SpiderMonkey Newsletter #6

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Jan de Mooij

Aug 28, 2020, 11:45:57 AM8/28/20
to dev-platform
(This newsletter is also available on our blog

SpiderMonkey is the JavaScript engine used in Mozilla Firefox. This
newsletter gives an overview of the JavaScript and WebAssembly work we’ve
done as part of the Firefox 80 and 81 Nightly release cycles. If you like
these newsletters, you may also enjoy Yulia’s Compiler Compiler live stream

With the recent changes at Mozilla, some may be worried about what this
means for SpiderMonkey. The team continues to remain strong, supported and
is excited to show off a lot of cool things this year and into the future.
JavaScript 👷🏽‍♀️ New features Firefox 80

- Ted added <https://bugzilla.mozilla.org/show_bug.cgi?id=1496852>
support for the export * as ns from “module” syntax in modules proposal

In progress

- Matthew implemented
<https://bugzilla.mozilla.org/show_bug.cgi?id=1642476> private class
behind a pref. He also added
<https://bugzilla.mozilla.org/show_bug.cgi?id=1644160> support for
adding private class fields to proxies.
- Adam implemented <https://bugzilla.mozilla.org/show_bug.cgi?id=1435826>
support for private methods and accessors
<https://github.com/tc39/proposal-private-methods>, behind a pref.
- Adam added <https://bugzilla.mozilla.org/show_bug.cgi?id=1647794> more
<https://bugzilla.mozilla.org/show_bug.cgi?id=1647796> Iterator helper
<https://tc39.es/proposal-iterator-helpers/> methods.
- Jeff landed <https://bugzilla.mozilla.org/show_bug.cgi?id=1502355>
more changes for ReadableStream pipeTo/pipeThrough.
- André implemented
Intl.DateTimeFormat.prototype.formatRange and formatRangeToParts.
- Yulia started <https://bugzilla.mozilla.org/show_bug.cgi?id=1519100>
implementing Top-level await

🗑️ Garbage Collection

- Jon moved <https://bugzilla.mozilla.org/show_bug.cgi?id=1652019> more
decommit code to the background thread.
- Jon added <https://bugzilla.mozilla.org/show_bug.cgi?id=1657850> prefs
and improved heuristics for the number of helper threads doing GC work.
- Yoshi added <https://bugzilla.mozilla.org/show_bug.cgi?id=1653914> GC
sub-categories to the profiler.
- Paul removed <https://bugzilla.mozilla.org/show_bug.cgi?id=1482089>
some unused GC telemetry code to improve memory usage.
- Jon enabled <https://bugzilla.mozilla.org/show_bug.cgi?id=1657122>
compacting for ObjectGroups.
- Yoshi fixed <https://bugzilla.mozilla.org/show_bug.cgi?id=1649702>
shutdown GCs to not start compression tasks.

❇️ Stencil

Stencil <https://bugzilla.mozilla.org/show_bug.cgi?id=1601332> is our
project to create an explicit interface between the frontend (parser,
bytecode emitter) and the rest of the VM, decoupling those components. This
lets us improve performance, simplify a lot of code and improve bytecode
caching. It also makes it possible to rewrite our frontend in Rust (see
SmooshMonkey item below).

We expect to switch <https://bugzilla.mozilla.org/show_bug.cgi?id=1660798>
the frontend to use ParserAtoms in the next Nightly cycle (Firefox 82). At
that point the frontend will work without doing any GC allocations and will
be more independent from the rest of the engine, unblocking further
(performance) improvements.

- Arai decoupled <https://bugzilla.mozilla.org/show_bug.cgi?id=1641202>
Stencil structures more from the frontend data structures.
- Ted made <https://bugzilla.mozilla.org/show_bug.cgi?id=1614041> module
parsing work with Stencils and without allocating GC things.
- Ted deferred <https://bugzilla.mozilla.org/show_bug.cgi?id=1652472> GC
allocation of ScriptSourceObject.
- Ted cleaned <https://bugzilla.mozilla.org/show_bug.cgi?id=1653248> up
scope/environment handling.
- Arai added <https://bugzilla.mozilla.org/show_bug.cgi?id=1654149> a
dumpStencil(code) function to the JS shell to print the Stencil data
- Kannan landed <https://bugzilla.mozilla.org/show_bug.cgi?id=1651750>
more <https://bugzilla.mozilla.org/show_bug.cgi?id=1652176> changes
<https://bugzilla.mozilla.org/show_bug.cgi?id=1654037> preparing for the
switch to ParserAtom.

🐒 SmooshMonkey

SmooshMonkey <https://github.com/mozilla-spidermonkey/jsparagus> is our
project to reimplement the frontend in a safe language (Rust) and will make
it easier to implement new features and improve long-term maintainability
of the code base.

- Arai hooked up <https://bugzilla.mozilla.org/show_bug.cgi?id=1648574>
the output of the Rust frontend to Stencil’s function data structures.

🚀 WarpBuilder

WarpBuilder <https://bugzilla.mozilla.org/show_bug.cgi?id=1613592> is the
JIT project to replace the frontend of our optimizing JIT (IonBuilder) and
the engine’s Type Inference mechanism with a new MIR builder based on
compiling CacheIR to MIR. WarpBuilder will let us improve security,
performance, memory usage and maintainability of the whole engine. Since
the previous newsletter we’ve ported a lot of optimizations to CacheIR and

- André, Caroline, Iain, Jan and Tom ported optimizations
<https://bugzilla.mozilla.org/show_bug.cgi?id=1638111> for many builtins
to CacheIR and Warp.
- They also implemented
<https://bugzilla.mozilla.org/show_bug.cgi?id=1628222> a lot of CacheIR
instructions in the transpiler.
- Iain landed <https://bugzilla.mozilla.org/show_bug.cgi?id=1646378>
support for Trial Inlining of JS functions.
- Tom optimized <https://bugzilla.mozilla.org/show_bug.cgi?id=1657088>
code <https://bugzilla.mozilla.org/show_bug.cgi?id=1658786> generated
for for-of loops and adding
<https://bugzilla.mozilla.org/show_bug.cgi?id=1659133> new properties to
- Caroline fixed <https://bugzilla.mozilla.org/show_bug.cgi?id=1656552>
and improved <https://bugzilla.mozilla.org/show_bug.cgi?id=1657022> JSON
output for the CacheIR health analysis tool.
- Jan made IsPackedArray
<https://bugzilla.mozilla.org/show_bug.cgi?id=1651645>, MaybeInIteration
<https://bugzilla.mozilla.org/show_bug.cgi?id=1655451> and the arguments
analysis <https://bugzilla.mozilla.org/show_bug.cgi?id=1657303> work
without depending on IonBuilder and TI.

🧹 Miscellaneous changes

- Tom fixed <https://bugzilla.mozilla.org/show_bug.cgi?id=1559253>
syntax errors in regular expressions to use the correct source location.
- Jan added <https://bugzilla.mozilla.org/show_bug.cgi?id=1657559> a
cache for atomizing strings to fix
<https://bugzilla.mozilla.org/show_bug.cgi?id=1654087> a performance
cliff affecting Reddit.
- Yoshi landed <https://bugzilla.mozilla.org/show_bug.cgi?id=1649887>
more <https://bugzilla.mozilla.org/show_bug.cgi?id=1650393> changes
<https://bugzilla.mozilla.org/show_bug.cgi?id=1651944> for integrating
<https://bugzilla.mozilla.org/show_bug.cgi?id=1559660> SpiderMonkey’s
helper threads with the browser’s thread pool.
- Philip Chimento added
<https://bugzilla.mozilla.org/show_bug.cgi?id=1606568> public APIs for
working with BigInts.
- Evan Welsh added <https://bugzilla.mozilla.org/show_bug.cgi?id=1654696>
a public API for enabling code coverage.
- Kanishk fixed <https://bugzilla.mozilla.org/show_bug.cgi?id=1527839>
an over-recursion bug in ExtractLinearSum.
- Logan cleaned up <https://bugzilla.mozilla.org/show_bug.cgi?id=1647342>
Debugger frames code.
- Jan optimized <https://bugzilla.mozilla.org/show_bug.cgi?id=1657830>
and cleaned up some jump relocation and jump code on x64 and ARM64.
- Tom removed <https://bugzilla.mozilla.org/show_bug.cgi?id=1651445> some
CallNonGenericMethod calls for Number and Date functions. This improved
certain error messages.
- André converted <https://bugzilla.mozilla.org/show_bug.cgi?id=1651732>
code from ValueToId to ToPropertyKey to fix subtle correctness bugs.
- Jeff moved <https://bugzilla.mozilla.org/show_bug.cgi?id=1656411> more
<https://bugzilla.mozilla.org/show_bug.cgi?id=1654927> code
<https://bugzilla.mozilla.org/show_bug.cgi?id=1659885> out of
jsfriendapi.h into smaller headers.
- Barun renamed <https://bugzilla.mozilla.org/show_bug.cgi?id=1483269>
gc::AbortReason to GCAbortReason to fix a conflict with jit::AbortReason.


- The Cranelift
<https://github.com/bytecodealliance/wasmtime/tree/main/cranelift> team
landed <https://bugzilla.mozilla.org/show_bug.cgi?id=1655928> various
<https://bugzilla.mozilla.org/show_bug.cgi?id=1649928> changes
<https://bugzilla.mozilla.org/show_bug.cgi?id=1656638> to prepare for
enabling <https://bugzilla.mozilla.org/show_bug.cgi?id=1649932>
Cranelift by default for ARM64 platforms in Nightly. On ARM64 we currently
only have the Wasm Baseline compiler so it’s the first platform where
Cranelift will be rolled out.
- Ryan implemented <https://bugzilla.mozilla.org/show_bug.cgi?id=1561521>
non-nullable references.
- Lars landed <https://bugzilla.mozilla.org/show_bug.cgi?id=1640669>
optimizations for SIMD and implemented
<https://bugzilla.mozilla.org/show_bug.cgi?id=1656226> some experimental
Wasm SIMD opcodes.
- Lars added <https://bugzilla.mozilla.org/show_bug.cgi?id=1646663>
alignment of loop headers to the Ion backend.
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