[LLVMdev] Casting.h illness

[Eugene Toder]

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).

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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