[llvm-dev] ORC and MCJIT clients: Heads up, API breaking changes in the pipeline.

[Lang Hames via llvm-dev]

Hi All,

Stage 1 landed just after I sent this and it looks like there was minimal
fallout from that for MCJIT users, but it broke common symbol support in
the ORC Lazy JIT. To fix this I've replaced RuntimeDyld::SymbolInfo with
the lazily-materializing JITSymbol (which I've moved from ORC to
ExecutionEngine) in r277386. Most clients won't see any breakage due to
this (neither Swift nor LLDB required updates), but ORC users will need to
rename RuntimeDyld::SymbolInfo to JITSymbol to get their code compiling
again. My apologies for the breakage, but this should be a big improvement
in the long run: it will allow me to fix common symbol support, and should
also make it possible to support weak symbols correctly.

The changes I mentioned in stages 2 and 3 in my original email are still in
the works, but I don't have schedule for them yet.

Cheers,
Lang.



On Fri, May 27, 2016 at 4:02 AM, Stefan Gränitz <stefan.graenitz at gmail.com>
wrote:

> Hi Lang, thanks for announcing. Would be great if you could send another
> short notice as soon as the actual patch exists.
> Best, Stefan
> Am 24.05.16 um 23:06 schrieb Lang Hames via llvm-dev:
>
> Hi All,
>
> I'm going to be making some API breaking changes to the ORC APIs, and to
> the RuntimeDyld class (which underlies MCJIT). The changes may affect MCJIT
> clients but are unlikely to. Where they do the fixes are likely to be
> trivial. ORC clients will be affected, but the fixes should also be
> straightforward.
>
> I have three upcoming changes in mind:
>
> 1) RuntimeDyld (the linker underlying MCJIT and ORC) will now search for
> symbols using the SymbolResolver's findSymbolInLogicalDylib first, before
> falling back to the findSymbol method if findSymbolInLogicalDylib returns a
> null result.
>
> This is a step towards making RuntimeDyld behave more like a static
> linker: we prefer to link against symbols defined in the same "logical
> dylib" as the module being JIT'd, before linking against external symbols.
>
> For clients that have not implemented findSymbolInLogicalDylib (I suspect
> this covers most clients) this change will have no effect: the default
> version of findSymbolInLogicalDylib will be called and will return null,
> and RuntimeDyld will fall back to calling findSymbol as it did before.
>
> For clients that have implemented findSymbolInLogicalDylib: Beware that
> any symbol found via this call will now shadow symbols that might have been
> found via findSymbol. It is unlikely that anyone is relying on
> non-shadowing so I don't expect any trouble here, but if you do see any
> changes in symbol resolution after this change (which I expect to land
> today) this is something to have in mind.
>
> 2) MemoryManager deregisterEHFrames will be removed.
>
> Memory managers should own their registered EH frames and deregister them
> on destruction. This will bring EH frame ownership in line with memory
> ownership: memory managers are already responsible for freeing the memory
> that they've allocated when they are destructed.
>
> To transition, clients should track the frames registered by their class,
> then deregister them in the destructor. For in-tree memory managers (and
> RuntimeDyld itself) this bug is being tracked by https://llvm.org/PR23991
> .
>
> 3) Some operations in the ORC APIs, RuntimeDyld, SymbolResolver and
> MemoryManager will be modified to return Error/Expected.
>
> MCJIT and ORC both support remote execution of JIT'd code via RPC, but
> there is currently no way to cleanly bail out and report failure in the
> case of an RPC error. Adding error returns provides the mechanism we need.
>
> When the interfaces are changed to add error returns clients will see
> compile-time errors for any custom ORC-based stacks, derived symbol
> resolvers, or memory managers. Most clients will just be able to update
> their offending return types and leave their function body as-is (for
> Expected<T> returns) or return llvm::Error::success() (for Error returns).
> Clients who are actually using the remote-JITing features will now be able
> to return any errors that they had previously been dropping or bailing out
> on. This change is being tracked by https://llvm.org/PR22612 .
>
> I've held off making these changes for a while, because I know there are a
> lot of out-of-tree JIT clients and I want to avoid breaking your code. I
> think this churn is worth the pain though - the resulting APIs will have
> less historical baggage, be more error-proof, and enable some new features.
>
> Please let me know if you have any questions or run into any trouble as
> these changes land - I'll be happy to help out.
>
> Cheers,
> Lang.
>
>
> _______________________________________________
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>
>
> -- https://about.me/stefan.graenitz
>
>
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