Re: [Haskell-cafe] Clearing up the status of GHCi on Raspberry Pi

Thanks so much for the detailed response. When I have a moment I will try to come up with some appropriate edits to the Wikis, I will run them by you first to make sure they're accurate. And even moreso, thanks for your work in this area. When is 7.8 looking like it's coming out? And, how hard would it be to install it onto my Pi? Would I have to compile it? I'm assuming it won't make it into the Jesse repository (or will it?) -Dan On Mon, Dec 16, 2013 at 8:22 AM, Ben Gamari wrote:

Dan Krol writes:

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I figure we should sort this out, I'd be happy to edit the wikis myself but I want to make sure I understand what's going on. I understand this probably comes down to some nuances that are not expressed here. For user friendliness of the language and ecosystem I think such things should be made clear. As somebody coming to this wanting to accomplish something, I want a clear answer on what is available to me. It's been a source of frustration for me.

Could somebody clear all this up for me? And while I'm at it, I would be grateful if somebody could explain to me how it is possible (if at all) to have ghci on my Raspberry Pi, short of compiling it myself. I'm not against compiling ghc, but I am against compiling it on my Raspberry Pi. Qemu I will consider, though.

The situation is a bit complicated and I've been pretty poor at keeping the existing documentation up-to-date. ARM support has in principle existed in the tree through the LLVM code generator for some time. The code generator itself is in my experience quite robust.

There are, however, a number of details in the runtime system which break GHCi. One of these is the runtime linker which until recently had effectively no support for ARM. I worked some initial ARM support in to 7.6.1 (b22501b408ddb0503a06a188b06d9cff9be697cd) and while things largely worked at the time, there were still some rough edges. For this reason, 7.6 can't really considered to support GHCi on ARM. Unfortunately at this point I became quite busy and didn't have time to look into the remaining issues. This was in late 2011.

In the last few weeks I've had time to have another look at this problem. It turns out one of the issues (lack of jump code, documented in bug #8380) was quite straightforward to fix (up to some cache coherency issues which I believe thoughtpolice has now sorted out, see 5bab1a57f572e29dfdffd6d1ce8e53a2772b18fd). Unfortunately after fixing this I found that there was still occassional crashes during the build process. I spent a fair bit of time poking around looking for the root cause but have still come up with no compelling leads. It's very likely that the culprit is the runtime linker, although I haven't found a way to narrow things down any further.

Frankly, implementing a runtime linker is non-trivial business and in my opinion the limited man-hours working on GHC are better spent elsewhere. Having our own runtime linker has its advantages, but for an architecture that is currently *barely* supported, it makes more sense to punt as much of this responsibility to other parties as possible. For this reason I think it would be wise to focus on moving this functionality to the system's runtime linker by using dynamic linking.

Dynamic linking has been working for some time now on x86 with the native code generator. Unfortunately, there have been rumors that things are broken when the LLVM code generator is used (which is the only option on ARM). I started looking into this late last week and believe I have the problem identified (thanks to help from Peter Wortmann, see [1]) and have something of a solution. At this point I'm running into build system issues[2] which prevent me from verifying my hacked work-around^H^H^Hsolution.

Assuming that I can validate the fix, I'm hoping there's a chance it (or something like it) can make it in to 7.8. As far as I can tell, this is the last major impediment to have GHC working well on ARM. Moreover, by switching to dynamic linking on ARM we will have eliminated a major source of trouble from the equation. This would mean that 7.8 would finally have (hopefully) robust support on ARM.

Cheers,

- Ben

[1] http://www.haskell.org/pipermail/ghc-devs/2013-December/003484.html [2] http://www.haskell.org/pipermail/ghc-devs/2013-December/003488.html