Re: [PATCH] Redesign jump threading profile updates

[Christophe Lyon <christophe.lyon@linaro.org>]

On 30 September 2014 20:20, Teresa Johnson <tejohnson@google.com> wrote:
> 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.
>

Since this commit, I can see all my builds for arm*linux* and
aarch64*linux* fail while building glibc:

/tmp/3496222_18.tmpdir/aci-gcc-fsf/builds/gcc-fsf-gccsrc/tools/bin/aarch64-none-linux-gnu-gcc
iso-2022-cn.c -c -std=gnu99 -fgnu89-inline  -O2 -Wall -Win
line -Wundef -Wwrite-strings -fmerge-all-constants -frounding-math -g
-Wstrict-prototypes   -fPIC        -I../include
-I/tmp/3496222_18.tmpdir/aci-gcc-f
sf/builds/gcc-fsf-gccsrc/obj-aarch64-none-linux-gnu/glibc-1/iconvdata
-I/tmp/3496222_18.tmpdir/aci-gcc-fsf/builds/gcc-fsf-gccsrc/obj-aarch64-none-linux
-gnu/glibc-1  -I../sysdeps/unix/sysv/linux/aarch64/nptl
-I../sysdeps/unix/sysv/linux/aarch64
-I../sysdeps/unix/sysv/linux/generic  -I../sysdeps/unix/s
ysv/linux/wordsize-64  -I../nptl/sysdeps/unix/sysv/linux
-I../nptl/sysdeps/pthread  -I../sysdeps/pthread
-I../sysdeps/unix/sysv/linux  -I../sysdeps/gn
u  -I../sysdeps/unix/inet  -I../nptl/sysdeps/unix/sysv
-I../sysdeps/unix/sysv  -I../nptl/sysdeps/unix  -I../sysdeps/unix
-I../sysdeps/posix  -I../sysd
eps/aarch64/fpu  -I../sysdeps/aarch64/nptl  -I../sysdeps/aarch64
-I../sysdeps/wordsize-64  -I../sysdeps/ieee754/ldbl-128
-I../sysdeps/ieee754/dbl-64/w
ordsize-64  -I../sysdeps/ieee754/dbl-64  -I../sysdeps/ieee754/flt-32
-I../sysdeps/aarch64/soft-fp  -I../sysdeps/ieee754
-I../sysdeps/generic  -I../npt
l  -I.. -I../libio -I. -nostdinc -isystem
/tmp/3496222_18.tmpdir/aci-gcc-fsf/builds/gcc-fsf-gccsrc/tools/lib/gcc/aarch64-none-linux-gnu/5.0.0/include
-i
system /tmp/3496222_18.tmpdir/aci-gcc-fsf/builds/gcc-fsf-gccsrc/tools/lib/gcc/aarch64-none-linux-gnu/5.0.0/include-fixed
-isystem /tmp/3496222_18.tmpdir
/aci-gcc-fsf/builds/gcc-fsf-gccsrc/sysroot-aarch64-none-linux-gnu/usr/include
 -D_LIBC_REENTRANT -include ../include/libc-symbols.h  -DPIC -DSHARED
-DNOT_IN_libc -o
/tmp/3496222_18.tmpdir/aci-gcc-fsf/builds/gcc-fsf-gccsrc/obj-aarch64-none-linux-gnu/glibc-1/iconvdata/iso-2022-cn.os
-MD -MP -MF /tmp/3
496222_18.tmpdir/aci-gcc-fsf/builds/gcc-fsf-gccsrc/obj-aarch64-none-linux-gnu/glibc-1/iconvdata/iso-2022-cn.os.dt
-MT /tmp/3496222_18.tmpdir/aci-gcc-fsf
/builds/gcc-fsf-gccsrc/obj-aarch64-none-linux-gnu/glibc-1/iconvdata/iso-2022-cn.os

In file included from iso-2022-cn.c:407:0:
../iconv/skeleton.c: In function 'gconv':
../iconv/skeleton.c:800:1: internal compiler error: in
check_probability, at basic-block.h:959
0xe4e2fb find_many_sub_basic_blocks(simple_bitmap_def*)
        /tmp/3496222_18.tmpdir/aci-gcc-fsf/sources/gcc-fsf/gccsrc/gcc/basic-block.h:959
0x6623f0 execute
        /tmp/3496222_18.tmpdir/aci-gcc-fsf/sources/gcc-fsf/gccsrc/gcc/cfgexpand.c:5916
Please submit a full bug report,
with preprocessed source if appropriate.
Please include the complete backtrace with any bug report.
See <http://gcc.gnu.org/bugs.html> for instructions.

Can you have a look?

Thanks,

Christophe.

> Thanks,
> Teresa
>
>>
>> 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