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

[Segher Boessenkool <segher@kernel.crashing.org>]

On Wed, Aug 04, 2021 at 05:20:58PM +0530, Prathamesh Kulkarni wrote:
> On Wed, 4 Aug 2021 at 15:49, Segher Boessenkool
> <segher@kernel.crashing.org> wrote:
> > 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.

That will depend on the vagaries of what optimisations the compiler
managed to do :-(

> Alternatively -- as you suggest, we could define a new builtin, say
> __builtin_ice(x) that returns true if 'x' is an ICE.

(That is a terrible name, it's not clear at all to the reader, just
write it out?  It is fun if you know what it means, but infuriating
otherwise.)

> And wrap the intrinsic inside a macro that would check if the arg is an ICE ?

That will work yeah.  Maybe not as elegant as you'd like, but not all
that bad, and it *works*.  Well, hopefully it does :-)

> 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.

You used __builtin_constant_p though, which works differently, so the
test is not conclusive, might not show what you want to show.

> Instead of __builtin_constant_p, we could use __builtin_ice.
> Would that be a reasonable approach ?

I think it will work, yes.

> 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.

Well, what happens if you call the actual builtin directly, with some
non-constant parameter?  That just fails with a more cryptic error,
right?  So you can view this as some syntactic sugar to make these
intrinsics easier to use.

Hrm I now remember a place I could have used this:

#define mtspr(n, x) do { asm("mtspr %1,%0" : : "r"(x), "n"(n)); } while (0)
#define mfspr(n) ({ \
	u32 x; asm volatile("mfspr %0,%1" : "=r"(x) : "n"(n)); x; \
})

It is quite similar to your builtin code really, and I did resort to
macros there, for similar reasons :-)


Segher