Fix bitmask vector bit order

[calebzulawski]

Fixes #379

[calebzulawski]

@RalfJung looks good?

[RalfJung]

I honestly can't tell, I don't really understand the code in this library. We'll see if this makes the Miri tests pass -- once it propagates into the rustc repository.

I also have no idea what should happen for cases like N == 12. Should the first or the last element of the array be the one that is only partially filled? I thought it'd have to be the first element, but then I looked at an example: on little endian, mask 0b001101001001 is the same as [0b01001001, 0b0011], and on big endian the same mask is written 0b100100101100 as a single integer and [0b10010010, 0b1100] as an array. Looks like indeed converting the arrays works by reversing the bits in each element, and then fixing up the last element as that is always the one that is "partial". But the order of array elements is the same in both cases. I don't quite understand why...

[calebzulawski]

Yeah, I'm not sure what makes the most sense either. Fwiw size 12 simply doesn't work yet, something is wrong with the intrinsic. When I fix the intrinsic I expect the last element to be the one needing fixing, because LLVM simply specifies the vector element ordering to be dependent on endianness (not just bitmasks, also concatenating vectors into a single integer). An alternative implementation could reverse the vector rather than the bitmask, if that makes it any clearer.

[RalfJung]

I'm fine with whatever as long as it is consistent between simd_bitmask and simd_select_bitmask, and documented properly. :)

Miri anyway only supports powers of 2 currently.

[workingjubilee]

We should explain why this is the correct behavior before we commit it, which means expanding our written spec for simd_select_bitmask and simd_bitmask.

[calebzulawski]

Perhaps we should expose the intrinsics in something like core::intrinsics::simd and use that documentation as the actual semantics

[RalfJung]

Any estimate when this will be propagated to the rustc repo? :)

[calebzulawski]

I just opened rust-lang/rust#121269