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Re: [PATCH] Sanitize block partitioning under -freorder-blocks-and-partition


On Tue, Sep 24, 2013 at 11:25 AM, Teresa Johnson <tejohnson@google.com> wrote:
> On Tue, Sep 24, 2013 at 10:57 AM, Jan Hubicka <hubicka@ucw.cz> wrote:
>>>
>>> I looked at one that failed after 100 as well (20031204-1.c). In this
>>> case, it was due to expansion which was creating multiple branches/bbs
>>> from a logical OR and guessing incorrectly on how to assign the
>>> counts:
>>>
>>>  if (octets == 4 && (*cp == ':' || *cp == '\0')) {
>>>
>>> The (*cp == ':' || *cp == '\0') part looked like the following going
>>> into RTL expansion:
>>>
>>>   [20031204-1.c : 31:33] _29 = _28 == 58;
>>>   [20031204-1.c : 31:33] _30 = _28 == 0;
>>>   [20031204-1.c : 31:33] _31 = _29 | _30;
>>>   [20031204-1.c : 31:18] if (_31 != 0)
>>>     goto <bb 16>;
>>>   else
>>>     goto <bb 19>;
>>>
>>> where the result of the OR was always true, so bb 16 had a count of
>>> 100 and bb 19 a count of 0. When it was expanded, the expanded version
>>> of the above turned into 2 bbs with a branch in between. Both
>>> comparisons were done in the first bb, but the first bb checked
>>> whether the result of the *cp == '\0' compare was true, and if not
>>> branched to the check for whether the *cp == ':' compare was true. It
>>> gave the branch to the second check against ':' a count of 0, so that
>>> bb got a count of 0 and was split out, and put the count of 100 on the
>>> fall through assuming the compare with '\0' always evaluated to true.
>>> In reality, this OR condition was always true because *cp was ':', not
>>> '\0'. Therefore, the count of 0 on the second block with the check for
>>> ':' was incorrect, we ended up trying to execute it, and failed.
>>
>> I see, we produce:
>> ;; if (_26 != 0)
>>
>> (insn 94 93 95 (set (reg:CCZ 17 flags)
>>         (compare:CCZ (reg:QI 107 [ D.2184 ])
>>             (const_int 0 [0]))) a.c:31 -1
>>      (nil))
>>
>> (insn 95 94 96 (set (reg:QI 122 [ D.2186 ])
>>         (eq:QI (reg:CCZ 17 flags)
>>             (const_int 0 [0]))) a.c:31 -1
>>      (nil))
>>
>> (insn 96 95 97 (set (reg:CCZ 17 flags)
>>         (compare:CCZ (reg:QI 122 [ D.2186 ])
>>             (const_int 0 [0]))) a.c:31 -1
>>      (nil))
>>
>> (jump_insn 97 96 98 (set (pc)
>>         (if_then_else (ne (reg:CCZ 17 flags)
>>                 (const_int 0 [0]))
>>             (label_ref 100)
>>             (pc))) a.c:31 -1
>>      (expr_list:REG_BR_PROB (const_int 6100 [0x17d4])
>>         (nil)))
>>
>> (insn 98 97 99 (set (reg:CCZ 17 flags)
>>         (compare:CCZ (reg:QI 108 [ D.2186 ])
>>             (const_int 0 [0]))) a.c:31 -1
>>      (nil))
>>
>> (jump_insn 99 98 100 (set (pc)
>>         (if_then_else (eq (reg:CCZ 17 flags)
>>                 (const_int 0 [0]))
>>             (label_ref 0)
>>             (pc))) a.c:31 -1
>>      (expr_list:REG_BR_PROB (const_int 3900 [0xf3c])
>>         (nil)))
>>
>> (code_label 100 99 0 14 "" [0 uses])
>>
>> That is because we TER together "_26 = _25 | _24" and "if (_26 != 0)"
>>
>> First I think the logic of do_jump should really be moved to trees.  It is not
>> doing things that can not be adequately represented by gimple.
>>
>> I am not that certain we want to move it before profiling though.
>>>
>>> Presumably we had the correct profile data for both blocks, but the
>>> accuracy was reduced when the OR was represented as a logical
>>> computation with a single branch. We could change the expansion code
>>> to do something different, e.g. treat as a 50-50 branch. But we would
>>> still end up with integer truncation issues when there was a single
>>> training run. But that could be dealt with conservatively in the
>>
>> Yep, but it is still better than what we have now - if the test above was
>> in hot part of program (i.e. not executed once), we will end up optimizing
>> the second conditional for size.
>>
>> So I think it is do_jump bug to not distribute probabilities across the two
>> conditoinals introduced.
>>> bbpart code as I suggested for the jump threading issue above. I.e. a
>>> cold block with incoming non-cold edges conservatively not marked cold
>>> for splitting.
>>
>> Yep, we can probably do that, but we ought to fix the individual cases
>> above at least for resonable number of runs.
>
> I made this change and it removed a few of the failures.
>
> I looked at another case that still failed with 1 train run but passed
> with 100. It turned out to be another truncation issue exposed by RTL
> expansion, where we created some control flow for a memset builtin
> which was in a block with an execution count of 1. Some of the blocks
> got frequencies less than half the original block, so the count was
> rounded down or truncated to 0. I noticed that in this case (as well
> as the jump threading case I fixed by looking for non-zero incoming
> edges in partitioning) that the bb frequency was non-zero.
>
> Why not just have probably_never_executed_bb_p return simply return
> false bb->frequency is non-zero (right now it does the opposite -
> returns true when bb->frequency is 0)? Making this change removed a
> bunch of other failures. With this change as well, there are only 3
> cases that still fail with 1 train run that pass with 100. Need to
> look at those.

FYI, These turned out to be more jump threading issues. I am currently
working on getting jump threading profile updates to work properly, I
think I'm pretty close. Haven't had a chance to look at do_jump yet.

Teresa

>
>>
>> Will you look into logic of do_jump or shall I try to dive in?
>
> I can take a look, but probably won't have a chance until late this
> week. If you don't get to it before then I will see if I can figure
> out why it is applying the branch probabilities this way.
>
> Teresa
>
>>
>> Honza
>
>
>
> --
> Teresa Johnson | Software Engineer | tejohnson@google.com | 408-460-2413



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


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