Encouraging preliminary results implementing memcpy in D

[David Nadlinger]

On Wednesday, 13 June 2018 at 06:46:43 UTC, Mike Franklin wrote:
> https://github.com/JinShil/memcpyD
>
> […]
>
> Feedback, advise, and pull requests to improve the 
> implementation are most welcome.

The memcpyD implementation is buggy; it assumes that all 
arguments are aligned to their size. This isn't necessarily true. 
For example, `ubyte[1024].alignof == 1`, and struct alignment can 
also be set explicitly using align(N).

On x86, you can get away with this in a lot of cases even though 
it's undefined behaviour [1], but this is not necessarily the 
case for SSE/AVX instructions. In fact, that's probably a pretty 
good guess as to where those weird crashes you mentioned come 
from.

On other architectures, unaligned accesses might be outright 
unsupported.

For loading into vector registers, you can use 
core.simd.loadUnaligned instead (ldc.simd.loadUnaligned for LDC – 
LDC's druntime has not been updated yet after {load, 
store}Unaligned were added upstream as well).

  — David



[1] This is applying C rules to D, where creation of unaligned 
pointers is undefined behaviour. The D specification doesn't 
mention