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

[Richard Biener <richard.guenther@gmail.com>]

On Mon, Jul 26, 2021 at 11:06 AM Prathamesh Kulkarni via Gcc
<gcc@gcc.gnu.org> wrote:
>
> 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 ?

I guess you want to mark it in a way that the frontend should force
constant evaluation and error if that's not possible?   C++ doesn't
allow to declare a parameter as 'constexpr' but something like

void foo (consteval int i);

since I guess you do want to allow passing constexpr arguments
in C++ or in C extended forms of constants like

static const int a[4];

foo (a[1]);

?  But yes, this looks useful to me.

Richard.

>
> 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