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Re: [PATCH i386] Enable -freorder-blocks-and-partition
- From: Teresa Johnson <tejohnson at google dot com>
- To: Andi Kleen <andi at firstfloor dot org>
- Cc: "gcc-patches at gcc dot gnu dot org" <gcc-patches at gcc dot gnu dot org>, David Li <davidxl at google dot com>, Jan Hubicka <hubicka at ucw dot cz>, Steven Bosscher <stevenb dot gcc at gmail dot com>
- Date: Tue, 19 Nov 2013 13:07:41 -0800
- Subject: Re: [PATCH i386] Enable -freorder-blocks-and-partition
- Authentication-results: sourceware.org; auth=none
- References: <CAAe5K+XeggoZGd8OgdGhh49RF-=5JbBdOEg+j2ZyYBCno0GfdA at mail dot gmail dot com> <8738msxg2o dot fsf at tassilo dot jf dot intel dot 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