[FFmpeg-devel] [PATCH 06/10] avfilter/avgblur_vulkan: call av_vkfmt_from_pixfmt only one time

[Andreas Rheinhardt]

Wu, Jianhua:
> Andreas Rheinhardt wrote:
>>
>> Wu Jianhua:
>>> There is a loop in av_vkfmt_from_pixfmt. And, even though some
>>> compilers will optimize it when the optimization option is on, to
>>> consider more situations, just call the function only one time here.
>>>
>>> Signed-off-by: Wu Jianhua <jianhua.wu at intel.com>
>>> ---
>>
>> What compilers optimize this? And how? After all, this function is not marked
>> as const.
> 
> When there is some same operations in the codes the compiler will help the
> developer to reuse the same result when the optimization is on, like -O3 options
> in GCC. However the -O# was not used, the function would be called again and
> again. And I check the assembly codes, it did. If someone compiled with the
> optimizations, this commit didn't changed anything in assembly codes. But it
> did help when there is no optimizations compiling.
> 

That is very surprising: Without a pure/const attribute the compiler is
not allowed to optimize multiple calls to the same functions into one;
and even with said attribute it is only allowed to do so if it can prove
that the argument (and in case of pure also the whole state of the
program) didn't change. I don't see how the compiler could know this, so
regardless of whether you add the attribute it would be a compiler bug
if the compiler actually optimized the code in that way.

> I am more prefer this way to just call the function only one time literally. I am
> not sure if anybody like this so I put it on the back of the patch set. And I check
> this function and it did return a const VkFormat * that supposed to
> mean we should not change the content.
> 
I also prefer to not emit unnecessary calls; it's just that the commit
message is surprising/unbelievable.

- Andreas