Re: Inter-CU DWARF size optimizations and gcc -flto

[Gary Benson <gbenson at redhat dot com>]

Daniel Jacobowitz wrote:
> There's also a lot of room for profiling to identify bad algorithms;
> I think we spend a lot of time reading the solib list from the
> inferior (something I thought I and others had fixed thoroughly
> already...) and I routinely hit inefficient algorithms e.g. during
> "next".

I did some work on this recently.  On my setup (with gdb and the
inferior on the same machine) it was spending a huge chunk of time
regenerating symbol tables every time the solib_event_breakpoint
hit.  The final patch I committed is here:

  http://www.cygwin.com/ml/gdb-patches/2011-10/msg00068.html

If you're seeing some sort of qsort comparison function at the top
of the profile it could be that something is bypassing this.

If you find the time is taken up mostly with transferring data from
the inferior to gdb (I never tried remote, for instance) then you
might be interested in some work I did last year on a SystemTap based
interface between glibc and gdb that should be able to be extended to
allow selective reading of the solib list.  That's waiting on Sergio's
SystemTap stuff... also the glibc maintainers seem hostile to the
idea of us inserting SystemTap probes in there.  I can dig up the code
I had for this if you're interested.

I also had a patch floating around that disabled the solib event
breakpoint under certain conditions, but I think the ambiguous
linespec stuff makes this patch invalid as you always have to be
looking out for new functions turning up.  If you're interested the
thread is http://www.cygwin.com/ml/gdb-patches/2011-09/msg00156.html
but it's probably useless :(

Cheers,
Gary

-- 
http://gbenson.net/