mini_racer VS Duktape

Some years ago I was on a shitty job - not technically, but the company turned out to be inhumane - at a Ruby shop, and on the side I was toying with mini_racer and I just upgraded to some macOS beta where it failed to build. A shitty +1-1 hack† for a compiler flag later and it was back flying.

A month later I received a cold email from a CTO to chat a bit about that PR, turns out they were using mini_racer heavily and forked it for their own purpose, and also created PyMiniRacer for the Python side of things. Next thing I know I got hired. Two years later the company got acquired.

Of course conditionally adding a compiler flag wasn't what got me hired per se, it only got my profile noticed. Probably side projects such as porting go by example to Ruby by implementing a ~1:1 CSP channel API[1], an Electron desktop client for Mattermost basically on a dare[2], ex mode for the Atom editor so that I could have that frackin' `:w`[3], leveraging Blocks to bolt on object-oriented-ness onto C because "closures are a poor man's object"[4], or reverse-engineering the Xbox One USB gamepad and writing a kext to turn it into a HID device on macOS from scratch on a lonely 7+h train ride with passengers judgementally staring at me sideways[4] probably contributed to it a bit.

My takeaway: luck is when preparation meets opportunity; but don't to side projects to get hired, because if you don't get hired then that time is lost. Rather, of all things, scratch your itch, have fun, embrace whatever quirkiness you fancy; no one can take that away from you.

[0]: https://github.com/rubyjs/mini_racer/commit/2086db1bbf2b5de4...

[1]: https://github.com/lloeki/normandy

[2]: https://github.com/lloeki/matterfront

[3]: https://github.com/lloeki/ex-mode

[4]: https://github.com/lloeki/cblocks-clobj/blob/master/main.c

[5]: https://github.com/lloeki/xbox_one_controller

Cue libv8-node+mini_racer from which PyMiniRacer was born. It is non-trivial but not as hard as one might think.

The most painful part is the libv8 build system and Google tooling, which makes it an absolute PITA for libv8 consumers that are not Chrome.

This is why the libv8 gem was atrocious to keep up to date and to build for several platforms, and why libv8-node was born, because the node build system and source distribution are actually sane.

Disclaimer: worked at Sqreen, now maintainer of libv8-node and collaborator of mini_racer

https://github.com/sqreen/PyMiniRacer

https://github.com/rubyjs/mini_racer

https://github.com/rubyjs/libv8-node

Eventually, we’ll want to call this JavaScript from a Rails controller using ExecJS or MiniRacer or some similar tool. None of the Ruby-to-JavaScript gems I found were sophisticated enough to know how to load npm modules with common.js or ES module syntax, so my solution is to just build the JavaScript with a watcher and have that run as part of bin/dev.

https://github.com/rubyjs/mini_racer/issues/190 has some info an then this PR: https://github.com/rubyjs/mini_racer/pull/210 that has been merged. Hope that all helps!

Im my projects I search for single file libs.(like https://github.com/svaarala/duktape etc...)

Use lrand48(), or better, implement a high-quality RNG like PCG or splitmix64.

You can also refer to the Unicode routines of other small JS engines[1,2], those don’t use ICU either, although the implementations are mercilessly size-optimized (to put it politely) and restricted to what the target JS version requires (e.g. casemapping but no normalization).

[1] https://github.com/bellard/quickjs/blob/master/libunicode.c

[2] https://github.com/svaarala/duktape/blob/master/src-input/du...

> memcpy from bytes to a NaN should work fine

Signaling NaNs are explicitly undefined in C11 F.2.1.: "This specification does not define the behavior of signaling NaNs." - and in practice may be "quieted" by conversion to Quiet NaNs, changing their bit patterns. Fast math optimization flags will also break the hell out of your code by assuming NaNs are impossible. I want to say there are more circumstances where optimizers and compiler generated code can butcher your NaN payloads, but I'd be working off recollected hearsay and I can't find a source, so don't quote me on that.

NaN boxing is common enough that, if you take the right precautions, a modern compiler should probably support it, maybe. NaN boxing is uncommon enough that, if your codebase needs to be sufficiently portable, you need an opt out for when it breaks. Let's review duktape's scars:

https://github.com/svaarala/duktape/blob/123d9426d5e5b36d5da...

https://github.com/svaarala/duktape/blob/5252b7a50611a3cb8bf...

https://github.com/svaarala/duktape/blob/224a0b89ca08a36e37e...

Note that "the right precautions" involve unions and proper integer types to avoid optimizer-invoked rewrites of the value and debugging when things go wrong, not simply YOLOing bytes into a double via memcpy. Note that debugging when it all goes terribly wrong can be quite painful. I've personally had the misfortune of being forced to debug duktape being built with fast math optimizatoins enabled on one "rare" platform + build configuration that wasn't caught by duktape's #if defined(__FAST_MATH__) checks linked above (wasn't Clang nor GCC, so go figure it didn't make the same #define)

I was expecting this to be about Duktape <https://github.com/svaarala/duktape>, but heh, for sure no. I'd bet $1 there's no way youtube-dl would switch, but I wonder if yt-dlp would?

Fast math optimizations can break code like this by breaking isNaN.

I was porting a C++ project to a certain platform - and that platform enabled a -ffast-math equivalent by default in Release (but not Debug) builds! This broke duktape, a JS engine said project embedded, in some nasty and subtle ways. Instead of storing a number/pointer/??? (8 bytes) + type tag (4? bytes) for each dynamically typed JS value, duktape can bit-pack values into a single 8 byte "double" value by storing object/string handles as NaN values - this isn't an uncommon trick for dynamically typed scripting stuff:

https://github.com/svaarala/duktape/blob/c3722054ea4a4e50f48...

Naturally, the -ffast-math equivalent broke isNaN checks, which caused random object/string handles to be mistakenly reinterpreted as "numbers" - but only in Release builds, for this one particular platform, in one rarely taken branch, so neither QA nor CI caught it, leading to hours of manufacturing a repro case, stepping through an absurd amount of code, and then finally looking at the default build rules and facepalming.

Cursing the platform vendor under my breath, I overrode the defaults to align with the defaults of every other config x platform combination we already had: no fast math. If you want those optimizations, use SSE-friendly NaN-avoiding intrinsics - or, if you must use the compiler flags, ensure you do so consistently across build configs and platforms, perhaps limited to a few TUs or modules if possible. This allows you to have a chance at using your Debug builds to debug the resulting "optimizations".

Sure. For example, DukTape is an implementation of Javascript designed to be embedded in other projects. Google's V8 Javascript engine (used in Chrome), can also be embedded, see Node.Js for example.

- Duktape (4.8k stars)