Constexpr in intrinsics?

[Marc Glisse]

On Mon, 28 Mar 2016, Allan Sandfeld Jensen wrote:

> On Sunday 27 March 2016, Marc Glisse wrote:
>> On Sun, 27 Mar 2016, Allan Sandfeld Jensen wrote:
>>> Would it be possible to add constexpr to the intrinsics headers?
>>>
>>> For instance _mm_set_XX and _mm_setzero intrinsics.
>>
>> Already suggested here:
>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65197
>>
>> A patch would be welcome (I started doing it at some point, I don't
>> remember if it was functional, the patch is attached).
>>
> That looks very similar to the patch I experimented with, and that at least
> works for using them in C++11 constexpr functions.
>
>>> Ideally it could also be added all intrinsics that can be evaluated at
>>> compile time, but it is harder to tell which those are.
>>>
>>> Does gcc have a C extension we can use to set constexpr?
>>
>> What for?
>
> To have similar functionality in C. For instance to explicitly allow those
> functions to be evaluated at compile time, and values with similar attributes
> be optimized completely out.

Those intrinsics that are implemented without builtins can already be 
evaluated at compile time.

#include <x86intrin.h>

__m128d f(){
   __m128d a=_mm_set_pd(1,2);
   __m128d b=_mm_setr_pd(4,3);
   return _mm_add_pd(a, b);
}

The generated asm is just

 	movapd	.LC0(%rip), %xmm0
 	ret

For the more esoteric intrinsics, what is missing is not in the parser, it 
is a folder that understands the behavior of each particular intrinsic.

> And of course avoid using precompiler noise, in
> shared C/C++ headers like these are.

-- 
Marc Glisse