Re: [PATCH i386] Enable -freorder-blocks-and-partition

[Teresa Johnson <tejohnson@google.com>]

On Tue, Nov 19, 2013 at 1:00 PM, Andi Kleen <andi@firstfloor.org> wrote:
> Teresa Johnson <tejohnson@google.com> writes:
>
>> This patch enables -freorder-blocks-and-partition by default for x86
>> at -O2 and up. It is showing some modest gains in cpu2006 performance
>> with profile feedback and -O2 on an Intel Westmere system. Specifically,
>> I am seeing consistent improvements in 401.bzip2 (1.5-3%), 483.xalancbmk
>> (1.5-3%), and 453.povray (2.5-3%), and no apparent regressions.
>
> One thing that worries me is what this will do to profilers.
>
> I had to hack some assembler code using out of line sections
> to able to handle the libunwind based perf dwarf unwinder.
>
> My understanding is that these out of line sections can be
> only described properly in dwarf3, and there's still some
> dwarf2 based unwinder code around.
>
> So this may cause problems with profiling and debugging.
>
> It's probably still a good idea, just may need some extra
> care.
>
> -Andi

Sri has approval for a patch that should address this by giving the
split cold sections a label. It should go in today as well:

http://gcc.gnu.org/ml/gcc-patches/2013-11/msg02143.html

Thanks,
Teresa
>
> --
> ak@linux.intel.com -- Speaking for myself only



-- 
Teresa Johnson | Software Engineer | tejohnson@google.com | 408-460-2413