[FFmpeg-devel] [PATCH] Extra build options for ALS (and others)

[Thilo Borgmann]

Michael Niedermayer schrieb:
> On Mon, Nov 30, 2009 at 04:09:23PM +0100, Thilo Borgmann wrote:
>> Thilo Borgmann schrieb:
>>> 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:
>>>>>>>>
>>>>>>>>> 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.
>>> Like Michael said, this would be a big test for different platforms and
>>> compilers which I cannot offer alone so several people would have to do
>>> this - if a benchmark would indicate that it might be worth testing.
>>>
>>> Also, I'm lacking a good idea of how to test this efficiently without
>>> having other factors like harddrives playing a predominant role which
>>> means testing execution time of ffmpeg.
>> I played around a little with the regression tests and audio decoders.
>> For most of my tests -finline-limit=4096 makes it a little faster, e.g.
>>
>> g726: 47001535 dezicycles -> 41628457 dezicycles (12%)
>> alac: 12855244 dezicycles -> 12849127 dezicycles ( 0%)
>> flac:   842020 dezicycles ->   786226 dezicycles ( 7%)
>> wma:   3663166 dezicycles ->  3197273 dezicycles (14%)
>>
>> which is not surprising. Inlining comes for a price, ffmpeg executable
>> growed from 5,4 MB to 6.1 MB.
>> Value used fro -finline-limit is 4096, default is 600 for gcc-4.0.
> 
> what about video codecs? h264, mpeg4, mpeg2 h263 ?

Can do tomorrow.

> and which cpu?

Intel Core 2 Duo 2.53 GHz.

-Thilo