Re: Status of "Improved LLVM backend"

Hi, I’m trying to implement a bitcode producing llvm backend[1], which would potentially allow to use a range of llvm versions with ghc. However, this is only tangentially relevant to the improved llvm backend, as Austin correctly pointed out[2], as there are other complications besides the fragility of the textual representation. So this is mostly only relevant to the improved ir you mentioned. The bitcode code gen plugin right now follows mostly the textual ir generation, but tries to prevent the ubiquitous symbol to i8* casting. The llvm gen turns cmm into ir, at this point however at that point, the wordsize has been embedded already, which means that the current textual llvm gen as well as the bitcode llvm gen try to figure out if relative access is in multiple wordsizes to use llvms getElementPointer. I don’t know if generating llvm from stg instead of cmm would be a better approach, which is what ghcjs and eta do as far as I know. Cheers, moritz — [1]: https://github.com/ghc-proposals/ghc-proposals/pull/25 [2]: https://github.com/ghc-proposals/ghc-proposals/pull/25#issuecomment-26169718...

On Nov 27, 2016, at 3:14 AM, Michal Terepeta wrote:

Hi all,

I was wondering what’s the current status of the “Improved LLVM backend” project ( https://ghc.haskell.org/trac/ghc/wiki/ImprovedLLVMBackend ). The page mentions a few main problems, but some seem to be already fixed: 1) Using/supporting only one version of LLVM. This has been done AFAIK. 2) Prebuilt binaries to be shipped together with GHC. I can't find anything about this. Is there a ticket? Has there been any progress on this? 3) Adding support for split-objs I found a ticket about it: https://ghc.haskell.org/trac/ghc/ticket/8300 which was closed as WONTFIX in favor of split-sections. So I guess this can also be considered as done. 4) Figuring out what LLVM optimizations are useful. Again, I can seem to find anything here. Has anyone looked at this? I only found an issue about this: https://ghc.haskell.org/trac/ghc/ticket/11295

The page also mentions that the generated IR could be improved in many cases, but it doesn't link to any tickets or discussions. Is there something I could read to better understand what are the main problems? The only thing I can recall is that proc point splitting is likely to cause issues for LLVM's ability to optimize the code. (but I only found a couple of email threads about this but couldn't find any follow-ups)

Thanks, Michal

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