[FFmpeg-devel] [PATCH 2/2] Use gcc/clang builtins for av_sat_(add|sub)_64_c if available.

[James Almer]

On 5/1/2020 4:46 PM, Carl Eugen Hoyos wrote:
> Am Fr., 1. Mai 2020 um 21:42 Uhr schrieb James Almer <jamrial at gmail.com>:
>>
>> On 5/1/2020 4:37 PM, Carl Eugen Hoyos wrote:
>>> Am Fr., 1. Mai 2020 um 21:34 Uhr schrieb James Almer <jamrial at gmail.com>:
>>>>
>>>> On 5/1/2020 4:27 PM, Carl Eugen Hoyos wrote:
>>>>> Am Fr., 1. Mai 2020 um 20:22 Uhr schrieb James Almer <jamrial at gmail.com>:
>>>>>>
>>>>>> On 5/1/2020 3:07 PM, Carl Eugen Hoyos wrote:
>>>>>>> Am Fr., 1. Mai 2020 um 19:24 Uhr schrieb Dale Curtis <dalecurtis at chromium.org>:
>>>>>>>>
>>>>>>>> Signed-off-by: Dale Curtis <dalecurtis at chromium.org>
>>>>>>>> ---
>>>>>>>>  libavutil/common.h | 10 ++++++++++
>>>>>>>>  1 file changed, 10 insertions(+)
>>>>>>>
>>>>>>> Imo, this is guaranteed to break x86 compilation with some unusual
>>>>>>> toolchain.
>>>>>>> I looked carefully at all occurrences of AV_GCC_VERSION_AT_LEAST()
>>>>>>> and I believe they are in fact different (not for x86 or combined with other
>>>>>>> checks).
>>>>>>
>>>>>> Can you elaborate on this? AV_GCC_VERSION_AT_LEAST() is a public macro
>>>>>> used in public headers to choose one or another path depending on if the
>>>>>> compiler is a recent enough GCC, or something else.
>>>>>
>>>>>> What toolchain could this break
>>>>>
>>>>> It definitely breaks icc compilation on Linux.
>>>>
>>>> So ICC on Linux defines __GNUC__ >= 5 yet doesn't support these builtins?
>>>
>>> No, but yes, this is the effect.
>>
>> So that means ICC is broken, defining support for features it doesn't
>> really has.
>> We can workaround that with a !defined(__INTEL_COMPILER) check, i guess,
>> but perhaps we may want to stop supporting broken compilers.
> 
> Or we add the necessary two lines of configure code necessary to avoid
> the issue.

No, a pre-processor check is more than enough. If we really care about
ICC, then Dale Curtis can add !defined(__INTEL_COMPILER) to the check,
like it's done in plenty other places for this exact same purpose.

> 
>>>>>> and why specifically x86?
>>>>>
>>>>> I mentioned x86 because afaics, all existing relevant uses of
>>>>> AV_GCC_VERSION_AT_LEAST() will not be triggered on x86.
>>>>
>>>> AV_GCC_VERSION_AT_LEAST is used in a lot of arch agnostic libavutil
>>>> headers, and in x86/intmath.h
>>>
>>> This is exactly what I meant above with "carefully".
>>
>> It would be nice if you could stop being so terse and explain what's on
>> your mind for once. You're not being clear at all, and i asked you to
>> elaborate, something you still haven't done.
> 
> To make this crystal-clear:
> It would be nice if you could stop writing offending and annoying mails as
> you did today.

How am i writing offending emails? I asked you to elaborate on something
and barely got any information in return, then asked you to be less
terse and actually elaborate on what you were trying to argue.

> 
> The intmath.h check is surrounded by an additional check that looks much
> stricter to me than the AV_GCC_VERSION_AT_LEAST() check there.

The AV_GCC_VERSION_AT_LEAST check in x86/intmath.h is wrapped in a
__GNUC__ check and a __BMI2__ check. The former has no effect as far as
the macro is concerned since it's already a part of it, and the latter
is yet another GCC define, hence being checked after __GNUC__.
And being in the x86 folder, it's of course triggered on x86 targets.

Meanwhile, all the AV_GCC_VERSION_AT_LEAST checks in attributes.h are
triggered for every single target, with considerations for clang and ICC
where required.

> 
> Carl Eugen
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