I propose adding the following global USE flag:

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


Current local flags that could probably be unified:

app-arch/libzpaq:jit - Enable just-in-time compilation for faster
compression (requires SSE2)

dev-libs/libpcre:jit - Enable Just-In-Time compilation of regexp
bytecode to machine code, through the SLJIT compiler. This feature might
conflict wtih security mitigation strategies such as NX/PaX as enabled
by Gentoo Hardened.

dev-python/pypy:jit - Enable the JIT compiler

dev-scheme/racket:jit - Enable just-in-time compiler

media-sound/csound:luajit - Use the lua just-in-time compiler
dev-lang/luajit instead of dev-lang/lua

net-libs/webkit-gtk:jit - Enable JIT javascript compiler (disabling it
will cause performance penalty)

www-client/epiphany:jit - Allow using net-libs/webkit-gtk that has the
JIT javascript compiler enabled

www-client/luakit:luajit - Use the lua just-in-time compiler
dev-lang/luajit instead of dev-lang/lua, which should make luakit
faster.

www-client/seamonkey:methodjit - Enable JIT for JavaScript using
MethodJIT for faster JS performance. Hardened users can disable this
USE-flag to use MPROTECT on grsecurity kernels.

www-servers/nginx:pcre-jit - Enable JIT for pcre

x11-libs/qt-core:jit - Enables JIT for Javascript usage inside Qt

x11-libs/qt-script:jit - Enables JIT for Javascript usage inside Qt

x11-libs/qt-webkit:jit - Enable JavaScriptCore just-in-time compiler for
faster JavaScript execution

-Alexandre.