The LLVM-based fourth tier is called FTL, for Fourth Tier LLVM
(and, of course, faster than light). It shares some portions
with the third stage, since the third stage already does important
code generating portion.
The result is a healthy performance boost. FTL produces code that
is more than 40 times faster than the interpreter, with
benchmarks taking about a third less time to run than the old
three tier system.
It’s pretty clever: the new FTL compiler takes longer to compile the code, but the code it generates runs much faster. But while the FTL compiler is working, WebKit will execute code generated by the (faster-to-compile) existing JIT compiler, then switch over to the FTL-generated code when it’s ready.