They might be referring to the ARM FJCVTZS instruction[0].

It speeds up converting JavaScript doubles to 32-bit integers.

[1] is the Webkit commit that added support for FJCVTZS, it was between a 0.5 to 2% speedup - that's a big win for a mature optimizer.

[0] https://stackoverflow.com/a/50966903/

[1] https://bugs.webkit.org/show_bug.cgi?id=184023#c24

In addition to FJCVTZS instruction, they added more floating point execution units and registers. Also did lower latency of pipelined fp instructions