Re: PR 23551: why should we coalesce inlined variables?

["Richard Guenther" <richard dot guenther at gmail dot com>]

On 7/9/07, Alexandre Oliva <aoliva@redhat.com> wrote:
> On Jul  9, 2007, "Richard Guenther" <richard.guenther@gmail.com> wrote:
>
> > On 7/9/07, Alexandre Oliva <aoliva@redhat.com> wrote:
> >> On Jul  9, 2007, "Richard Guenther" <richard.guenther@gmail.com> wrote:
> >>
> >> > So improve the way we are collecting information to produce debug info,
> >> > but not disable optimizations.
> >>
> >> +1
> >>
> >> The patch I proposed has already been shown to not disable
> >> optimizations, so I don't see what you're arguing for.
>
> > It disables variable coalescing which is an optimization.
>
> Make that "transformation".  It was shown to decrease overall
> performance on few-register arches, and to do so even on many-register
> arches for some specific testcases.
>
> > Well, I see no reason we need to increase memory usage for the case
> > '-g' is not specified.
>
> Because GCC is by design a compiler that generates the very same code
> regardless of -g.  We can't refrain from making certain
> transformations depending on -g, because they might change the
> generated code.

My suggestion isn't to make a transformation dependent on g.

> The mere presence of line number notes or other
> annotations has been shown to have effects on certain other
> transformations.  For example, the presence of trees holding location
> information could affect decisions about inlining or loop unrolling,
> and the presence of line number notes in RTL did cause some
> differences in code with or without -g in the past.

I'm very well ware of this.  I suggested to have a on-the-side mapping (you
need it for each function), like a hashtable that maps the SSA_NAME version
to a list of user-visible DECLs.  This doesn't change the IL and so doesn't
create different code if you are using -g or not.  You would need to maintain
this mapping during optimization passes, but this isn't different to the problem
you are facing now.  Just it would not be business of the IL to hold
the information.

Richard.