Optimize 128-bit integer formatting

[jhpratt]

The compiler is unaware of the restricted range of the input, so it is unable to optimize out the final division and modulus. By doing this manually, we get a nontrivial performance gain.

r? @dtolnay

(copied from dtolnay/itoa#68)

[rustbot]

Some changes occurred in integer formatting

cc @tgross35

[dtolnay]

Thanks!

I confirmed that the only 2 callers pass a value which is the remainder part of div_rem_1e16, which is always less than 10¹⁶.

rust/library/core/src/fmt/num.rs

Lines 664 to 677 in fd0c901

	let (quot_1e16, mod_1e16) = div_rem_1e16(self);
	let (mut remain, mut offset) = if quot_1e16 == 0 {
	(mod_1e16, U128_MAX_DEC_N)
	} else {
	// Write digits at buf[23..39].
	enc_16lsd::<{ U128_MAX_DEC_N - 16 }>(buf, mod_1e16);
	// Take another 16 decimals.
	let (quot2, mod2) = div_rem_1e16(quot_1e16);
	if quot2 == 0 {
	(mod2, U128_MAX_DEC_N - 16)
	} else {
	// Write digits at buf[7..23].
	enc_16lsd::<{ U128_MAX_DEC_N - 32 }>(buf, mod2);

Optional suggestion: debug_assert!(n < 1_0000_0000_0000_0000) inside enc_16lsd.

View changes since this review

[dtolnay]

@bors r+

[rust-bors]

📌 Commit ebb1f47 has been approved by dtolnay

It is now in the queue for this repository.