Re: GHC status report

On 02/05/2014 01:09, Dominick Samperi wrote:

If I understand your last comment correctly linking to libR should continue to work, even if you revert to static linking of Haskell compiled code via RTS linker. Since you say this is the way things have always been, perhaps the real explanation for why 7.8 fixed the issue is that some bug was fixed along the way...

Indeed. To know for sure we would have to test 7.8 with DYNAMIC_GHC_PROGRAMS=NO with your setup - is there a way to do that? Cheers, Simon

Note that R is a C library, so the C++ issues that Carter mentions are not a factor here.

Thanks, Dominick

On Thu, May 1, 2014 at 5:27 PM, Simon Marlow wrote:

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On 01/05/14 14:48, Dominick Samperi wrote:

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The problem with some graphics libraries used via FFI (and other libraries that are not thread-safe), if I understand the situation correctly, is that ghci forks a thread when it shouldn't, causing some programs to miscalculate the available stack space (because they think there is only one thread).

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The new dynamic linking support and the flag -fno-ghci-sandbox fixes this problem, and I would not vote for the removal of these features.

So I understand how -fno-ghci-sandbox avoids problems with GUI libraries that use thread-local state. But how is dynamic linking involved here? What improved in GHC 7.8 relative to 7.6 for you? And could you clarify "miscalculate the available stack space"? What needs to calculate stack space? Why? C stack space?

We can certainly make a smoother experience around -fno-ghci-sandbox for using GUI libraries.

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It is not clear to me from Simon's original post how linking to all of those C++ libs can continue to work if dynamic linking is removed in 7.10? Perhaps I misunderstand what you mean by "revert to static linking"?

When I say "revert to static linking" I mean make GHCi static linked, and have it load Haskell code compiled for static linking using the RTS linker (like it did in 7.6). Foreign libraries would still be loaded using the system linker, as they always have been.

To be clear, I'm not officially proposing that we drop dynamic linking, but I think it's worthwhile exploring the design space again, given that we know dynamic linking has been tougher than we expected.

Cheers, Simon

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Thanks, Dominick

On Wed, Apr 30, 2014 at 9:26 PM, George Colpitts wrote:

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To elaborate, in the past, I had a lot of problems using libraries from the ghci prompt on the Mac but I haven't tried recently.

As an example, on the web page for the book the Haskell School of Expression it says:

Note for OS X users: running graphics applications from GHCi is no longer supported. Instead, one has to compile a graphics program using GHC in order to run it (see example/GMIExamples.lhs for an example).

I had similar problems using the Yale Euterpea music program from ghci. When I inquired I was referred to https://ghc.haskell.org/trac/ghc/ticket/4244 and https://ghc.haskell.org/trac/ghc/ticket/781. I see that the latter is now scheduled for 7.10.1

On Wed, Apr 30, 2014 at 1:45 PM, Simon Marlow wrote:

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On 30/04/2014 01:35, George Colpitts wrote:

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It doesn't have anything about the dynamic linking changes made for 7.8. I think it's worth mentioning the improvements we expect to get from that. The highlights of the release notes do mention it, so maybe that suffices.

In particular, I'm hoping that it is going to fix a lot of problems with using foreign libraries such as OpenGL from ghci. I could be wrong about that though.

I'd like to understand more about what those problems are. As a data point, at Facebook we're using static linking (I compiled GHC with DYNAMIC_GHC_PROGRAMS=NO), we're loading upwards of 50 3rd-party C++ libraries and one gigantic shared library consisting of a ton of in-house C++ code, together with all our Haskell code into GHCi, and it works perfectly. The key to using the static linker is to not use it for C++ code - you want all your external C++ code in shared libraries and load those using the system linker.

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 think Austin is going to update https://ghc.haskell.org/trac/ghc/wiki/DynamicGhcPrograms, and then we'll see where we stand.

Cheers, Simon

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On Tue, Apr 29, 2014 at 6:13 PM, Simon Peyton Jones mailto:simonpj@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____

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