There’s no instructions for GC’d languages. That was the old Jazelle ARM extension (which could microcode some of java’s bytecode for direct execution).

The “javascript instruction” is FJCVTZS, which is a rounding mode matching x86 semantics, which is incidentally what JS specifies for double -> int32 conversions, and soft-coding it on top of FCVTZS it is rather expensive (it requires a dozen additional instructions to fix up edge cases).

This is beneficial to javascript (on the order of a percentage point on some benchmarks suites, however pure javascript crypto can get high double digits gains), but it’s also beneficial for any replication of x86 rounding on ARM, including but not limited to emulating x86 on arm (aka Rosetta 2).