I see that JIT isn't supported for the Sparc targets for example.
What is the scope of the change to add JIT support.
I see, for example, that for X86 target class X86JITInfo is defined with 10 methods. It's only used in one place: setPICBase. Also createX86JITCodeEmitterPass creates JIT pass -- very simple.
So is implementing these steps enough to add JIT, or there are some hidden difficulties that aren't obvious?
The biggest piece is adding hooks for instruction encoding. Sparc should be mostly autogenerated from the .td files like the PPC backend, so take a look at lib/Target/PowerPC/PPCCodeEmitter.cpp.
However, in the mid-term, we're planning on eliminating the JIT target interfaces as they are now and replacing them with an MC implementation. If you're interested in getting the JIT up, I'd strongly suggest working on converting the sparc backend to start using the MC interfaces (e.g. switch SparcAsmPrinter.cpp to print instructions by lowering them to MCInst, then add a new SparcMCInstPrinter interface). There is some information about MC here: http://blog.llvm.org/2010/04/intro-to-llvm-mc-project.html and I'm happy to answer any questions about it.