Are there any alignment guarantees for .data declarations?

[lorcanmooney]

I'd like to do something like the following, but safely on all architectures.

static ReadOnlySpan<byte> data => new byte[] { ... };
MemoryMarshal.Cast<byte, uint>(data);

I see that accessing data loads the address of a compiler-synthesized static field (and type), but what influences the alignment of a static field? Does the runtime provide any useful guarantees I can work with?

[jkotas]

Does the runtime provide any useful guarantees I can work with?

There is no useful guarantee today. (Nit: The alignment has to be guaranteed by the compiler. Runtime cannot do much about it.)

Dealing with the alignment guarantees has been a significant part of the work required to make #60948 and associated Roslyn feature happen. Look at the linked issue to see all places that had to be fixed to make the alignment work.

[lorcanmooney]

Understood. Thanks.

[lorcanmooney]

That was an interesting read.

I see a bunch of runtime work to deal with endianess concerns. I took a different approach, using a set of wrapper structs (UInt16LE, UInt32BE, etc) with implicit conversions to the matching runtime types which took care of any byte-swapping, and some basic interface implementations (IComparable, IEquatable). The JIT is shockingly good at removing the unecessary cruft and, as far as I could measure, basic operations on ReadOnlySpan such as BinarySearch and IndexOf saw no performance loss for little-endian data. Out of curiosity I tried some big-endian data, and the overhead was only about 3%.

I've opened dotnet/roslyn/issues/61259 to request the compiler to provide some sort of alignment guarantee.