[FFmpeg-devel] [PATCH] configure+libm.h: add fmin/fmax/fminf/fmaxf emulation

Muhammad Faiz mfcc64 at gmail.com
Tue Nov 3 18:54:03 CET 2015

On Tue, Nov 3, 2015 at 3:47 AM, Ganesh Ajjanagadde <gajjanag at mit.edu> wrote:
> On Tue, Nov 3, 2015 at 5:06 AM, Muhammad Faiz <mfcc64 at gmail.com> wrote:
>> On Sat, Oct 31, 2015 at 10:15 AM, Ganesh Ajjanagadde <gajjanag at mit.edu> wrote:
>>> On Fri, Oct 30, 2015 at 7:29 PM, Michael Niedermayer
>>> <michael at niedermayer.cc> wrote:
>>>> On Fri, Oct 30, 2015 at 06:53:34PM -0400, Ganesh Ajjanagadde wrote:
>>>>> On Fri, Oct 30, 2015 at 6:35 PM, Michael Niedermayer <michaelni at gmx.at> wrote:
>>>>> > From: Michael Niedermayer <michael at niedermayer.cc>
>>>>> >
>>>>> > This should fix the build failure of avf_showcqt.c
>>>>> >
>>>>> > An alternative solution would be to add a check for fmin/fmax to fate-source and
>>>>> > then to replace them by FFMIN/FFMAX, i can do that if preferred?
>>>>> >
>>>>> > Untested due to lack of a affected platform
>>>>> I recall some interest on my end to get fmin, fmax etc for different
>>>>> reasons, and it was remarked that commit
>>>>> 4436a8f44dedc83767b3d9da9beb85d1fae2ca30 may be relevant. The summary
>>>>> seems to be that getting it to work on all platforms is not so simple.
>>>> :/
>>>> ill replace the problematic ones by FFMIN/MAX for now so the build
>>>> failure on the affected msvc platforms is fixed
>>>> tieing a build fix to this complexity would be unwise
>>>>> I am definitely interested in getting it to work in order to replace
>>>>> FFMAX/FFMIN for floating point in especially libavfilter. This will
>>>>> allow better nan signalling at a slight performance cost.
>>>> would that performance cost affect all systems or just ones not
>>>> having fmin/fmax ?
>>>> i think affecting all systems would be bad
>>> A correct fmin and fmax will be slower than FFMIN/FFMAX, simply
>>> because they do NaN handling. How much slower, I have not tested. Also
>>> note that flags may be passed to the compiler to ignore NaN's,
>>> something which can possibly be done locally if desired. However, I
>>> view FFMAX/FFMIN as the cleaner solution if NaN signalling/propagation
>>> is not an issue, so I may not pursue this.
>> Common problem with FFMIN/FFMAX (and other macros that evaluates
>> arguments more than once):
>> FFMAX(a, myfunc(b))
>> will call myfunc twice (OK compiler may optimize it and call myfunc once)
>> and often people don't care about it.
> That goes without saying, but sometimes compiler can't optimize due to
> sequence point stuff. This is why the macro should be used cautiously.
> However, FFmpeg devs are generally cautious about these things so I no
> longer press this that hard. The real technical benefit (if any) is
> the different NaN handling as noted above.

Here some examples:
libavfilter/g729postfilter.c: 544
    *voicing = FFMAX(*voicing, long_term_filter(adsp, pitch_delay_int,
residual_filt_buf + 10,

others that call sqrt, strlen, etc (altough possibly compiler can optimize it)

commit 94494dab910133106e80ece0f79935d78138e415
+        volume = FFABS(av_expr_eval(volume_expr, expr_vars_val, NULL));
I posted the patch, and I didn't noticed or warned that it is wrong.


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