[cfe-dev] Allow constexpr in vector extensions

[Tom Westerhout via cfe-dev]

Greetings!


This is my first post on this ML so, please, do tell me if I'm doing it
wrong.

I've noticed the following difference between GCC and Clang. Consider
this piece of code

```
typedef int v4si __attribute__ ((vector_size (16)));

int main()
{
    constexpr v4si a = {1,2,3,4};
    constexpr v4si b = {2,0,0,0};
    v4si c = a + b;
}
```

It compiles cleanly with both GCC and Clang. However, if I try to make c
constexpr, Clang tells me that operator+ on vectors is not constexpr. I'm
wondering if there's a reason for that. There are no constrains from the
Standard as these are intrinsics, so I see no reason why we couldn't
allow constexpr code benefit from SIMD.


Tom