Low Level Tools/2019Q2

Nick

Implement "pipelined" rustc compilation (compiler side).

100% completed.
Don't generate unnecessary rmeta files. A precursor.
In JSON output, emit a directive after metadata is generated. The part that lets rustc tell Cargo when metadata has been written.
Emit metadata files earlier. So that significant crate overlapping can occur.
Tracking issue for stabilizing "pipelined compilation" has been opened.
Announcement email to dev-platform
Speed-ups depend on program structure and configuration, but up to 1.84x has been observed.
Speedups for opt builds of notable projects on my 14-core Linux box:
- webrender: 1.27x
- cranelift-codegen: 1.20x
- Firefox: 1.07x
- script-servo: 1.07x
- style-servo: 1.06x

Ship GWP in Nightly.

Land 5 changes that speed up or reduce peak memory usage of rustc.

100% completed.
Increase `Span` from 4 bytes to 8 bytes. A big win across lots of workloads, especially larger crates: some script-servo builds are 15% faster.
Avoid `hygiene_data` lookups. Up to 10% speed-up on packed-simd, up to 3% on numerous others.
Avoid more hygiene lookups. Up to 2% speed-up on many benchmarks.
Reduce the `DepNode` pre-allocation ratio. Reduces peak memory use by 20 MiB on some workloads.
Use `Symbol` more. Up to 1% speed-up, as well as making the code nicer. Should also help with parallel rustc.
Use `Symbol` even more. Up to 1% speed-up. Should also help with parallel rustc.