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

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

On Fri, 23 Jul 2021 at 23:29, Andrew Pinski <pinskia@gmail.com> wrote:
>
> 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.
Hi Andrew,
Thanks for the suggestions. IIUC, we could use this approach to check
if the argument
falls within a certain range (min / max), but I am not sure how it
will help to determine
if the arg is a constant immediate ? AFAIK, vshl_n intrinsics require
that the 2nd arg is immediate ?

Even the current RTL builtin checking is not consistent across
optimization levels:
For eg:
int32x2_t f(int32_t *restrict a)
{
  int32x2_t v = vld1_s32 (a);
  int b = 2;
  return vshl_n_s32 (v, b);
}

With pristine trunk, compiling with -O2 results in no errors because
constant propagation replaces 'b' with 2, and during expansion,
expand_builtin_args is happy. But at -O0, it results in the error -
"argument 2 must be a constant immediate".

So I guess we need some mechanism to mark a parameter as a constant ?

Thanks,
Prathamesh
>
> 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