Patrick Steinhardt <ps@xxxxxx> 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.
