perf: fix encode_varint{32,64}

[morrisonlevi]

The previous code was full of jumps that aren't easily predictable. This version is less code, generates less code, and should perform better (well, on x86_64 anyway, I am not an aarch64 expert). All while using safe Rust.

You can compare assembly on godbolt. The current version of encode_varint64 is 99 lines long and has ~10 labels, the new version is 27 lines long with only 3 labels.

I haven't included benchmarking numbers because I wasn't sure what characteristics you'd be looking for. Based on the x86_64 assembly, it should definitely out-perform. Anything in particular you are looking for there?

Edit: I also simplified encoded_varint64_len while I was at it. It gets nicer assembly as a result as well but obviously matters less.

[morrisonlevi]

Interestingly, the miri failure is in std:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=92348df8e81c457c63c78c0845bf9e3b.

fn main() {
    match 2u64.checked_pow(64) {
        Some(n) => println!("2^64: {n}"),
        None => {}
    };
}

Given this is a checked_* operation, I would not expect an overflow related issue:

   Compiling playground v0.0.1 (/playground)
    Finished dev [unoptimized + debuginfo] target(s) in 0.33s
     Running `/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/cargo-miri runner target/miri/x86_64-unknown-linux-gnu/debug/playground`
error: Undefined Behavior: overflowing shift by 64 in `unchecked_shl`
    --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/num/mod.rs:1181:5
     |
1181 | /     uint_impl! {
1182 | |         Self = u64,
1183 | |         ActualT = u64,
1184 | |         SignedT = i64,
...    |
1198 | |         bound_condition = "",
1199 | |     }
     | |_____^ overflowing shift by 64 in `unchecked_shl`
     |
     = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
     = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
     = note: BACKTRACE:
     = note: inside `core::num::<impl u64>::checked_pow` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/num/uint_macros.rs:1382:31: 1385:18
note: inside `main`
    --> src/main.rs:2:11
     |
2    |     match 2u64.checked_pow(64) {
     |           ^^^^^^^^^^^^^^^^^^^^
     = note: this error originates in the macro `uint_impl` (in Nightly builds, run with -Z macro-backtrace for more info)

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

error: aborting due to 1 previous error

I have not reported this upstream nor looked for existing issues yet.

[morrisonlevi]

I opened an issue: rust-lang/rust#120537.

[stepancheg]

Merged the part about encoded_varint64_len. Can you please rebase and confirm miri is no longer an issue?

[morrisonlevi]

I merged in master, and Miri looks good now 👍🏻

[stepancheg]

OK, if this is still an issue, some benchmark is needed. Smaller machine code does not mean code is faster.

(Project is not very well maintained now, so long responses, sorry.)

[morrisonlevi]

Re-analyzing this, I made a mistake previously: the jumps are actually quite predictable. With this in mind, I would expect this PR to be a slight regression (since it isn't unrolled), but it gave me some ideas to improve it...

...but then I realized that someone already beat me to these ideas: https://github.com/as-com/varint-simd. I think it's worth considering using their implementation on x86_64, but there's no point in doing it in this PR.

So for now, we'll just count the encoded_varint64_len as a win and not merge the rest.