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);
}
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
