Commits on Mar 23, 2021

1. Optimize calling a WebAssembly function …

   This commit implements a few optimizations, mainly inlining, that should
   improve the performance of calling a WebAssembly function. This code
   path can be quite hot depending on the embedding case and we hadn't
   really put much effort into optimizing the nitty gritty.
   
   The predominant optimization here is adding `#[inline]` to trivial
   functions so performance is improved without having to compile with LTO.
   Another optimization is to call `lazy_per_thread_init` when traps are
   initialized per-thread (when a `Store` is created) rather than each time
   a function is called. The next optimization is to change the unwind
   reason in the `CallThreadState` to `MaybeUninit` to avoid extra checks
   in the default case about whether we need to drop its variants (since in
   the happy path we never need to drop it). The final optimization is to
   optimize out a few checks when `async` support is disabled for a small
   speed boost.
   
   In a small benchmark where wasmtime calls a simple wasm function my
   macOS computer dropped from 110ns to 86ns overhead, a 20% decrease. The
   macOS overhead is still largely dominated by the global lock acquisition
   and hash table management for traps right now, but I suspect the Linux
   overhead is much better (should be on the order of ~30 or so ns).
   
   We still have a long way to go to compete with SpiderMonkey which, in
   testing, seem to have ~6ns overhead in calling the same wasm function on
   my computer.

   

   alexcrichton committed Mar 23, 2021
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