Unbox small records

[raviqqe]

Problem

• We allocate memory blocks for records of any size unless they have no field.
• Those extra heap allocations are sometimes expensive.

Solution

• Unbox small records that have only a field of a size less than a word.
  • Those fields are all simple concrete types (i.e. non-union types.)
• This is safe even for recursive types but might need some tweaks for a compiler.
  • For example, LLVM might complain about a self-recursive type of type foo { x foo }.
  • They can be represented as type foo { x usize } instead.
• Do we want to change the FFI library too to implement easy-to-use new() constructors automatically.
  • They box or unbox records depending on a number of fields.

References

• Blocked by Efficient C calling convention in FFI #444.

[sammy-hughes]

What about bigger primitives that aren't implemented by the language, such as complex numbers? Would such a target qualify for this optimization?

[raviqqe]

Thanks for your question!

What about bigger primitives that aren't implemented by the language, such as complex numbers?

It doesn't matter how big the primitives in the language are because this optimization is totally internal and not actually limited to small records but also applicable to any multi-word values. We can just box or unbox big primitives on demand at boundaries if necessary (e.g. to box big primitives into "variant" types or to make big primitives FFI-safe.)

Would such a target qualify for this optimization?

So yes!

But of course, data transfer between Pen and foreign languages like Rust is another topic on optimization!