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

[Andrew Pinski <pinskia@gmail.com>]

On Fri, Jul 23, 2021 at 3:55 AM Prathamesh Kulkarni via Gcc
<gcc@gcc.gnu.org> wrote:
>
> Hi,
> Continuing from this thread,
> https://gcc.gnu.org/pipermail/gcc-patches/2021-July/575920.html
> The proposal is to provide a mechanism to mark a parameter in a
> function as a literal constant.
>
> Motivation:
> Consider the following intrinsic vshl_n_s32 from arrm/arm_neon.h:
>
> __extension__ extern __inline int32x2_t
> __attribute__  ((__always_inline__, __gnu_inline__, __artificial__))
> vshl_n_s32 (int32x2_t __a, const int __b)
> {
>   return (int32x2_t)__builtin_neon_vshl_nv2si (__a, __b);
> }
>
> and it's caller:
>
> int32x2_t f (int32x2_t x)
> {
>    return vshl_n_s32 (x, 1);
> }

Can't you do similar to what is done already in the aarch64 back-end:
#define __AARCH64_NUM_LANES(__v) (sizeof (__v) / sizeof (__v[0]))
#define __AARCH64_LANE_CHECK(__vec, __idx)      \
        __builtin_aarch64_im_lane_boundsi (sizeof(__vec),
sizeof(__vec[0]), __idx)

?
Yes this is about lanes but you could even add one for min/max which
is generic and such; add an argument to say the intrinsics name even.
You could do this as a non-target builtin if you want and reuse it
also for the aarch64 backend.

Thanks,
Andrew Pinski

>
> The constraint here is that, vshl_n<type> intrinsics require that the
> second arg (__b),
> should be an immediate value.
> Currently, this check is performed by arm_expand_builtin_args, and if
> a non-constant
> value gets passed, it emits the following diagnostic:
>
> ../armhf-build/gcc/include/arm_neon.h:4904:10: error: argument 2 must
> be a constant immediate
>  4904 |   return (int32x2_t)__builtin_neon_vshl_nv2si (__a, __b);
>       |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> However, we're trying to replace builtin calls with gcc's C vector
> extensions where
> possible (PR66791), because the builtins are opaque to the optimizers.
>
> Unfortunately, we lose type checking of immediate value if we replace
> the builtin
> with << operator:
>
> __extension__ extern __inline int32x2_t
> __attribute__  ((__always_inline__, __gnu_inline__, __artificial__))
> vshl_n_s32 (int32x2_t __a, const int __b)
> {
>   return __a << __b;
> }
>
> So, I was wondering if we should have an attribute for a parameter to
> specifically
> mark it as a constant value with optional range value info ?
> As Richard suggested, sth like:
> void foo(int x __attribute__((literal_constant (min_val, max_val)));
>
> Thanks,
> Prathamesh