Portable bitwise operations library (was Re: Portability Considerations)

[Alice Osako]

I've written what are working ISO-compatible implementations of BIT, 
SETBIT, BWNOT, BWAND, BWOR, BWXOR, SHL, and SHR, using only the ISO 
primitives, though they are painful kludges which I am frankly 
embarrassed by - especially the shifts.

https://github.com/Schol-R-LEA/Modula-2-Portable-Bit-Manipulation/tree/main

There has to be a better solution, IMO, but this is what I was able to 
come up with without resorting to either the GCC-specific libraries, or 
importing assembly code. Even then, the claim to being 'portable' is 
probably questionable at best. The fact that there is a constant which 
defines the word width, which would have to be manually modified for the 
correct value, makes the question of 'portability' pointless.

And yes, I did have to resort to using variant record aliasing to write 
the two specialized cast utility functions, and worse, to looping on the 
bits for the shifts. I am truly uncomfortable with this approach, but 
couldn't think of a better one within the constraints of generic ISO code.

I will get to ASHR, ROTL, and ROTR eventually, but this is about the 
most I can manage tonight. Similarly, a GCC specific version, hopefully 
less grotesque, will have to wait, as will a more comprehensive suite of 
tests.

Perhaps someone need to write a unit test framework for Modula-2, but 
that won't be me right now.

By the way, does the GCC implementation of Modula-2 support inline 
assembly, or would I have to wrap the assembly/C code into functions and 
import them? Neither solution really appeals to me, but they would be 
better than the unholy mockery I've already written. I know that the 
ByteBitOps and WordBitOps modules exist, but those are undocumented and 
that's the last thing I need to be messing with right now.

I am not certain how I would proceed with a version that is compatible 
with PIM (any edition); it is probably something that would have to be 
done on a per-implementation basis.