Huge slowdown since 6.0

[Daniel Jacobowitz <drow at false dot org>]

I finished prototyping my partial-symbol-sorting changes, as discussed on
gdb-patches, and went to measure their impact.  The results were a little
confusing, but I've tracked them down.  At the bottom of my message are
profiling numbers if you want to see them.  The summary is that I hope they
won't be needed.

Basically, the bottleneck in strcmp_iw_ordered that I am attacking was not
there in 6.0.  This baffled me.  There were just as many psymbols and there
should have been just as many calls; the change to use strcmp_iw_ordered
predates 6.0.

The only reasonable explanation is that the number of global symbols has
vastly increased.  This appears to be the case.  David, the blame appears to
be yours, in dwarf2read.c revision 1.120:

@@ -1519,14 +1556,16 @@ add_partial_symbol (struct partial_die_i
          /* For C++, these implicitly act as typedefs as well. */
          add_psymbol_to_list (actual_name, strlen (actual_name),
                               VAR_DOMAIN, LOC_TYPEDEF,
-                              &objfile->static_psymbols,
+                              &objfile->global_psymbols,
                               0, (CORE_ADDR) 0, cu_language, objfile);
        }
       break;
     case DW_TAG_enumerator:
       add_psymbol_to_list (actual_name, strlen (actual_name),
                           VAR_DOMAIN, LOC_CONST,
-                          &objfile->static_psymbols,
+                          cu_language == language_cplus
+                          ? &objfile->static_psymbols
+                          : &objfile->global_psymbols,
                           0, (CORE_ADDR) 0, cu_language, objfile);
       break;
     default:

Could you re-explain the need for this change, please?  You said:

+           /* NOTE: carlton/2003-11-10: C++ class symbols shouldn't
+              really ever be static objects: otherwise, if you try
+              to, say, break of a class's method and you're in a file
+              which doesn't mention that class, it won't work unless
+              the check for all static symbols in lookup_symbol_aux
+              saves you.  See the OtherFileClass tests in
+              gdb.c++/namespace.exp.  */
+

but is that also necessary for enumerators?  As things stand, one large enum
in a used header can lead to worse-than-linear slowdown of GDB startup for
DWARF-2, and a couple megabytes of wasted memory.


Profiling information:

HEAD:
% time /opt/src/gdb/x86-as/gdb/gdb -batch /usr/lib/debug/libc.so.6
Using host libthread_db library "/lib/tls/i686/cmov/libthread_db.so.1".
/opt/src/gdb/x86-as/gdb/gdb -batch /usr/lib/debug/libc.so.6  3.35s user 0.32s system 97% cpu 3.751 total

With my changes:
drow@nevyn:/big/fsf/projects/sym-full-name/ot% time ../obj/gdb/gdb -batch /usr/lib/debug/libc.so.6
Using host libthread_db library "/lib/tls/i686/cmov/libthread_db.so.1".
../obj/gdb/gdb -batch /usr/lib/debug/libc.so.6  2.43s user 0.21s system 99% cpu 2.656 total

Awesome!  28% speedup.  But:

6.0:
% time ./gdb/gdb -batch /usr/lib/debug/libc.so.6
./gdb/gdb -batch /usr/lib/debug/libc.so.6  1.95s user 0.26s system 96% cpu 2.288 total


So it recovers a bare half of what we've lost since GDB 6.0.  All binaries
built using the same compiler and CFLAGS (-O2).

Profiles for 6.0:
  %   cumulative   self              self     total           
 time   samples   samples    calls  T1/call  T1/call  name    
 28.46   9128.00  9128.00                             bcache
 10.92  12631.00  3503.00                             read_partial_die
  9.10  15549.00  2918.00                             hash
  8.13  18155.00  2606.00                             htab_find_slot_with_hash
  5.57  19943.00  1788.00                             read_attribute_value
  5.11  21581.00  1638.00                             htab_hash_string
  3.80  22799.00  1218.00                             read_unsigned_leb128
...
  0.04  31769.00    14.00                             strcmp_iw_ordered
...
  0.01  32011.00     2.00                             compare_psymbols

For HEAD:
  %   cumulative   self              self     total           
 time   samples   samples    calls  T1/call  T1/call  name    
 19.00   8696.00  8696.00                             strcmp_iw_ordered
 16.31  16162.00  7466.00                             bcache_data
  7.50  19594.00  3432.00                             read_partial_die
  6.63  22630.00  3036.00                             htab_find_slot_with_hash
  4.93  24888.00  2258.00                             hash
  4.87  27118.00  2230.00                             symbol_natural_name
  3.94  28923.00  1805.00                             read_attribute_value
  3.69  30611.00  1688.00                             htab_hash_string
  3.28  32111.00  1500.00                             _init
  2.95  33461.00  1350.00                             compare_psymbols


With my changes:
  %   cumulative   self              self     total           
 time   samples   samples    calls  T1/call  T1/call  name    
 17.53   8015.00  8015.00                             bcache_data
  7.52  11455.00  3440.00                             htab_find_slot_with_hash
  7.31  14799.00  3344.00                             msymbol_hash_iw
  7.18  18082.00  3283.00                             read_partial_die
  5.13  20426.00  2344.00                             hash
  4.30  22392.00  1966.00                             compare_psymbols
  4.23  24328.00  1936.00                             gnu_debuglink_crc32
  3.92  26122.00  1794.00                             read_attribute_value
  3.67  27800.00  1678.00                             htab_hash_string

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer