> "There are several theories as to why the M1 does so well on on JavaScript benchmarks."

A major reason it does so well is the native Javascript support added in ARMv8.3-A.

Specifically, FJCVTZS (Floating-point Javascript Convert to Signed fixed-point, rounding toward Zero)[1] which is a Javascript-specific variant of FCVTZS implementing the overflow and exception handling behaviour that Javascript wants.

Javascript needs to do these conversions a lot, since it doesn't have integer types, and it's much faster to have it implemented in silicon rather than using the old instructions and having to handle the overflow/error checking.

[1] https://developer.arm.com/documentation/100076/0100/a64-inst...