> Am 05.06.2024 um 16:08 schrieb Michael Matz <m...@suse.de>:
>
> Hey,
>
>> On Wed, 5 Jun 2024, David Brown wrote:
>>
>> The ideal here would be to have some way to tell gcc that a given
>> function has the semantics of a different function. For example, a
>> programmer might have several implementations of "memcpy" that are
>> optimised for different purposes based on the size or alignment of the
>> arguments. Maybe some of these are written with inline assembly or work
>> in a completely different way (I've used DMA on a microcontroller for
>> the purpose). If you could tell the compiler that the semantic
>> behaviour and results were the same as standard memcpy(), that could
>> lead to optimisations.
>>
>> Then you could declare your "isinf" function with
>> __attribute__((semantics_of(__builtin_isinf))).
>>
>> And the feature could be used in any situation where you can write a
>> function in a simple, easy-to-analyse version and a more efficient but
>> opaque version.
>
> Hmm, that actually sounds like a useful feature. There are some details
> to care for, like what to do with arguments: e.g. do they need to have the
> same types as the referred builtin, only compatible ones, or even just
> convertible ones, and suchlike, but yeah, that sounds nice.
There’s the difficulty to avoid having GCC replace it with a semantically
equivalent call (that would be no longer optimized). That is, your intent is
to enable a subset of optimizations only enabled by denoting the semantic
equivalence. That subset at least needs documenting.
Richard
>
> Ciao,
> Michael.
