[LLVMdev] New JIT APIs

[Lang Hames]

Hi Dave,

To confirm - I have no plans to remove MCJIT. I don't want to change any
>> behavior for existing clients. The new stuff is opt-in.
>>
>
> Why not? We did work to remove the legacy JIT in favor of MCJIT for the
> usual reasons (less code/maintenance burden/etc) - it'd seem unfortunate to
> then go back to maintaining two JITs again.
>
> You mention the intent to provide a superset of MCJIT's behavior, at which
> point it seems it'd be preferable to kill of MCJIT in favor of ORC (heck,
> we killed of the legacy JIT before MCJIT had feature parity).
>
>

Not having plans at the moment doesn't preclude making plans in the future,
it's just premature to think about replacing MCJIT when the "replacement"
hasn't even been submitted to llvm-commits yet. :)

The bar for transitioning is higher now, since MCJIT has more substantial
clients than the legacy JIT had. The impetus for transitioning is also
lower: The legacy JIT required a lot of custom infrastructure to be kept
around. MCJIT is much more lightweight, and shares most of its foundation
(RuntimeDyld) with Orc.

If MCJITReplacement reaches full feature and performance parity with MCJIT
(which I do actually want to see), and the transition can be done either
transparently (by having ExecutionEngineBuilder return an MCJITReplacement
instead of an MCJIT), or in a manual way that all clients are happy to buy
into, then I'd be ok with deprecating and eventually removing MCJIT. That's
a discussion for the future though.

So clients should rest easy: We just went through a difficult transition
from the legacy JIT, and I don't want to put you through that again any
time soon.

- Lang.
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