Re: [PATCH][RFC] Re-structure tree_ssa_useless_type_conversion_1 to work towards a middle-end type system

[Richard Guenther <rguenther at suse dot de>]

On Wed, 20 Jun 2007, Michael Matz wrote:

> Hi,
> 
> On Wed, 20 Jun 2007, Richard Guenther wrote:
> 
> >       /* If the outer type is (void *), then the conversion is not
> >          necessary.
> >          ???  This makes tree_ssa_useless_type_conversion_1 not
> >          transitive.  */
> 
> Not this line itself makes it not transitive, but the fact that it still 
> relies on the frontends langhooks makes it so.  Document that fact so it's 
> clear that when the final goal is implemented (langhook removed) this 
> doesn't violate transitivity.

Done.
 
> >       if (TREE_CODE (TREE_TYPE (outer_type)) == VOID_TYPE)
> >         return true;
> > 
> >   /* Return true if the conversion from INNER_TYPE to OUTER_TYPE is a
> > !    useless type conversion, otherwise return false.
> > !    This function implicitly defines the middle-end type system.  The
> > !    following invariants shall be fulfilled:
> > ! 
> > !      1) tree_ssa_useless_type_conversion_1 is transitive.  If
> > ! 	a < b and b < c then a < c.
> > ! 
> > !      2) tree_ssa_useless_type_conversion_1 is not communtative.
> > ! 	From a < b does not follow a > b.
> > ! 
> > !      3) Conversions are useless only if with the resulting type
> 
> Something is missing in this sentence.  Perhaps "if _values_ with the 
> resulting type ..."?  The whole sentence reads strange, though.  
> Operations are not applied to types but to values.  Types actually _are_ 
> exactly the set of operations applicable to values.
> 
> Perhaps reformulate the whole thing to something like:
> 
> 3) Types define the available set of operations applicable to values.  A 
>    type conversion is useless if the operations for the target type is a 
>    subset of the operations for the source type.  For example casts to 
>    void* are useless, casts from void* are not (void* can't be 
>    dereferenced or offsetted, but copied, hence its set of operations is 
>    a strict subset of that of all other data pointer types).  Casts to
>    const T* are useless (can't be written to), casts from const T* to T* 
>    are not.

I've copied your rewrite.

Thanks,
Richard.