We weren't using the "union" cargo feature for the smallvec crate, which
reduces the size of a SmallVec by one machine word. This feature
requires Rust 1.49 but we already require much newer versions.
When using Wasmtime to compile pulldown-cmark from Sightglass, this
saves a decent amount of memory allocations and writes. According to
`valgrind --tool=dhat`:
- 6.2MiB (3.69%) less memory allocated over the program's lifetime
- 0.5MiB (4.13%) less memory allocated at maximum heap size
- 5.5MiB (1.88%) fewer bytes written to
- 0.44% fewer instructions executed
Sightglass reports a statistically significant runtime improvement too:
compilation :: cycles :: benchmarks/pulldown-cmark/benchmark.wasm
Δ = 24379323.60 ± 20051394.04 (confidence = 99%)
shrink-abiarg-0406da67c.so is 1.01x to 1.13x faster than main-be690a468.so!
[227506364 355007998.78 423280514] main-be690a468.so
[227686018 330628675.18 406025344] shrink-abiarg-0406da67c.so
compilation :: cycles :: benchmarks/spidermonkey/benchmark.wasm
Δ = 360151622.56 ± 278294316.90 (confidence = 99%)
shrink-abiarg-0406da67c.so is 1.01x to 1.07x faster than main-be690a468.so!
[8709162212 8911001926.44 9535111576] main-be690a468.so
[5058015392 8550850303.88 9282148438] shrink-abiarg-0406da67c.so
compilation :: cycles :: benchmarks/bz2/benchmark.wasm
Δ = 6936570.28 ± 6897696.38 (confidence = 99%)
shrink-abiarg-0406da67c.so is 1.00x to 1.08x faster than main-be690a468.so!
[155810934 175260571.20 234737344] main-be690a468.so
[119128240 168324000.92 257451074] shrink-abiarg-0406da67c.so
