C bitfields guarantees

[Walter Bright]

On 7/5/2024 12:35 PM, Steven Schveighoffer wrote:
> What if you need > 32 bits or want to pack into a `ulong`? Is the behavior sane 
> across compilers?

Yes. The trouble happens when you mix different field types. There are also 
differences when declaring "packed" bit fields - a C extension that ImportC does 
not implement.

You can see which cases are different in:

ImportC:

https://github.com/dlang/dmd/blob/master/compiler/test/runnable/bitfieldsms.c
https://github.com/dlang/dmd/blob/master/compiler/test/runnable/bitfieldsposix32.c
https://github.com/dlang/dmd/blob/master/compiler/test/runnable/bitfieldsposix64.c

D:

https://github.com/dlang/dmd/blob/master/compiler/test/runnable/dbitfieldsms.c
https://github.com/dlang/dmd/blob/master/compiler/test/runnable/dbitfieldsposix32.c
https://github.com/dlang/dmd/blob/master/compiler/test/runnable/dbitfieldsposix64.c