Re: [gcc15] nested functions in C

[Siddhesh Poyarekar <siddhesh@gotplt.org>]

On 2023-12-04 13:51, Jakub Jelinek wrote:
> Why?  The syntax doesn't seem to be something unexpected, and as C doesn't
> have lambdas, one can use the nested functions instead.
> The only problem is if you need to pass function pointers somewhere else
> (and target doesn't have function descriptors or something similar), if it
> is only done to make code more readable compared to say use of macros, I
> think the nested functions are better, one doesn't have to worry about
> multiple evaluations of argument side-effects etc.  And if everything is
> inlined and SRA optimized, there is no extra cost.
> The problem of passing it as a function pointer to other functions is
> common with C++, only lambdas which don't capture anything actually can be
> convertible to function pointer, for anything else you need a template and
> instantiate it for a particular lambda (which is something you can't do in
> C).

I think from a language standpoint, the general idea that nested 
functions are just any functions inside functions (which is how the C 
nested functions essentially behave) is too broad and they should be 
restricted to minimal implementations that, e.g. don't have side-effects 
or if they do, there's explicit syntactic sugar to make it clearer.

If (like Martin stated earlier), nested functions are in fact going to 
enter the C standard in future then maybe this discussion is moot and we 
probably are better off implementing descriptor support for C to replace 
the on-stack trampolines instead of adding -Werror=trampolines in a hurry.

Thanks,
Sid