GHC status report

Thu May 1 23:46:17 UTC 2014

With the old custom linker we weren't able to get our custom MPFR linked in
properly on all platforms for use in ghci.

On Macs we ran into some rather interesting problems. We could get it to
work for actual executables, but ghci would segfault with stuff resolved to
clearly wrong addresses. If I recall correctly it may have been some kind
of MachO symbol type that wasn't being resolved properly by the custom GHC
linker, perhaps? We chased after it off and on for a long time to no avail.

It was complicated a great deal by having to build their library and merge
it directly into ours, because we couldn't use system MPFR, due to the
issues with the GHC garbage collector hooks into GMP.

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> wrote:

> On 01/05/14 15:27, Edward Kmett wrote:
>
>> Figured I'd make one case for dynamic linking:
>>
>> 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>> 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>>> 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____
>>     | >
>>     | >     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____
>>     | >
>>     | >     __ __
>>     | >
>>     | >
>>     | >     _______________________________________________
>>     | >     ghc-devs mailing list
>>     | > ghc-devs at haskell.org <mailto:ghc-devs at haskell.org>
>>     <mailto:ghc-devs at haskell.org <mailto:ghc-devs at haskell.org>>
>>
>>     | > http://www.haskell.org/mailman/listinfo/ghc-devs
>>     | >
>>     | >
>>     | >
>>     | >
>>     | > _______________________________________________
>>     | > ghc-devs mailing list
>>     | > ghc-devs at haskell.org <mailto:ghc-devs at haskell.org>
>>     | > http://www.haskell.org/mailman/listinfo/ghc-devs
>>     | >
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>>
>>
>
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