way to accomplish this?
>> 
>> Cheers,
>> 
>>        Gabor
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>> 
> 
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