AVX stream intrinsics signatures are incorrect, maybe unsound

[gnzlbg]

In the intel intrinsics guide, the signature is:

void _mm256_stream_si256 (__m256i * mem_addr, __m256i a)

but in Rust the signature is:

pub unsafe fn _mm256_stream_si256(mem_addr: *const __m256i, a: __m256i)

Note that the mem_addr pointer is const in Rust, but not in C. Since the only purpose of this function is to write through this pointer, it is probably unsound for it to take a const pointer.

EDIT: _mm256_stream_pd and _mm256_stream_ps also have this same issue. The SSE _mm_stream_ps, _mm_stream_pd, and _mm_stream_pi intrinsics correctly use *mut here.

A tangential bug is that the intrinsic verification failed hard here, since the C intrinsics do not use const pointers, it would have been safer to require these intrinsics to use mut pointers.

[alexcrichton]

Oh dear sounds bad! It may be the case though we can fix this without practical breakage. Want to send a PR and we can crater?

[gnzlbg]

Will do.

[RalfJung]

Notice that *const T and *mut T are equivalent in terms of UB, so this is not -- on its own -- unsound.

However, it could lead to people using &T on the call site, and that would be unsound.

[gnzlbg]

Yes, we should just have had a single pointer type :/