[llvm-dev] MCJIT versus Orc

[Russell Wallace via llvm-dev]

When writing a JIT compiler using LLVM, as I understand it, you can use two alternative APIs, MCJIT and Orc. The latter offers lazy compilation. Would it be accurate to say that if you want eager compilation - always compile an entire module upfront - you should use MCJIT?

[Justin Bogner via llvm-dev]

+lang.

My understanding is that Orc is strictly more flexible than MCJIT. You
can, in fact, exactly implement the MCJIT API using the Orc APIs.

I think the general advice is that MCJIT's a bit more mature and stable,
but you probably want Orc in new code unless you really need that
maturity.
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[Russell Wallace via llvm-dev]

Right, but is there any known use case where Orc's flexibility allows something to be done that couldn't be with MCJIT, apart from lazy compilation?

[Lang Hames via llvm-dev]

Hi Russell,

Apologies for the delayed reply.

Orc's current advantages over MCJIT are:

(1) Built-in support for lazy compilation.

(2) Cleaner support for multi-module management, including un-loading modules.

(3) Much better memory management (depending on use-case, can consume far less memory for JIT'd code).

I expect Orc to gain further advantages over MCJIT in the future too.

I'd recommend using Orc rather than MCJIT.

- Lang.