Global USE flag

Enable just-in-time compilation for improved performance. May prevent use of some PaX memory protection features in Gentoo Hardened.

Packages describing “jit” as local USE flag

Package “jit” Flag Description
dev-lang/moarvm Enable Just-In-Time-Compiler. Has no effect except on AMD64 and Darwin.
net-im/prosody Use LuaJIT instead of vanilla Lua
sys-devel/gcc Enable libgccjit so other applications can embed gcc for Just-In-Time compilation. This will slow down the compiler a bit as it forces all of the toolchain to be shared libs.
www-apache/mod_security Add support for the PCRE Just-in-Time optimisation, as enabled by dev-libs/libpcre with jit USE flag enabled. Might not be available on hardened systems.
www-apps/cgit If the lua flag is enabled, jit uses LuaJIT instead of vanilla Lua