gecko-dev/third_party/rust/base64-0.9.3

base64

It's base64. What more could anyone want?

Example

extern crate base64;

use base64::{encode, decode};

fn main() {
    let a = b"hello world";
    let b = "aGVsbG8gd29ybGQ=";

    assert_eq!(encode(a), b);
    assert_eq!(a, &decode(b).unwrap()[..]);
}

See the docs for all the details.

Purpose

I have a fondness for small dependency footprints, ecosystems where you can pick and choose what functionality you need, and no more. Unix philosophy sort of thing I guess, many tiny utilities interoperating across a common interface. One time making a Twitter bot, I ran into the need to correctly pluralize arbitrary words. I found on npm a module that did nothing but pluralize words. Nothing else, just a couple of functions. I'd like for this to be that "just a couple of functions."

Developing

Benchmarks are in benches/. Running them requires nightly rust, but rustup makes it easy:

rustup run nightly cargo bench

Decoding is aided by some pre-calculated tables, which are generated by:

cargo run --example make_tables > src/tables.rs.tmp && mv src/tables.rs.tmp src/tables.rs

Profiling

On Linux, you can use perf for profiling. Then compile the benchmarks with rustup nightly run cargo bench --no-run.

Run the benchmark binary with perf (shown here filtering to one particular benchmark, which will make the results easier to read). perf is only available to the root user on most systems as it fiddles with event counters in your CPU, so use sudo. We need to run the actual benchmark binary, hence the path into target. You can see the actual full path with rustup run nightly cargo bench -v; it will print out the commands it runs. If you use the exact path that bench outputs, make sure you get the one that's for the benchmarks, not the tests. You may also want to cargo clean so you have only one benchmarks- binary (they tend to accumulate).

sudo perf record target/release/deps/benchmarks-* --bench decode_10mib_reuse

Then analyze the results, again with perf:

sudo perf annotate -l

You'll see a bunch of interleaved rust source and assembly like this. The section with lib.rs:327 is telling us that 4.02% of samples saw the movzbl aka bit shift as the active instruction. However, this percentage is not as exact as it seems due to a phenomenon called skid. Basically, a consequence of how fancy modern CPUs are is that this sort of instruction profiling is inherently inaccurate, especially in branch-heavy code.

 lib.rs:322    0.70 :     10698:       mov    %rdi,%rax
    2.82 :        1069b:       shr    $0x38,%rax
         :                  if morsel == decode_tables::INVALID_VALUE {
         :                      bad_byte_index = input_index;
         :                      break;
         :                  };
         :                  accum = (morsel as u64) << 58;
 lib.rs:327    4.02 :     1069f:       movzbl (%r9,%rax,1),%r15d
         :              // fast loop of 8 bytes at a time
         :              while input_index < length_of_full_chunks {
         :                  let mut accum: u64;
         :
         :                  let input_chunk = BigEndian::read_u64(&input_bytes[input_index..(input_index + 8)]);
         :                  morsel = decode_table[(input_chunk >> 56) as usize];
 lib.rs:322    3.68 :     106a4:       cmp    $0xff,%r15
         :                  if morsel == decode_tables::INVALID_VALUE {
    0.00 :        106ab:       je     1090e <base64::decode_config_buf::hbf68a45fefa299c1+0x46e>

Fuzzing

This uses cargo-fuzz. See fuzz/fuzzers for the available fuzzing scripts. To run, use an invocation like these:

cargo +nightly fuzz run roundtrip
cargo +nightly fuzz run roundtrip_no_pad
cargo +nightly fuzz run roundtrip_mime -- -max_len=10240
cargo +nightly fuzz run roundtrip_random_config -- -max_len=10240

License

This project is dual-licensed under MIT and Apache 2.0.