From: Junio C Hamano <gitster@pobox.com>
Date: Fri, 05 Sep 2025 21:46:18 GMT
Subject: Re: [PATCH RFC v2 4/7] rust: implement a test balloon via the "varint" subsystem
Message-ID: <xmqqy0qs1sk5.fsf@gitster.g>
In-Reply-To: <20250905-b4-pks-rust-breaking-change-v2-4-6939cbf4a0b8@pks.im>

Patrick Steinhardt <ps@pks.im> writes:

> diff --git a/src/varint.rs b/src/varint.rs
> new file mode 100644
> index 00000000000..3d41760a555
> --- /dev/null
> +++ b/src/varint.rs
> @@ -0,0 +1,92 @@
> +use std::os::raw::c_int;
> +use std::os::raw::c_uchar;
> +
> +#[no_mangle]
> +pub unsafe extern "C" fn decode_varint(bufp: *mut *const c_uchar) -> usize {
> +    let mut buf = *bufp;
> +    let mut c = *buf;
> +    let mut val = usize::from(c & 127);
> +
> +    buf = buf.add(1);
> +
> +    while (c & 128) != 0 {
> +        val += 1;
> +        if val == 0 || val.leading_zeros() < 7 {
> +            return 0; // overflow
> +        }
> +
> +        c = *buf;
> +        buf = buf.add(1);
> +
> +        val = (val << 7) + usize::from(c & 127);
> +    }
> +
> +    *bufp = buf;
> +    val
> +}

This (and the encoding side) looks quite faithful translation of the
original in C.  Interestingly, disassembly I saw looked a lot more
optimized than the C variant compiled with clang-19 -O2.  The
difference probably is largely due to its omitting frame pointer.

The comparison to detect overflow compiled to direct comparison with
0x1ffffffffffffff (both in C and rustc/LLVM), which was amusing,
too.