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

[Richard Earnshaw]

On 04/08/2021 13:46, Segher Boessenkool wrote:
> 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
> 

We don't want to have to resort to macros.  Not least because at some
point we want to replace the content of arm_neon.h with a single #pragma
directive to remove all the parsing of the header that's needed.  What's
more, if we had a suitable pragma we'd stand a fighting chance of being
able to extend support to other languages as well that don't use the
pre-processor, such as Fortran or Ada (not that that is on the cards
right now).

R.