gen4: implement a growable semantics.TableSet

[vmg]

Description

This was requested by @systay on Slack. We need a TableSet data structure that can support more than 64 tables. We're doing this right here in the smallest possible space: TableSet is now 16 bytes large and packs a small, single-word bitmap that is used when the set contains less than 64 tables, and a pointer to an implementation of an arbitrarily large set.

This implementation is pretty straightforward -- I don't believe it can be optimized further like we'd do in C/C++/Rust. For completeness, these kind of implementations are usually done like this in non-GC'ed languages:

Instead of the 16-byte struct we have here with struct TableSet{ small uint64; large *largeTableSet }, we'd have a single-field struct with a single word in it: struct TableSet { set uintptr }. When trying to read or write to this set, we'd check if ts.set & 0x1 != 0; if this is true, then we have a 63 bit tableset by doing ts.set >> 1. If it's false, then we'd have a large tableset by doing (*largeTableSet)(ts.set). This works in practice because memory allocators will never return pointers with a 1-off alignment (their minimal alignment is at the very least the system's word size, 4/8), so we can store a bitmap in the pointer as long as the last bit of the bitmap is always 1. 👌

...Of course this doesn't work in a garbage collected language like Go because if you leave a *largeTableSet pointed at by an uintptr, the GC won't find it and it'll be freed on a later GC pass. So we don't get to to pack pointers and we need to run with 16-byte TableSets.

Good enough!

cc @systay @frouioui

Related Issue(s)

#7280

Checklist

• Tests were added or are not required
• Documentation was added or is not required

[frouioui]

The following errors appear when compiling:

abstract/concatenate.go:41:12: invalid operation: tableSet |= source.TableID() (operator | not defined on struct)
abstract/querygraph.go:81:29: invalid operation: lhs | rhs (operator | not defined on struct)

[systay]

Love it! Thank you thank you thank you