[FFmpeg-devel] [PATCH] Extra build options for ALS (and others)
Måns Rullgård
mans
Fri Nov 27 17:39:30 CET 2009
Thilo Borgmann <thilo.borgmann at googlemail.com> writes:
> M?ns Rullg?rd schrieb:
>> Thilo Borgmann <thilo.borgmann at googlemail.com> writes:
>>
>>> M?ns Rullg?rd schrieb:
>>>> Thilo Borgmann <thilo.borgmann at googlemail.com> writes:
>>>>
>>>>> M?ns Rullg?rd schrieb:
>>>>>> Thilo Borgmann <thilo.borgmann at googlemail.com> writes:
>>>>>>
>>>>>>> Hi,
>>>>>>>
>>>>>>> recently the need for an extra build option for the ALS decoder arose.
>>>>>> Is it impossible to achieve the desired outcome with some combination
>>>>>> of always_inline, noinline, and flatten attributes?
>>>>> No. See [PATCH] Split reading and decoding of blocks in ALS.
>>>>>
>>>>> Although I've managed to have the functions from the alsdec.c inlined
>>>>> manually according to the grep'ed output of the assembler code, it seems
>>>>> like it is not enough to manually inline functions from within that .c
>>>>> file only using these technique.
>>>> I'm confused. Can it be done in the C code only or not? This kind of
>>>> issue should really not be solved in the makefile.
>>> The issue is the big slowdown. The patch that causes this splits a big
>>> function into two, which are then called successively.
>>>
>>> To overcome the slowdown issue, I inspected the functions being inlined
>>> with and without the -finline-limit option. I can use av_always_inline
>>> for many functions within alsdec.c to have the same functions inlined
>>> like -finline-limit does.
>>>
>>> Unfortunately, using -finline-limit removes the slowdown introduced by
>>> the patch while using av_always_inline does not.
>>
>> So it's not doing the same thing. What is it doing differently?
>> Where did you get the limit number from?
>>
>
> All function calls within alsdec.s when using -finline-limit=4096:
> 1 call L1102
> 1 call L138
> 1 call L456
> 2 call L___udivdi3$stub
> 10 call L_av_freep$stub
> 1 call L_av_get_bits_per_sample_format$stub
> 12 call L_av_log$stub
> 5 call L_av_log_missing_feature$stub
> 8 call L_av_malloc$stub
> 2 call L_av_mallocz$stub
> 1 call L_ff_mpeg4audio_get_config$stub
> 6 call L_memcpy$stub
> 2 call L_memmove$stub
> 1 call L_memset$stub
> 2 call _decode_blocks_ind
> 4 call _decode_end
> 36 call _decode_rice
> 10 call _get_bits_long
> 11 call _parse_bs_info
> 2 call _zero_remaining
>
> All function calls within alsdec.s when using many av_always_inline's.
> This is designed to inline the same functions from alsdec.c like the
> unpatched alsdec.c would yield without any extra build option:
> 1 call L1561
> 1 call L176
> 1 call L21
> 2 call L___udivdi3$stub
> 10 call L_av_freep$stub
> 1 call L_av_get_bits_per_sample_format$stub
> 13 call L_av_log$stub
> 5 call L_av_log_missing_feature$stub
> 8 call L_av_malloc$stub
> 2 call L_av_mallocz$stub
> 1 call L_ff_mpeg4audio_get_config$stub
> 1 call L_memcpy$stub
> 1 call L_memmove$stub
> 2 call L_memset$stub
> 8 call ___inline_memcpy_chk
> 2 call ___inline_memmove_chk
> 6 call _align_get_bits
> 5 call _av_ceil_log2
> 4 call _av_clip
> 4 call _decode_end
> 47 call _get_bits
> 90 call _get_bits1
> 3 call _get_bits_count
> 61 call _get_bits_left
> 39 call _get_bits_long
> 4 call _get_sbits_long
> 60 call _get_unary
> 2 call _init_get_bits
> 3 call _parse_bs_info
> 3 call _read_time
> 7 call _skip_bits
> 2 call _skip_bits1
> 5 call _skip_bits_long
Not inlining those get_bits etc will certainly slow things down,
that's for sure.
> So -finline-limit can inline many functions in the object file which are
> not part of alsdec.c. Which might be the reason for the performance
> difference.
>
> But using -finline-limit does not yield a speed gain for the unpatched
> file! So there might be something else but I don't see.
>
> The value of 4096 has been choosen randomly. As long as I don't know
> exactly why -finline-limit removes the slowdown and that it cannot be
> replaced by another approach, there is no need to figure out a more
> optimal value...
We should do some benchmarks using that flag globally and see what
happens. Maybe we'd gain from using it everywhere.
--
M?ns Rullg?rd
mans at mansr.com
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