[LLVMdev] New JIT APIs

[Lang Hames]

> p.s. I don't think we know what the "right" interface is for the JIT yet.

Orc was actually motivated in part by this. Having compassable components
makes it much easier for clients to experiment with different JIT API
designs, as well as new features. Hopefully any helpful lessons learned can
be fed back into the tree.

Again - keeping the existing MCJIT API is a fundamental requirement for the
foreseeable future*.

- Lang.

* The caveat, which I mentioned in a previous message, is that I'm happy to
discuss replacing the *implementation* of MCJIT with MCJITReplacement
if/when clients are satisfied that the latter provides full feature and
performance parity and compatible behavior. Such a discussion is premature
though.

On Wed, Jan 14, 2015 at 2:46 PM, Philip Reames <listmail at philipreames.com>
wrote:

>
> On 01/14/2015 02:33 PM, David Blaikie wrote:
>
>
>
> On Wed, Jan 14, 2015 at 2:22 PM, Lang Hames <lhames at gmail.com> wrote:
>
>> 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. :)
>>
>
> Not necessarily - it doesn't seem unreasonable to make a plan to ensure we
> don't end up with duplicate functionality to debug/test/fix indefinitely
> before adding the duplicate. Seems to be common in the project to make
> replacements, introduce them as an alternative but with an explicit
> goal/plan from the start that this is not a perpetual state. (for example,
> Chandler's pass manager work and I think most of the bits that Chandler's
> rewritten (shuffling, inlining, etc) were this way - maybe there are
> counterexamples where similar/duplicate functionality was introduced
> without such a goal, but none come to my mind)
>
> But I dunno, maybe other people find that to be an OK state of affairs,
> I'm not a code owner/authority in much/any of this.
>
> As a user of the JIT, I am *very* strongly in favour of Lang's espoused
> position.
>
> p.s. I don't think we know what the "right" interface is for the JIT yet.
> Until we do, having multiple interfaces (with a single shared
> implementation, based on the rest of LLVM) seems entirely reasonable and
> appropriate.
>
> p.p.s. If Lang was proposing the replacement of MCJIT - he's not! - the
> review barrier would be far far higher.  He'd have to satisfy all existing
> - well, all vocal - users of the old interface that his new one met their
> needs.  This would be a much slower process and we want to let things
> evolve more quickly than that.  We *don't* want to be looking at an old-JIT
> retirement v2.  That took literally years and blocked a lot of useful work
> on the JIT infrastructure.
>
>
> - David
>
>
>>   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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