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Latest Revision to ARM Instruction Set Includes Optimizations for JavaScript (daringfireball.net)
19 points by mariuz 5 months ago | hide | past | web | favorite | 7 comments




That seems a little hyperbolic. Jazelle DBX is an example of "optimized for X language".

This instruction disproportionately helps dynamic languages that convert between int and float (of which JS is definitely most popular), but basically every program that does casting from float to int stands to benefit. That's much better design in my opinion.


The insn is definitely js-specific though. The architecture has int-float conversion insns already which work fine for the purposes of almost all languages (dynamic or not); but js requires some specific oddball semantics which this new insn provides.

There are some details in this 2016 blogpost -- https://community.arm.com/processors/b/blog/posts/armv8-a-ar...


This "the new iPhone is insanely fast at Javascript" story needs to die. It has always been wrong conclusions drawn from the wrong benchmarks. Speedometer is a DOM benchmark with very little actual Javascript weight. DOM performance on mobile Safari is relatively good, considering that the DOM in general is a performance trainwreck. Apple has really done a good job optimizing this so that the "modern web" runs on their devices in an acceptable fashion. It probably has a lot to do with efficient GPU interfacing.

Furthermore, it is impossible to properly benchmark any JIT language on a non-jailbroken iOS device, because JIT is not allowed on iOS, only Apple's implementations like JavaScriptCore are available. It's pointless to make such benchmarks and draw conclusions about the hardware. Even if you benchmarked JavaScriptCore on different platforms, it wouldn't necessarily be fair, because you don't know how much more effort went e.g. into the ARM backend.


One an run a JIT on an iOS device, it is that the app will get denied on submission. So it would be possible to benchmark a JITed language on iOS. There are a ton of other ARM targets that could be used as well.


You can't make a page executable on iOS, which is an OS-level security measure that also rules out JIT compilation.

Separate from that, apps that download and run scripts may or may not be rejected based on App Store terms.


Why is this getting downvoted? If there's an inaccuracy here, please point it out.




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