serialize_u64 is slow

[dtolnay]

It currently looks like this:

fn serialize_u64(&mut self, value: u64) -> Result<()> {
    write!(&mut self.writer, "{}", value).map_err(From::from)
}

I tested the following variation and it made an astonishing 20% difference in the bench_log benchmarks (294 MB/s vs 244 MB/s).

if value == 0 {
    self.writer.write_all(b"0").map_err(From::from)
} else {
    let mut scratch = [0u8; 20];
    let mut len = 0;
    while value > 0 {
        scratch[19 - len] = (value % 10) as u8 + b'0';
        value /= 10;
        len += 1;
    }
    self.writer.write_all(&scratch[20-len..20]).map_err(From::from)
}

That is enough to motivate looking for a better solution, or contributing a fix to Rust for whatever is making write! slow.

[dtolnay]

Here is the code that write! is using: num.rs#L201-L254.

[dtolnay]

The fmt method I linked to is way faster than mine. All the slowness comes from pad_integral which we shouldn't even need because we want the default format. I will look for a way to avoid going through pad_integral.

[dtolnay]

Fixed in #87.