[LLVMdev] New JIT APIs

[Philip Reames]

On 01/14/2015 02:56 PM, David Blaikie wrote:
>
>
> On Wed, Jan 14, 2015 at 2:46 PM, Philip Reames 
> <listmail at philipreames.com <mailto: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
>>     <mailto: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.
>
>
> Not necessarily - It could simply be the stated plan (& he has stated 
> it) to reach feature parity. At that point it seems it'd be hard to 
> justify keeping both around when one has a superset of features of the 
> other.
>
>       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.
>
>
> Not sure I follow quite why the old JIT retirement was a blocker, but 
> introducing a new JIT API with the intention to maintain both would 
> block less work. Could you describe this in more detail?
I think Lang's response covered all the relevant points and we're far 
off topic at this point.  Let me know if there was something you think 
Lang's comments didn't address that you'd like me to.
>
> - David
>
>
>>
>>     - 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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