Re: [gcc15] nested functions in C

[Martin Uecker <uecker@tugraz.at>]

Am Montag, dem 04.12.2023 um 19:51 +0100 schrieb Jakub Jelinek:
> On Mon, Dec 04, 2023 at 01:27:32PM -0500, Siddhesh Poyarekar wrote:
> > [Branching this into a separate conversation to avoid derailing the patch,
> > which isn't directly related]
> > 
> > On 2023-12-04 12:21, Martin Uecker wrote:
> > > I do not really agree with that.  Nested functions can substantially
> > > improve code quality and in C can avoid type unsafe use of
> > > void* pointers in callbacks. The code is often much better with
> > > nested functions than without.  Nested functions and lambdas
> > > (i.e. anonymous nested functions) are used in many languages
> > > because they make code better and GNU's nested function are no
> > > exception.
> > > 
> > > So I disagree with the idea that discouraging nested functions leads
> > > to better code - I think the exact opposite is true.
> > 
> > I would argue that GNU's nested functions *are* an exception because they're
> > like feathers stuck on a pig to try and make it fly; I think a significant
> > specification effort is required to actually make it a cleanly usable
> > feature.
> 
> 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).

In C++ you can erase the type with std::function.  C is missing a 
function pointer type which can encapsulate the static chain on
all archs (not only for nested functions, also for language 
interoperability).

Martin

>