[PATCH] Redesign jump threading profile updates

Teresa Johnson tejohnson@google.com
Tue Sep 30 18:20:00 GMT 2014

On Mon, Sep 29, 2014 at 9:33 PM, Jeff Law <law@redhat.com> wrote:
> On 09/29/14 08:19, Teresa Johnson wrote:
>>> Just an update - I found some good test cases by compiling the
>>> c-torture tests with profile feedback with and without my patch. But
>>> in the cases I pulled out I saw that there were still a couple profile
>>> or probability insanities introduced by jump threading (albeit far
>>> less than before), so I wanted to investigate before I commit. I ran
>>> out of time this week and will not get to this until I get back from
>>> vacation the week after next.
>> Hi Jeff,
>> I finally had a chance to get back to this and look at the remaining
>> insanities in the new test cases I created. It turns out that there
>> were still a few issues in the case where there were guessed
>> frequencies and no profile counts. The two test cases I created do use
>> FDO, and the insanities in the routines with profile counts went away
>> with my patch. But the outlined copies of routines that were also
>> inlined into the main routine still had estimated frequencies, and
>> these still had a few issues.
>> The problem is that the profile updates are done incrementally as we
>> walk and update the paths in ssa_fix_duplicate_block_edges, including
>> the block and edge counts, the block frequencies and the
>> probabilities. This is very difficult to do when only operating on
>> frequencies since the edge frequencies are derived from the source
>> block frequency and the probability. Therefore, once the source block
>> frequency is updated, the edge frequency is also affected, and it is
>> really difficult to figure out what the update to the edge frequency
>> (essentially the probability) is using the same incremental update
>> approach. I was attempting to handle this with the routine
>> deduce_freq, for example, but this turned out to have issues for
>> certain types of paths. I tried a few other approaches, but they start
>> looking really ugly and I didn't want to add a parallel but different
>> algorithm in the case of no profile counts.
>> So by far the simplest approach was simply to take a snapshot of the
>> existing block and edge frequencies along the path before we start the
>> updates in ssa_fix_duplicate_block_edges, by copying them into the
>> profile count fields of those blocks and edges. Then the existing
>> algorithm operates the same as when we do have counts, and can
>> essentially operate incrementally on the edge frequencies since they
>> live in the count field of the edge and are no longer affected anytime
>> the source block is updated. Since the algorithm does update block
>> frequencies and probabilities as well (based on the count updates
>> performed), we can simply clear out these fake count fields at the end
>> of ssa_fix_duplicate_block_edges. This takes care of the remaining
>> insanities introduced by jump threading from these test cases. During
>> testing I also added in some checking to ensure that the count fields
>> for the whole routine were cleared properly to make sure the new
>> clear_counts_path was not missing anything (checking is a little too
>> heavyweight to add in normally).
>> New patch below (also attached since my mailer sometimes eats spaces).
>> The differences between the old patch and the new one:
>> - removed deduce_freq (which was my least favorite part of the patch
>> anyway!), and its call from recompute_probabilities, since it is no
>> longer necessary.
>> - two new routines freqs_to_counts_path and clear_counts_path, invoked
>> from ssa_fix_duplicate_block_edges.
>> - two new tests
>> Bootstrapped and tested on x86_64-unknown-linux-gnu, ok for trunk?
>> Thanks,
>> Teresa
>> gcc:
>> 2014-09-29  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.
>>          (recompute_probabilities): Ditto.
>>          (update_joiner_offpath_counts): Ditto.
>>          (freqs_to_counts_path): Ditto.
>>          (clear_counts_path): 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.
>> gcc/testsuite:
>> 2014-09-29  Teresa Johnson  <tejohnson@google.com>
>>          * testsuite/gcc.dg/tree-prof/20050826-2.c: New test.
>>          * testsuite/gcc.dg/tree-prof/cmpsf-1.c: Ditto.
> Given I'd already been through this pretty thoroughly, I just gave this a
> cursory review.
> clear_counts_path needs a function comment.  It's pretty obvious what it's
> doing, but for completeness let's go ahead and get the obvious comment in
> there.

Done and committed as r215739.


> With that fix, approved for the trunk.  Thanks for taking the time to sort
> out all these issues.
> jeff

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

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