Re: questioning glib r5316 by mclasen

[Stefan Kost <ensonic hora-obscura de>]

Hi,

Quoting Behdad Esfahbod <behdad behdad org>:

> On Mon, 2007-05-14 at 06:49 -0400, Tim Janik wrote:
> > hi Matthias.
> >
> > can you please explain your glib change from 2007-01-26 where you're making
> > function-scoped symbols non-static without any obvious need?
> >
> > i'm sorry to have to bring this up, but your ChangeLog entry is very
> > uninformative on this matter:
> >    +	* gutils.c: Make some structs which are used only once
> >    +	non-static.
> > because it uselessly states *what* was done (the diff can tell that
> > already), instead of describing *why* the change was done.
> >
> > thanks for consideration.
>
> Hi Tim,
>
> I'll give a shot at explaining this if you don't mind.
>
> A const struct when defined static will hopefully end up in the
> readonly .data section of the binary and process and everything will be
> perfect.  However, if the struct contains pointer arguments (function or
> data) that are initialized to some other symbol, it will need link-time
> relocation and hence cannot live in the readonly data segment anymore,
> and will be moved to regular, writable, .data.  The effect is that its
> memory will not be shared across processes.
>
> Now if you make that struct non-static, it will be allocated off the
> stack and initialized at run-time, and disposed when the stack frame
> goes out of scope.  No per-process penalty.
>
> In the cases that Matthias committed, the functions in question were
> only run once during a process's lifecycle, so there is not much runtime
> penalty being paid, in return for less link-time overhead and less
> per-process memory use.
Thanks for the great explanation. I am experimenting withsimillar  
changes in gstreamer. A gst_type_register_static_full() show similar  
effect like the g_type_register_static_simple(), so it would be good  
to have that in glib too. I would also like to see the same for  
interfaces. There it actually brings some more benefits too:
 #1 saving relocs
 #2 G_IMPLEMENT_INTERFACE can transparaently use it
 #3 G_IMPLEMENT_INTERFACE can be used multiple times in  
G_DEFINE_TYPE_WITH_CODE

From the above explanation I understand that all this only makes  
sense to structures that are initialized with pointers. I ave onecase  
in gstreamer that I don't fully understand (sorry for offtopic). Each  
element (aka widget) has
  static const GstElementDetails details = GST_ELEMENT_DETAILS (
      "bla1",  "bla2", "bl3", "bla4");
  gst_element_class_set_details (gstelement_class, &details);

I changed that into
  gst_element_class_set_details_simple (gstelement_class,
    "bla1",  "bla2", "bl3", "bla4");

where the function makes a const GstElementDetails details and fills  
it. This saves lots of relocs, but increases the binary size. When I  
dump section, the sectionsizes do not sum up to the binary size and  
they even get smaller. So whats the extra space - any ideas?

I will check whats the difference if I just drop the 'static' from version 1.

Stefan
> Regards,
>
> behdad
>
>
> > ---
> > ciaoTJ
> >
> > Index: /usr/src/gtk+head/glib/glib/gmessages.c
> > ===================================================================
> > --- /usr/src/gtk+head/glib/glib/gmessages.c	(revision 5315)
> > +++ /usr/src/gtk+head/glib/glib/gmessages.c	(revision 5316)
> > @@ -144,7 +144,7 @@
> >
> >         if (val)
> >   	{
> > -	  static const GDebugKey keys[] = {
> > +	  const GDebugKey keys[] = {
> >   	    { "error", G_LOG_LEVEL_ERROR },
> >   	    { "critical", G_LOG_LEVEL_CRITICAL },
> >   	    { "warning", G_LOG_LEVEL_WARNING },
> > @@ -1062,7 +1062,7 @@
> >     val = g_getenv ("G_DEBUG");
> >     if (val != NULL)
> >       {
> > -      static const GDebugKey keys[] = {
> > +      const GDebugKey keys[] = {
> >   	{"fatal_warnings", G_DEBUG_FATAL_WARNINGS},
> >   	{"fatal_criticals", G_DEBUG_FATAL_CRITICALS}
> >         };
> > Index: /usr/src/gtk+head/glib/glib/gslice.c
> > ===================================================================
> > --- /usr/src/gtk+head/glib/glib/gslice.c	(revision 5315)
> > +++ /usr/src/gtk+head/glib/glib/gslice.c	(revision 5316)
> > @@ -277,7 +277,7 @@
> >     /* don't use g_malloc/g_message here */
> >     gchar buffer[1024];
> >     const gchar *val = _g_getenv_nomalloc ("G_SLICE", buffer);
> > -  static const GDebugKey keys[] = {
> > +  const GDebugKey keys[] = {
> >       { "always-malloc", 1 << 0 },
> >       { "debug-blocks",  1 << 1 },
> >     };
> > Index: /usr/src/gtk+head/glib/glib/gmem.c
> > ===================================================================
> > --- /usr/src/gtk+head/glib/glib/gmem.c	(revision 5315)
> > +++ /usr/src/gtk+head/glib/glib/gmem.c	(revision 5316)
> > @@ -689,7 +689,7 @@
> >   {
> >     gchar buffer[1024];
> >     const gchar *val;
> > -  static const GDebugKey keys[] = {
> > +  const GDebugKey keys[] = {
> >       { "gc-friendly", 1 },
> >     };
> >     gint flags;
> > Index: /usr/src/gtk+head/glib/ChangeLog
> > ===================================================================
> > --- /usr/src/gtk+head/glib/ChangeLog	(revision 5315)
> > +++ /usr/src/gtk+head/glib/ChangeLog	(revision 5316)
> > @@ -1,3 +1,11 @@
> > +2007-01-26  Matthias Clasen <mclasen redhat com>
> > +
> > +	* gmem.c:
> > +	* gslice.c:
> > +	* gmessages.c:
> > +	* gutils.c: Make some structs which are used only once
> > +	non-static.
> > +
> >   2007-01-24  Benjamin Otte <otte gnome org>
> >
> >   	* glib/gprintf.c (g_sprintf): Clarify the documentation
> > _______________________________________________
> > gtk-devel-list mailing list
> > gtk-devel-list gnome org
> > http://mail.gnome.org/mailman/listinfo/gtk-devel-list
> --
> behdad
> http://behdad.org/
>
> "Those who would give up Essential Liberty to purchase a little
>  Temporary Safety, deserve neither Liberty nor Safety."
>         -- Benjamin Franklin, 1759
>
>
>
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