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

[Michael Matz]

Hi,

On Wed, 20 Jun 2007, Andrew Pinski wrote:

> On 6/20/07, Michael Matz <matz@suse.de> 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.
> 
> Huh?  Yes it does violate transivity.  int *a; void *b;
> "b = a;" vs "a = (int*)b;"  (this is IR form I am talking about).

That example shows that uselessness is not symmetric, it doesn't seem to 
talk about transitivity, but we already knew that this relation isn't 
symmetric.  And no, regarding conversions _to_ void* as useless in itself 
doesn't destroy transitivity anywhere.  Except that currently the whole 
thing still uses the langhook, which sometimes regards conversion _from_ 
void* as useless, which obviously is broken.


Ciao,
Michael.