Re: questioning glib r5316 by mclasen


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.





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
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