Re: [RFC] Adding a new attribute to function param to mark it as constant

[Prathamesh Kulkarni <prathamesh.kulkarni@linaro.org>]

On Wed, 4 Aug 2021 at 03:27, Segher Boessenkool
<segher@kernel.crashing.org> wrote:
>
> Hi!
>
> On Fri, Jul 23, 2021 at 04:23:42PM +0530, Prathamesh Kulkarni via Gcc wrote:
> > The constraint here is that, vshl_n<type> intrinsics require that the
> > second arg (__b),
> > should be an immediate value.
>
> Something that matches the "n" constraint, not necessarily a literal,
> but stricter than just "immediate".  It probably is a good idea to allow
> only "integer constant expression"s, so that the validity of the source
> code does not depend on what the optimisers do with the code.
>
> > As Richard suggested, sth like:
> > void foo(int x __attribute__((literal_constant (min_val, max_val)));
>
> The Linux kernel has a macro __is_constexpr to test if something is an
> integer constant expression, see <linux/const.h> .  That is a much
> better idea imo.  There could be a builtin for that of course, but an
> attribute is less powerful, less usable, less useful.
Hi Segher,
Thanks for the suggestions. I am not sure tho if we could use a macro
similar to __is_constexpr
to check if parameter is constant inside an inline function (which is
the case for intrinsics) ?

For eg:
#define __is_constexpr(x) \
        (sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))

inline int foo(const int x)
{
  _Static_assert (__is_constexpr (x));
  return x;
}

int main()
{
  return foo (1);
}

results in:
foo.c: In function ‘foo’:
foo.c:8:3: error: static assertion failed
    8 |   _Static_assert (__is_constexpr (x));

Initially we tried to use __Static_assert (__builtin_constant_p (arg))
for the same purpose but that did not work
because while parsing the intrinsic function, the FE cannot determine
if the arg is indeed a constant.
I guess the static assertion or __is_constexpr would work only if the
intrinsic were defined as a macro instead of an inline function ?
Or am I misunderstanding ?

Thanks,
Prathamesh
>
>
> Segher