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

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/ https://stackoverflow.com/a/50966903/ [1] https://bugs.webkit.org/show_bug.cgi?id=184023#c24 https://bugs.webkit.org/show_bug.cgi?id=184023#c24