[LLVMdev] Out-of-tree passes (Was: LLVM Weekly - #43, Oct 27th 2014)

Philip Reames listmail at philipreames.com
Mon Oct 27 09:41:25 PDT 2014

On 10/27/2014 08:46 AM, David Blaikie wrote:
> On Mon, Oct 27, 2014 at 2:59 AM, David Chisnall 
> <David.Chisnall at cl.cam.ac.uk <mailto:David.Chisnall at cl.cam.ac.uk>> wrote:
>     On 27 Oct 2014, at 09:33, Alex Bradbury <asb at asbradbury.org
>     <mailto:asb at asbradbury.org>> wrote:
>     > The Haskell community have put together a [proposal for an
>     improved LLVM
>     > backend to
>     GHC](https://ghc.haskell.org/trac/ghc/wiki/ImprovedLLVMBackend).
>     > They intend to ship GHC with its own local LLVM build.
>     This post brings up an interesting point:
>     > However, the framework is modular - we can extend LLVM with
>     plugins. For example, several years ago, Max Bolingbroke ​wrote a
>     plugin for LLVM's alias analysis that improved the generated code
>     in some cases by 12%, just by teaching it GHC-specific code
>     generation needs.
>     >
>     > However, due to lack of API guarantees mentioned above, it
>     becomes difficult to support such analysis for arbitrary end
>     users, and we cannot fix or tune analysis results to specific
>     versions of LLVM or GHC.
>     This is a problem for anyone with an out-of-tree LLVM front end,
>     or library, that would benefit from some custom optimisations. 
>     Without even a nod towards API (let alone ABI) stability for the
>     core IR classes, it's very hard for people to gain the full
>     benefit of using LLVM, unless their code is part of the LLVM tree
>     and follows the same release cycle as LLVM (which doesn't scale).
> I don't know that many of the major contributors follow the LLVM 
> release cycle, fwiw - one of the reasons we all care about stability 
> on ToT so very much.
+1 on this.  I use Clang on the release schedule, but our LLVM work 
tracks TOT.  IMHO, trying to do anything else for an embedded compiler 
in a VM is pure folly and will lead to worlds of pain.
> (& at least Apple likely has lots of fun internal toys and they manage 
> to follow ToT pretty closely, not sure about other major contributors 
> - kind of the nature of many groups who keep their work out of ToT is 
> that they're not involved in the community)
> - David
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