[rfc][patch] Eliminate quadratic slow-down on number of solibs.

[ppluzhnikov@google.com (Paul Pluzhnikov)]

Greetings,

We have a test case which uses ~2800 shared libraries.

Running this test with '--version' (i.e. run to main and exit) takes about
40 seconds.

Running it under GDB (CVS Head) with a breakpoint set on main takes 15
minutes, with significant CPU usage by GDB itself. Profiling GDB, I see
that the 2 top contributors are:

 time   seconds   seconds    calls   s/call   s/call  name    
 61.66    728.26   728.26     2794     0.26     0.26  find_methods
 24.21   1014.18   285.91  3904625     0.00     0.00  lookup_minimal_symbol_text

The former call comes from breakpoin_re_set(), as GDB scans all libraries
loaded so far, looking to see if any of them define Objective-C methods
which match "main" as a selector.

The latter call comes from repeaded calls to create_overlay_event_breakpoint,
which again scans all libraries loaded so far for "_ovly_debug_event".

AFAICT, both of the above scans are quadratic in number of solibs; and we
don't use either overlays or Objective-C, so this is "pure waste" for us.

To address the repeated scanning for objective-c, I propose adding
n_objc_syms to the struct objstats, initializing that field to ~0,
and counting them in find_methods the first time we encounter a given
objfile. If we see that objfile again, and n_objc_syms == 0, find_methods
could return immediately.

To address the overlay_event_breakpoint, I propose attached patch.
[Tested on Linux/x86_64 with no regressions.]

Comments?

Thanks,
--
Paul Pluzhnikov

ChangeLog:

2009-04-20  Paul Pluzhnikov  <ppluzhnikov@google.com>

	* breakpoint.h: Add breakpoint_re_set_objfile prototype.

	* breakpoint.c (breakpoint_re_set_objfile): Don't rescan
	all objfiles unnecessarily.
	(breakpoint_re_set): New function.

	* symfile.c (new_symfile_objfile): Call breakpoint_re_set_objfile
	instead of breakpoint_re_set.
	
	* objfiles.c (objfile_relocate): Likewise.


Index: breakpoint.h
===================================================================
RCS file: /cvs/src/src/gdb/breakpoint.h,v
retrieving revision 1.90
diff -u -p -u -r1.90 breakpoint.h
--- breakpoint.h	31 Mar 2009 16:44:17 -0000	1.90
+++ breakpoint.h	20 Apr 2009 23:02:11 -0000
@@ -687,7 +687,7 @@ extern int breakpoint_thread_match (CORE
 extern void until_break_command (char *, int, int);
 
 extern void breakpoint_re_set (void);
-
+extern void breakpoint_re_set_objfile (struct objfile *);
 extern void breakpoint_re_set_thread (struct breakpoint *);
 
 extern struct breakpoint *set_momentary_breakpoint
Index: breakpoint.c
===================================================================
RCS file: /cvs/src/src/gdb/breakpoint.c,v
retrieving revision 1.390
diff -u -p -u -r1.390 breakpoint.c
--- breakpoint.c	31 Mar 2009 16:44:17 -0000	1.390
+++ breakpoint.c	20 Apr 2009 23:02:11 -0000
@@ -7711,7 +7711,7 @@ breakpoint_re_set_one (void *bint)
 
 /* Re-set all breakpoints after symbols have been re-loaded.  */
 void
-breakpoint_re_set (void)
+breakpoint_re_set_objfile (struct objfile *objfile)
 {
   struct breakpoint *b, *temp;
   enum language save_language;
@@ -7730,8 +7730,17 @@ breakpoint_re_set (void)
   }
   set_language (save_language);
   input_radix = save_input_radix;
-  
-  create_overlay_event_breakpoint ("_ovly_debug_event");
+
+  if (objfile != NULL)
+    create_overlay_event_breakpoint_1 ("_ovly_debug_event", objfile);
+  else
+    create_overlay_event_breakpoint ("_ovly_debug_event");
+}
+
+void
+breakpoint_re_set (void)
+{
+  breakpoint_re_set_objfile (NULL);
 }
 \f
 /* Reset the thread number of this breakpoint:
Index: symfile.c
===================================================================
RCS file: /cvs/src/src/gdb/symfile.c,v
retrieving revision 1.224
diff -u -p -u -r1.224 symfile.c
--- symfile.c	7 Apr 2009 20:43:51 -0000	1.224
+++ symfile.c	20 Apr 2009 23:02:11 -0000
@@ -922,7 +922,7 @@ new_symfile_objfile (struct objfile *obj
     }
   else
     {
-      breakpoint_re_set ();
+      breakpoint_re_set_objfile (objfile);
     }
 
   /* We're done reading the symbol file; finish off complaints.  */
Index: objfiles.c
===================================================================
RCS file: /cvs/src/src/gdb/objfiles.c,v
retrieving revision 1.82
diff -u -p -u -r1.82 objfiles.c
--- objfiles.c	11 Mar 2009 20:26:02 -0000	1.82
+++ objfiles.c	20 Apr 2009 23:02:11 -0000
@@ -674,7 +674,7 @@ objfile_relocate (struct objfile *objfil
     }
 
   /* Relocate breakpoints as necessary, after things are relocated. */
-  breakpoint_re_set ();
+  breakpoint_re_set_objfile (objfile);
 }
 \f
 /* Many places in gdb want to test just to see if we have any partial