Annotations incur huge overhead

[asl]

It seems that Annotations / Annotation incur lots of overheads due to cloning during visiting:

As one can see, the Annotations is behind the largest amount of memory allocations (and total size). Annotation itself is also results in large memory overhead.

There are multiple things that contribute to this:

• Annotation itself is large. It is supposed to be a union of different things (vector of tokens, vector of expressions or indexed vector), but instead it just hold all of them. As a result, Annotation itself is 576 bytes and this is quite large node.
• Annotations is also large: ~200 bytes, but more importantly it is cloned every time IAnnotated node is visited in the Transform. The reason is simple: while it is optional, it is always initialized to Annotations::empty resulting to explicit copies during visiting.

[jafingerhut]

@asl Can this issue be closed now that #4992 is merged?

[asl]

@asl Can this issue be closed now that #4992 is merged?

Not yet. Annotation itself is still huge. Fix is WIP :)