Am Montag, dem 15.12.2025 um 14:00 +0100 schrieb Richard Biener:
> On Sun, Dec 14, 2025 at 3:33 PM Martin Uecker via Gcc <[email protected]> wrote:
> >
> > Am Sonntag, dem 14.12.2025 um 15:10 +0100 schrieb Andreas Schwab:
> > > On Dez 14 2025, Martin Uecker via Gcc wrote:
> > >
> > > > One could use templates in C++ or macros, but then this is
> > > > rather heavy weight and also rigid as one has to structure the
> > > > code around it. I am rather looking for some lightweight
> > > > solution such as annotation I could put on the argument or variable
> > > > that says: If this is value is known to be a compile-time
> > > > constant, please create a specialized code path when optimizing.
> > > > (maybe with some limit on recursion), maybe only for specified
> > > > parameter ranges.
> > > >
> > > > Or are there other better ways to do this already?
> > >
> > > Is __builtin_constant_p what you are looking for?
> >
> > Not really. I know I can use it in some inline wrapper to
> > add my own compile-time specialization, but I want the compiler
> > to create the specialized code for me without having to duplicate
> > it the source. It can specialize code already just fine if it
> > likes to do so, but there seems to be no good way to
> > enforce this.
>
> It only specializes at calls with constant arguments. And it will not
> replace calls to with non-constant arguments with if (val == constant)
> specialization(); else regular();
> either. IIRC at some point we've talked about collecting
> "interesting" constants and directing
> all calls to a dispatcher. There might be a PR about this.
>
> So, do you actually want to have the unspecialized function replaced by
> a dispatcher that has a few values specialized?
I am not 100% sure myself what I want/need, but something like a
dispatcher based on some manually specified constants is indeed
what I had in mind, e.g.
int foo(int n)
[[gnu::specialize(0)]]
{
...
}
or maybe even inside functions
int foo(int n)
{
...
[[gnu::specialize(n == 3)]];
...
}
Martin
