Commits on Feb 9, 2022

1. Implement lazy funcref table and anyfunc initialization. …

   During instance initialization, we build two sorts of arrays eagerly:
   
   - We create an "anyfunc" (a `VMCallerCheckedAnyfunc`) for every function
     in an instance.
   
   - We initialize every element of a funcref table with an initializer to
     a pointer to one of these anyfuncs.
   
   Most instances will not touch (via call_indirect or table.get) all
   funcref table elements. And most anyfuncs will never be referenced,
   because most functions are never placed in tables or used with
   `ref.func`. Thus, both of these initialization tasks are quite wasteful.
   Profiling shows that a significant fraction of the remaining
   instance-initialization time after our other recent optimizations is
   going into these two tasks.
   
   This PR implements two basic ideas:
   
   - The anyfunc array can be lazily initialized as long as we retain the
     information needed to do so. For now, in this PR, we just recreate the
     anyfunc whenever a pointer is taken to it, because doing so is fast
     enough; in the future we could keep some state to know whether the
     anyfunc has been written yet and skip this work if redundant.
   
     This technique allows us to leave the anyfunc array as uninitialized
     memory, which can be a significant savings. Filling it with
     initialized anyfuncs is very expensive, but even zeroing it is
     expensive: e.g. in a large module, it can be >500KB.
   
   - A funcref table can be lazily initialized as long as we retain a link
     to its corresponding instance and function index for each element. A
     zero in a table element means "uninitialized", and a slowpath does the
     initialization.
   
   Funcref tables are a little tricky because funcrefs can be null. We need
   to distinguish "element was initially non-null, but user stored explicit
   null later" from "element never touched" (ie the lazy init should not
   blow away an explicitly stored null). We solve this by stealing the LSB
   from every funcref (anyfunc pointer): when the LSB is set, the funcref
   is initialized and we don't hit the lazy-init slowpath. We insert the
   bit on storing to the table and mask it off after loading.
   
   We do have to set up a precomputed array of `FuncIndex`s for the table
   in order for this to work. We do this as part of the module compilation.
   
   This PR also refactors the way that the runtime crate gains access to
   information computed during module compilation.
   
   Performance effect measured with in-tree benches/instantiation.rs, using
   SpiderMonkey built for WASI, and with memfd enabled:
   
   ```
   BEFORE:
   
   sequential/default/spidermonkey.wasm
                           time:   [68.569 us 68.696 us 68.856 us]
   sequential/pooling/spidermonkey.wasm
                           time:   [69.406 us 69.435 us 69.465 us]
   
   parallel/default/spidermonkey.wasm: with 1 background thread
                           time:   [69.444 us 69.470 us 69.497 us]
   parallel/default/spidermonkey.wasm: with 16 background threads
                           time:   [183.72 us 184.31 us 184.89 us]
   parallel/pooling/spidermonkey.wasm: with 1 background thread
                           time:   [69.018 us 69.070 us 69.136 us]
   parallel/pooling/spidermonkey.wasm: with 16 background threads
                           time:   [326.81 us 337.32 us 347.01 us]
   
   WITH THIS PR:
   
   sequential/default/spidermonkey.wasm
                           time:   [6.7821 us 6.8096 us 6.8397 us]
                           change: [-90.245% -90.193% -90.142%] (p = 0.00 < 0.05)
                           Performance has improved.
   sequential/pooling/spidermonkey.wasm
                           time:   [3.0410 us 3.0558 us 3.0724 us]
                           change: [-95.566% -95.552% -95.537%] (p = 0.00 < 0.05)
                           Performance has improved.
   
   parallel/default/spidermonkey.wasm: with 1 background thread
                           time:   [7.2643 us 7.2689 us 7.2735 us]
                           change: [-89.541% -89.533% -89.525%] (p = 0.00 < 0.05)
                           Performance has improved.
   parallel/default/spidermonkey.wasm: with 16 background threads
                           time:   [147.36 us 148.99 us 150.74 us]
                           change: [-18.997% -18.081% -17.285%] (p = 0.00 < 0.05)
                           Performance has improved.
   parallel/pooling/spidermonkey.wasm: with 1 background thread
                           time:   [3.1009 us 3.1021 us 3.1033 us]
                           change: [-95.517% -95.511% -95.506%] (p = 0.00 < 0.05)
                           Performance has improved.
   parallel/pooling/spidermonkey.wasm: with 16 background threads
                           time:   [49.449 us 50.475 us 51.540 us]
                           change: [-85.423% -84.964% -84.465%] (p = 0.00 < 0.05)
                           Performance has improved.
   ```
   
   So an improvement of something like 80-95% for a very large module (7420
   functions in its one funcref table, 31928 functions total).

   

   cfallin committed Feb 9, 2022
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