[rfc] Search only one (well, two) objfiles for a minimal symbol

[Daniel Jacobowitz <drow at false dot org>]

lookup_minimal_symbol_by_pc_section already calls find_pc_section.
For reasons described by Andrew's 2004-01-27 comment, we don't require
the found symbol to be in that section.  However, that objection
doesn't mean we have to ignore pc_section completely; I think it's
safe to require that we find the same objfile.

I have a simple testcase where this helps.  It's nothing but "set
obj.z.y.x = 1" repeated a couple thousand times in a command file.
Before this patch, lookup_minimal_symbol_by_pc_section takes 17% of
execution time; after this patch it takes about 2%, and the slowest
part is now c_lex.  That's because most of the minsyms were in
libc.so.6, which is clearly not the same objfile as the found section.
There's still room to speed things up, but not by a lot.

Any comments?  Joel, do you think you could test this on IRIX for me?
Thanks!

-- 
Daniel Jacobowitz
CodeSourcery

2007-06-24  Daniel Jacobowitz  <dan@codesourcery.com>

	* minsyms.c (lookup_minimal_symbol_by_pc_section): Search fewer
	objfiles.

Index: minsyms.c
===================================================================
RCS file: /cvs/src/src/gdb/minsyms.c,v
retrieving revision 1.50
diff -u -p -r1.50 minsyms.c
--- minsyms.c	15 Jun 2007 22:39:52 -0000	1.50
+++ minsyms.c	24 Jun 2007 18:44:08 -0000
@@ -387,20 +387,20 @@ lookup_minimal_symbol_by_pc_section (COR
   if (pc_section == NULL)
     return NULL;
 
-  /* NOTE: cagney/2004-01-27: Removed code (added 2003-07-19) that was
-     trying to force the PC into a valid section as returned by
-     find_pc_section.  It broke IRIX 6.5 mdebug which relies on this
-     code returning an absolute symbol - the problem was that
-     find_pc_section wasn't returning an absolute section and hence
-     the code below would skip over absolute symbols.  Since the
-     original problem was with finding a frame's function, and that
-     uses [indirectly] lookup_minimal_symbol_by_pc, the original
-     problem has been fixed by having that function use
-     find_pc_section.  */
-
-  for (objfile = object_files;
-       objfile != NULL;
-       objfile = objfile->next)
+  /* We can not require the symbol found to be in pc_section, because
+     e.g. IRIX 6.5 mdebug relies on this code returning an absolute
+     symbol - but find_pc_section won't return an absolute section and
+     hence the code below would skip over absolute symbols.  We can
+     still take advantage of the call to find_pc_section, though - the
+     object file still must match.  In case we have separate debug
+     files, search both the file and its separate debug file.  There's
+     no telling which one will have the minimal symbols.  */
+
+  objfile = pc_section->objfile;
+  if (objfile->separate_debug_objfile)
+    objfile = objfile->separate_debug_objfile;
+
+  for (; objfile != NULL; objfile = objfile->separate_debug_objfile_backlink)
     {
       /* If this objfile has a minimal symbol table, go search it using
          a binary search.  Note that a minimal symbol table always consists