zflecs: fix wrong alignment issue

[LukasKastern]

This is the flecs issue for reference.
Compiling flecs using -DFLECS_USE_OS_ALLOC already works around the issue.

But I've also implemented the allocation related os calls using a gpa to track memory leaks and guarantee correct alignment.
I haven't benchmarked the performance impact of using it. So it might make sense to make it optional 🤷.

This change should fix #284.

[michal-z]

Thanks for the PR!

So, this adds 128 bytes to each allocation, right? This seems expensive to me.

In other libs I use a hash map to track size and alignment for each allocation. Like here:
https://github.com/michal-z/zig-gamedev/blob/main/libs/zphysics/src/zphysics.zig#L625

So my approach adds 8 bytes per allocation + some hash map lookups.

I think this PR is good idea but I would prefer to use approach with the hash map. What do you think? Would you like to modify this PR?

[LukasKastern]

So, this adds 128 bytes to each allocation, right? This seems expensive to me.

Oh sorry. I mixed bits/bytes up in this pull request. The max alignment should be 16 bytes not 128 lol.
I'll fix it up later today 😅.

Do you think 16 bytes of extra space for the allocation header is fine or should we use a hashmap anyways?

[michal-z]

Maybe we could use 8 bytes for a header and store there size (u48) and alignment (u16) that would be the best solution.

[LukasKastern]

Maybe we could use 8 bytes for a header and store there size (u48) and alignment (u16) that would be the best solution.

That would also mean that the maximum alignment that flecs would be able to request is 8 bytes.
I'm gonna try that out and see if it works.

Glibc malloc seems to align to 16 bytes on 64bit platforms by default.

I guess we can expose the alignment as a build_option in case you have a component that requires a greater alignment.
At that point it might be simpler to just use a hashmap like you suggested tho'.

[michal-z]

I thought that flecs passes alignment for each allocation.

Let's go with 16 bytes headers.

[LukasKastern]

Should be good now. Let me know if you would like me to change something.