perf(lexer): pad `Token` to 16 bytes

[overlookmotel]

#9918 caused a small regression on lexer and parser benchmarks because it introduced an uninitialized padding byte into Token.

Add another dummy u8 to fill the uninitialized hole.

[overlookmotel]

• #9926 👈 (View in Graphite)
• main

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[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #9926 will not alter performance

_{Comparing 03-20-perf_lexer_pad_token_to_16_bytes (c54c02a) with main (33c1c76)}

Summary

✅ 33 untouched benchmarks

[Boshen]

I'm not seeing any difference, both are size = 16 (0x10), align = 0x4. What are property do I need to inspect? Can this be a compile time check?

We already have this

#[cfg(test)]
mod size_asserts {
    use super::Token;
    const _: () = assert!(std::mem::size_of::<Token>() == 16);
}

why is this not enough?

[overlookmotel]

Unfortunately, Rust offers no tools to detect whether a type contains padding. So I don't think it's possible to implement a test.

This PR does not affect type size or alignment, only whether any of the bytes representing the type are uninitialized. When they are, creating and copying a Token is more instructions, as compiler avoids ever reading or writing the uninitialized byte. So e.g. copying a Token is 2 x 8-byte reads + 2 x 8-byte writes (overlapping). Whereas with the _padding: u8, _padding2: u16 fields added, there are no unitialized bytes, so copying Token is just 1 x 16-byte read + 1 x 16-byte write.

This is a micro-micro-optimization, but creating and copying Tokens is such an extremely hot path that it makes a measureable difference. #3283 was the one that revealed this - 5% speed-up just due to the fields of Token getting re-arranged.

But this PR isn't succeeding in fixing the perf loss. Looks like changing the u32 field to a u16 has caused the other fields to move around, which is also a regression. I'll try to fix it.