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Re: [PATCH] Sanitize block partitioning under -freorder-blocks-and-partition
- From: Teresa Johnson <tejohnson at google dot com>
- To: Martin LiÅka <marxin dot liska at gmail dot com>
- Cc: Jan Hubicka <hubicka at ucw dot cz>, Bernhard Reutner-Fischer <rep dot dot dot nop at gmail dot com>, "gcc-patches at gcc dot gnu dot org" <gcc-patches at gcc dot gnu dot org>, Steven Bosscher <stevenb dot gcc at gmail dot com>, Jeff Law <law at redhat dot com>, Sriraman Tallam <tmsriram at google dot com>
- Date: Fri, 9 Aug 2013 14:03:50 -0700
- Subject: Re: [PATCH] Sanitize block partitioning under -freorder-blocks-and-partition
- References: <CAAe5K+U6+xyy95KSeA7+SZ0tUdFt2dmF-vSxNsBsqg53NSyU3Q at mail dot gmail dot com> <CAC1BbcSnwiAfzKqngLvLBGnmPDLNhYdwbyWDMJ++PzsMd=M-3Q at mail dot gmail dot com> <CAAe5K+XM_RCJBmtKSyQCfD86hi2Ls_BCyuZkd2WKSoGUFm84DA at mail dot gmail dot com> <20130802150529 dot GC15776 at kam dot mff dot cuni dot cz> <CAAe5K+WiR02Rs1jYMFRF2F8ey60UO7LwRa8WWq7coQ5Pq8HhiQ at mail dot gmail dot com> <CAAe5K+WToUznwFFfm5beapXAOOrOgxHR8LXmYBTL70C4VVsT+w at mail dot gmail dot com> <20130808222332 dot GA31755 at kam dot mff dot cuni dot cz> <CAAe5K+W+8borbPkt4BB1ayRgDbFBtd6oyZsGuUiC854o9t0Rjg at mail dot gmail dot com> <20130809095843 dot GC31755 at kam dot mff dot cuni dot cz> <CAAe5K+XXT6t5CXBDXPWMNSrLWwqfw8F_J2fNUAN2afqb5qPhzQ at mail dot gmail dot com> <20130809152804 dot GA6579 at kam dot mff dot cuni dot cz> <CAObPJ3NAAJ6iRvi9pnUAiUx56hW8F7QNT5-0n5eaQF+3m8K4Xg at mail dot gmail dot com>
On Fri, Aug 9, 2013 at 8:54 AM, Martin LiÅka <firstname.lastname@example.org> wrote:
> On 9 August 2013 17:28, Jan Hubicka <email@example.com> wrote:
>>> > Do we sanity check that the cold partition does not contain any blocks of
>>> > count 0? It may be that the profile is broken enough to make partitioning
>>> > not work.
>>> Do you mean sanity check that the cold partition does not contain any
>>> blocks of count > 0? (they should all be zero) I don't think that
>>> sanity check is there, but I can try adding that.
>> Thanks, lets start with this - I suppose we need to figure out if
>> 1) the reachable blocks goes to cold section because partitioning decides
>> so even if they have non-0 count.
>> 2) the reachable blocks goes to cold section because they have incorrectly
>> updated count to 0 by someone
>> 3) profiling gets some blocks wrong.
>>> The issue with such a sanity check may be due to the later fixup I
>>> have in this patch (fixup_partitions). It is invoked after certain
>>> optimizations on the cfg that may make hot blocks previously reached
>>> by both hot and cold edges only reachable by cold blocks. These blocks
>>> are remarked cold. If the profile data hasn't been updated correctly
>>> it is possible that they would still have a non-0 count, although they
>>> are essentially cold after the cfg transformation.
>> Well, or the other posibility is that the edges was updated wrong
>> and the blocks are really cold. We need to figure out if that happens
>> commonly enough.
>> I will try to think of some artificial testcases.
>>> But certainly such a sanity check should always succeed after the
>>> original partitioning.
>>> > I can think of inlining where the count gets scaled all way down to 0. Perhaps
>>> > count scaling code can be modified to never round towards 0 for block executing
>>> > non-0 times...
>>> This reminds me of why this situation could happen. When I have been
>>> testing this on the google branch I found situations where COMDAT
>>> routines have 0 profile counts (if I remember correctly, this happens
>>> when profile-gen binary has call to out-of-line copy of COMDAT in
>>> module A, linker chooses the out-of-line copy from module B, therefore
>>> the profile data for COMDAT in module A is 0). When the COMDAT gets
>>> inlined, the 0 counts on its bbs are scaled to 0, even though the
>>> callsite is non-zero. I have a patch that I was planning to send as a
>>> follow-up that handles this case by propagating the callsite bb's
>>> count to the inlined code when it has 0 counts, scaling by the edge
>>> frequencies. I can either include that patch in this one, or send it
>>> for review separately right now. Do you want to give it a try with
>>> this one to see if it addresses the issue?
>> This scenario should not happen with LTO setup: the LTO symbol tables contains
>> code before early optimization and should be identical with profiling or
>> without (modulo the new references and call from profile code).
>> But this patch seems useful as a backup solution for non-LTO, so yes, please
>> send it separately and I can try to double check that it really do not happen
>> with LTO.
>> (acutally LTO symtab may just chose COMDAT from module that has counts with it.
>> It has all the info for it. I was thinkin about it few weeks back. It is
>> bit hard to do - you need to verify that all references from the function are
>> the same or linking might fail if you overwrite linker's decisiosns).
>>> Also, can you send me reproduction instructions for gimp? I don't
>>> think I need Martin's patch, but which version of gimp and what is the
>>> equivalent way for me to train it? I have some scripts to generate a
>>> similar type of instruction heat map graph that I have been using to
>>> tune partitioning and function reordering. Essentially it uses linux
>>> perf to sample on instructions_retired and then munge the data in
>>> several ways to produce various stats and graphs. One thing that has
>>> been useful has been to combine the perf data with nm output to
>>> determine which cold functions are being executed at runtime.
> I use gimp from git repository, commit:
> 88ecd59c3436d302b644a5d25c1938c0e7b60ae0 (from Fet 5 2013)
> Link: http://www.gimp.org/source/#gimp_from_git
Thanks. Were you building with LTO? And just -O2, or any other options
I should use?
>>> However, for this to tell me which split cold bbs are being executed I
>>> need to use a patch that Sri sent for review several months back that
>>> gives the split cold section its own name:
>>> Steven had some follow up comments that Sri hasn't had a chance to address yet:
>>> (cc'ing Sri as we should probably revive this patch soon to address
>>> gdb and other issues with detecting split functions properly)
>> Intresting, I used linker script for this purposes, but that his GNU ld only...
>>> > Honza
>>> >> Thanks,
>>> >> Teresa
>>> >> > I think we are really looking primarily for dead parts of the functions (sanity checks/error handling)
>>> >> > that should not be visited by train run. We can then see how to make the heuristic more aggressive?
>>> >> >
>>> >> > Honza
>>> >> --
>>> >> Teresa Johnson | Software Engineer | firstname.lastname@example.org | 408-460-2413
>>> Teresa Johnson | Software Engineer | email@example.com | 408-460-2413
Teresa Johnson | Software Engineer | firstname.lastname@example.org | 408-460-2413