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 15:49, Segher Boessenkool
<segher@kernel.crashing.org> wrote:
>
> On Wed, Aug 04, 2021 at 03:20:45PM +0530, Prathamesh Kulkarni wrote:
> > On Wed, 4 Aug 2021 at 03:27, Segher Boessenkool
> > <segher@kernel.crashing.org> wrote:
> > > 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) ?
>
> I said we can make a builtin that returns if its arg is an ICE -- we do
> not have to do tricky tricks :-)
>
> The macro would work fine in an inline function though, or, where do you
> see potential problems?
>
> > 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));
>
> And that is correct, x is *not* an integer constant expression here.
> Because it is a variable, instead :-)
>
> If you do this in a macro it should work though?
>
> > 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.
>
> Yes.  If you want something like that you need to test very late during
> compilation whether something is a constant then: it will not be earlier.
>
> > 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 ?
>
> Both __builtin_constant_p and __is_constexpr will not work in your use
> case (since a function argument is not a constant, let alone an ICE).
> It only becomes a constant value later on.  The manual (for the former)
> says:
>   You may use this built-in function in either a macro or an inline
>   function. However, if you use it in an inlined function and pass an
>   argument of the function as the argument to the built-in, GCC never
>   returns 1 when you call the inline function with a string constant or
>   compound literal (see Compound Literals) and does not return 1 when you
>   pass a constant numeric value to the inline function unless you specify
>   the -O option.
Indeed, that's why I was thinking if we should use an attribute to mark param as
a constant, so during type-checking the function call, the compiler
can emit a diagnostic if the passed arg
is not a constant.

Alternatively -- as you suggest, we could define a new builtin, say
__builtin_ice(x) that returns true if 'x' is an ICE.
And wrap the intrinsic inside a macro that would check if the arg is an ICE ?

For eg:

__extension__ extern __inline int32x2_t
__attribute__  ((__always_inline__, __gnu_inline__, __artificial__))
vshl_n_s32_1 (int32x2_t __a, const int __b)
{
  return __builtin_neon_vshl_nv2si (__a, __b);
}

#define vshl_n_s32(__a, __b) \
({ typeof (__a) a = (__a); \
   _Static_assert (__builtin_constant_p ((__b)), #__b " is not an
integer constant"); \
   vshl_n_s32_1 (a, (__b)); })

void f(int32x2_t x, const int y)
{
  vshl_n_s32 (x, 2);
  vshl_n_s32 (x, y);

  int z = 1;
  vshl_n_s32 (x, z);
}

With this, the compiler rejects vshl_n_s32 (x, y) and vshl_n_s32 (x,
z) at all optimization levels since neither 'y' nor 'z' is an ICE.
Instead of __builtin_constant_p, we could use __builtin_ice.
Would that be a reasonable approach ?

But this changes the semantics of intrinsic from being an inline
function to a macro, and I am not sure if that's a good idea.

Thanks,
Prathamesh

> An integer constant expression is well-defined whatever the optimisation
> level is, it is a feature of the language.
>
> If some x is an ICE you can do
>   asm ("" :: "n"(x));
> and if it is a constant you can do
>   asm ("" :: "i"(x));
> (not that that gets you much further, but it might help explorng this).
>
>
> Segher