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

[Martin Uecker <ma.uecker@gmail.com>]

> 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 ?

You have to use it at the call site:

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

#define foo(x) foo(({ _Static_assert(__is_constexpr(x), "no ICE"); (x); }))

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

int main()
{
  foo(1);
  int n = 1;
  foo(n);	// error
}


--Martin