GHC status report
Simon Marlow
marlowsd at gmail.com
Fri May 2 13:47:39 UTC 2014
On 02/05/2014 14:28, Edward Kmett wrote:
> Perhaps. We actually tried that originally, but had issues about
> where and how to get cabal to place it. We'd need it to go somewhere
> installed rather than the local build dir lest it not be there when
> we go to use the lib, but IIRC, cabal couldn't/wouldn't tell me where
> it was putting the final installed version and then there is the
> issue of the local in place runs vs. post cabal install runs and
> referencing that dir from subsequent projects. If we want a
> transparent ’cabal install rounded` It isn't clear to me how to get
> there down this path, but it is entirely possible I just missed
> something obvious.
>
> MPFR/rounded was just an off the cuff example. Another place where
> the dynamic linker really helps is external c++ libraries which
> should now actually get all of their initializers called in the right
> order when launched from ghci.
External C++ libraries work just fine with the statically linked GHCi -
it uses the system linker to load them, and all the initalizers get
called, in the right order, as they should. So this is where I'm
confused about what the problems actually are - there seems to be this
perception that GHCi didn't work with external C++ code, but as far as I
know it works just fine (indeed we're doing it a lot at Facebook, so
there's at least an existence proof that depending on a non-trivial
amount of external C++ with a statically-linked GHCi can work).
Cheers,
Simon
>
>
>> Cheers,
>> Simon
>>
>>> Switching to the system dynamic linker fo ghci seems to have resolved
>>> all of that effortlessly.
>>>
>>> Dan Peebles has been talking to the MPFR folks to see if we can get them
>>> to expose enough information about the 'hidden' allocations they use
>>> that we can make them visible to GHC or have them do what our local fix
>>> does and avoid using the MPFR allocator for their hidden constant cache.
>>>
>>> If they do that then we can actually link to the library like normal
>>> rather than link it in directly, but it isn't clear to me what would
>>> happen even with those hooks if we rolled back to something like the old
>>> custom linker.
>>>
>>> -Edward
>>>
>>>
>>> On Thu, May 1, 2014 at 5:30 PM, Simon Marlow <marlowsd at gmail.com
>>> <mailto:marlowsd at gmail.com>> wrote:
>>>
>>> On 01/05/14 15:27, Edward Kmett wrote:
>>>
>>> Figured I'd make one case for dynamic linking:
>>>
>>> https://github.com/ekmett/__rounded
>>> <https://github.com/ekmett/rounded>
>>>
>>> Dynamic linking is finally enabling us to build a version of MPFR
>>> bindings for Haskell for scientific/high precision computing
>>> with 7.8. I
>>> would really hate to lose it after all of these years trying to
>>> get it
>>> work, as I have a rather large edifice being built atop that
>>> platform.
>>> We tried and failed due to limitations of the old linker for
>>> almost 3 years.
>>>
>>>
>>> I understand the issues with MPFR. But how is dynamic linking helping?
>>>
>>>
>>> That said, -dynamic-too seems to cause me all sorts of problems
>>> elsewhere. ^C'ing out of a build and restarting it will often
>>> make a .o
>>> but lose the .dyn_o, leading to GHC + cabal getting confused and
>>> refusing to build until I clean. This hits me several times a day.
>>>
>>>
>>> We should fix this (or at least make it a lot less likely). Is
>>> there a ticket?
>>>
>>> Cheers,
>>> Simon
>>>
>>>
>>> -Edward
>>>
>>>
>>> On Thu, May 1, 2014 at 3:29 AM, Simon Peyton Jones
>>> <simonpj at microsoft.com <mailto:simonpj at microsoft.com>
>>> <mailto:simonpj at microsoft.com <mailto:simonpj at microsoft.com>>__>
>>> wrote:
>>>
>>> | Dynamic linking has been a huge headache in GHC, and it's not
>>> clear that
>>> | it's an overall improvement compared with the static
>>> linker. Now that
>>> | 7.8 is out of the way, it's time to have a conversation about
>>> whether we
>>> | want to do dynamic linking again for 7.10, or revert to
>>> static
>>> linking.
>>>
>>> I echo this. Dynamic linking has had many un-anticipated
>>> costs and
>>> it is still very far from sorted out. It originally felt
>>> like a
>>> Fantastic Idea to give up our own linker and adopt the system
>>> linker, but it now feels to me like a black hole, endlessly
>>> sucking
>>> effort and increasing complexity.
>>>
>>> My viewpoint is highly un-informed about details; I just
>>> watch the
>>> traffic going by. And of course it does have benefits that
>>> doubtless generate less traffic.
>>>
>>> Simon
>>>
>>> |
>>> |
>>> |
>>> | >
>>> | > On Tue, Apr 29, 2014 at 6:13 PM, Simon Peyton Jones
>>> | > <simonpj at microsoft.com <mailto:simonpj at microsoft.com>
>>> <mailto:simonpj at microsoft.com <mailto:simonpj at microsoft.com>>
>>> <mailto:simonpj at microsoft.com
>>> <mailto:simonpj at microsoft.com> <mailto:simonpj at microsoft.com
>>> <mailto:simonpj at microsoft.com>>__>> wrote:
>>> | >
>>> | > As Austin has told us, there's a draft of the *GHC
>>> Status Report
>>> | for
>>> | > the HCAR*, here:____
>>> | >
>>> | >
>>> https://ghc.haskell.org/trac/__ghc/wiki/Status/May14____
>>> <https://ghc.haskell.org/trac/ghc/wiki/Status/May14____>
>>> | >
>>> | > Have we missed out something you have been working
>>> hard on? Do
>>> | > take a moment to add a bullet in an appropriate
>>> place (it's a
>>> | > wiki). I'd like to be sure that we are giving
>>> credit to all the
>>> | > appropriate people, so please help us fix that too.
>>> GHC is
>>> a team
>>> | > effort.____
>>> | >
>>> | > Deadline is 1 May I think.____
>>> | >
>>> | > Thanks____
>>> | >
>>> | > Simon____
>>> | >
>>> | > __ __
>>> | >
>>> | >
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