jstring

JavaString

The JavaString uses short string optimizations and a lack of a "capacity" field to reduce struct size and heap fragmentation in certain cases.

Features

• Supports String API (very little at the moment but steadily growing)
• Smaller size than standard string (16 vs 24 bytes on 64-bit platforms)
• String interning for up to 15 bytes on 64-bit architectures (or 7 bytes on 32-bit)

How it works

Here's how it works:

1. We store len, the length of the string, and data, the pointer to the string itself.
2. We maintain the invariant that data is a valid pointer if and only if it points to something that's aligned to 2 bytes.
3. Now, any time we wanna read the string, we first check the lowest significance bit on data, and use that to see whether or not to dereference it.
4. Since data only uses one bit for its flag, we can use the entire lower order byte for length information when it's interned. We do this with a bitshift right.
5. When interning, we have std::mem::size_of::<usize>() * 2 - 1 bytes of space. On x64, this is 15 bytes, and on 32-bit architectures, this is 7 bytes.

License: MIT