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Re: [PATCH] Redesign jump threading profile updates


On Tue, Jul 22, 2014 at 7:29 PM, Jeff Law <law@redhat.com> wrote:
> On 03/26/14 17:44, Teresa Johnson wrote:
>>
>> Recently I discovered that the profile updates being performed by jump
>> threading were incorrect in many cases, particularly in the case where
>> the threading path contains a joiner. Some of the duplicated
>> blocks/edges were not getting any counts, leading to incorrect
>> function splitting and other downstream optimizations, and there were
>> other insanities as well. After making a few attempts to fix the
>> handling I ended up completely redesigning the profile update code,
>> removing a few places throughout the code where it was attempting to
>> do some updates.
>>
>> The biggest complication (see the large comment and example above the
>> new routine compute_path_counts) is that we duplicate a conditional
>> jump in the joiner case, possibly multiple times for multiple jump
>> thread paths through that joiner, and it isn't trivial to figure out
>> what probability to assign each of the duplicated successor edges (and
>> the original after threading). Each jump thread path may need to have
>> a different probability of staying on path through the joiner in order
>> to keep the counts going out of the threading path sane.
>>
>> The patch below was bootstrapped and tested on
>> x86_64-unknown-linux-gnu, and also tested with a profiledbootstrap. I
>> additionally tested with cpu2006, confirming that the amount of
>> resulting cycle samples in the split cold sections reduced, and
>> through manual inspection that many different cases were now correct.
>> I also measured performance with cpu2006, running each benchmark
>> multiple times on a Westmere and see some speedups (453.povray 1-2%,
>> 403.gcc 1-1.5%, and noisy but positive speedups in 471.omnetpp and
>> 483.xalancbmk).
>>
>> Looks like my mailer is corrupting the spacing, which makes it harder
>> to look at the CFG examples in the big header comment block I added.
>> So I have also included the patch as an attachment.
>>
>> Ok for stage 1?
>>
>> Thanks,
>> Teresa
>>
>>   2014-03-26  Teresa Johnson  <tejohnson@google.com>
>>
>>          * tree-ssa-threadupdate.c (struct ssa_local_info_t): New
>>          duplicate_blocks bitmap.
>>          (remove_ctrl_stmt_and_useless_edges): Ditto.
>>          (create_block_for_threading): Ditto.
>>          (compute_path_counts): New function.
>>          (update_profile): Ditto.
>>          (deduce_freq): Ditto.
>>          (recompute_probabilities): Ditto.
>>          (update_joiner_offpath_counts): Ditto.
>>          (ssa_fix_duplicate_block_edges): Update profile info.
>>          (ssa_create_duplicates): Pass new parameter.
>>          (ssa_redirect_edges): Remove old profile update.
>>          (thread_block_1): New duplicate_blocks bitmap,
>>          remove old profile update.
>>          (thread_single_edge): Pass new parameter.
>
> First off, sorry this took so long to get reviewed.
>
> Most of what's going on in here is similar to something I sketched out, but
> never coded up a while back -- with the significant difference that you're
> handling joiner blocks as well.
>
> Everything looks to be well thought through and documented in the code at a
> level I wish existed throughout GCC.
>
> The only thing I see missing is regression tests.  I don't think you need to
> do anything huge here, but it ought to be possible to set up relatively
> simple cases which show the probabilities/counts being updated properly.
>
> Otherwise it looks excellent.  It's pre-approved once you've added some kind
> of testing and fixed the nits noted below.

Thanks! I will fix the issues you note below and create some test
cases before I commit.
Teresa

>
>
>
>> +   In the aboe example, after all jump threading is complete, we will
>
> s/aboe/above/
>
>
>
>> +  struct el *next, *el;
>> +  bitmap in_edge_srcs = BITMAP_ALLOC (NULL);
>> +  for (el = rd->incoming_edges; el; el = next)
>> +    {
>> +      next = el->next;
>> +      bitmap_set_bit (in_edge_srcs, el->e->src->index);
>> +    }
>
> Please add vertical whitespace after this loop, but before declaring
> variables for the next loop.
>
> Jeff
>



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


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