February 2019 - ghc-devs

The future of Phabricator
by Ben Gamari 18 Sep '21

18 Sep '21

TL;DR. For several reasons I think we should consider alternatives to Phabricator. My view is that GitLab seems like the best option. Hello everyone, Over the past year I have been growing increasingly weary of our continued dependence on Phabricator. Without a doubt, its code review interface is the best I have used. However, for a myriad of reasons I am recently questioning whether it is still the best tool for GHC's needs. # The problem There are a number of reasons why I am currently uncertain about Phabricator. For one, at this point we have no options for support in the event that something goes wrong as the company responsible for Phabricator, Phacility, has closed their support channels to non-paying customers. Furthermore, in the past year or two Phacility has been placing their development resources in the parts their customers pay them for, which appear to be much different that the parts that we actively use. For this reason, some parts that we rely on seem oddly half-finished. This concern was recently underscored by some rather unfortunate misfeatures in Harbormaster which resulted in broken CI after the Hadrian merge and now apparent bugs which have made it difficult to migrate to the CircleCI integration we previously prepared. Perhaps most importantly, in our recent development priorities survey our use of Phabricator was the most common complaint by a fair margin, both in case of respondents who have contributed patches and those who have not. On the whole, past contributors and potential future contributors alike have strongly indicated that they want a more Git-like experience. Of course, this wasn't terribly surprising; this is just the most recent case where contributors have made this preference known. Frankly, in a sense it is hard to blame them. The fact that users need to install a PHP tool, Arcanist, to contribute anything but documentation patches has always seemed like unnecessary friction to me and I would be quite happy to be rid of it. Indeed we have had a quite healthy number of GitHub documentation patches since we started accepting them. This makes me thing that there may indeed be potential contributoes that we are leaving on the table. # What to do With Rackspace support ending at the end of year, now may be a good time to consider whether we really want to continue on this road. Phabricator is great at code review but I am less and less certain that it is worth the maintenance uncertainty and potential lost contributors that it costs. Moreover, good alternatives seem closer at-hand than they were when we deployed Phabricator. ## Move to GitHub When people complain about our infrastructure, they often use GitHub as the example of what they would like to see. However, realistically I have a hard time seeing GitHub as a viable option. Its feature set is simply insufficient enough to handle the needs of a larger project like GHC without significant external tooling (as seen in the case of Rust-lang). The concrete reasons have been well-documented in previous discussions but, to summarize, * its review functionality is extremely painful to use with larger patches * rebased patches lead to extreme confusion and lost review comments * it lacks support for pre-receive hooks, which serve as a last line of defense to prevent unintended submodule changes * its inability to deal with external tickets is problematic * there is essentially no possibility that we could eventually migrate GHC's tickets to GitHub's ticket tracker without considerable data loss (much less manage the ticket load that would result), meaning that we would forever be stuck with maintaining Trac. * on a personal note, its search functionality has often left me empty-handed On the whole, these issues seem pretty hard to surmount. ## Move to GitLab In using GitLab for another project over the past months I have been positively surprised by its quality. It handles rebased merge requests far better than GitHub, has functional search, and quite a usable review interface. Furthermore, upstream has been extremely responsive to suggestions for improvement [1]. Even out-of-the-box it seems to be flexible enough to accommodate our needs, supporting integration with external issue trackers, having reasonable release management features, and support for code owners to automate review triaging (replacing much of the functionality of Phabricator's Herald). Finally, other FOSS projects' [3] recent migrations from Phabrictor to GitLab have shown that GitLab-the-company is quite willing to offer help when needed. I took some time this weekend to try setting up a quick GHC instance [2] to play around with. Even after just a few hours of playing around I think the result is surprisingly usable. Out of curiosity I also played around with importing some tickets from Trac (building on Matt Pickering's Trac-to-Maniphest migration tool). With relatively little effort I was even able to get nearly all of our tickets (as of 9 months ago) imported while preserving ticket numbers (although there are naturally a few wrinkles that would need to be worked out). Naturally, I think we should treat the question of ticket tracker migration as an orthogonal one to code review, but it is good to know that this is possible. ## Continue with Phabricator Continuing with Phabricator is of course an option. Its review functionality is great and it has served us reasonably well. However, compared to GitLab and even GitHub of today its features seem less distinguished than they once did. Moreover, the prospect of having to maintain a largely stagnant product with no support strikes me as a slightly dangerous game to play. Working around the issues we have recently encountered has already cost a non-negligible amount of time. # The bottom line If it wasn't clear already, I think that we should strongly consider a move to GitLab. At this point it seems clear that it isn't going to vanish, has a steady pace of development, is featureful, and available. However, these are just my thoughts. What do you think? Cheers, - Ben [1] 11.4 will ship with a file tree view in the code review interface, which I reported (https://gitlab.com/gitlab-org/gitlab-ce/issues/46474) as being is one of the Phabricator features I missed the most during review [2] https://gitlab.home.smart-cactus.org/ghc/ghc/issues/14641 [3] The GNOME and freedesktop.org projects have recently migrated, the former from a hodge-podge of self-hosted services and the latter from Phabricator

17 40

Residency profiles
by Simon Peyton Jones 23 Jan '20

23 Jan '20

Simon, Ben, Omer As you'll see in comments 55-72 of https://ghc.haskell.org/trac/ghc/ticket/9476, Sebastian has been a bit flummoxed by the task of measure residency profiles; that is, how much data is truly live during execution. A major GC measures that, but we are vulnerable to exactly when it happens (even with -G1) and that can lead to irreproducible results. I think what we want is a way to trigger GC at very regular intervals, after (say) each 10kbytes or 100kbytes or 1Mbyte of allocation. That might be expensive, but we'd get reproducible results. I don't think that is possible right now - see the ticket - but it would be easy enough to do wouldn't it? Just give only 10k or 100k or 1M to the allocator when setting it running again. Would you consider this? Or are we just missing something obvious? Needless to say, we want to do all this with full optimisation on, no cost-centre profiling. Thanks Simon

4 8

GHC's module hierarchy
by Simon Peyton Jones 02 Oct '19

02 Oct '19

Dear ghc-devs hsyl20 proposes a radical overhaul of the module structure of GHC itself. He or she suggested it six months ago in https://ghc.haskell.org/trac/ghc/ticket/13009 and has now offered a monster patch https://phabricator.haskell.org/D3647 It's clearly the result of a lot of work, but I was the only one who responded on the original ticket, and it'll affect all of your lives in a very immediate way. So, would you like to * consider the idea * look at the actual re-mapping of modules hsyl20 proposals * express an opinion about whether to go ahead Probably the ticket, rather than Phab, is the best place to comment on the general idea. I'd like to thank hsyl20. GHC's rather flat module structure has grown incrementally over years. But still, there are pros and cons. Simon

2 1

Min closure payload size?
by Ömer Sinan Ağacan 11 Sep '19

11 Sep '19

Hi, I was trying to understand why some info tables that have no ptrs and nptrs like GCD_CAF end up with 1 nptrs in the generated info table and found this code in Constants.h: /* ----------------------------------------------------------------------------- Minimum closure sizes This is the minimum number of words in the payload of a heap-allocated closure, so that the closure has enough room to be overwritten with a forwarding pointer during garbage collection. -------------------------------------------------------------------------- */ #define MIN_PAYLOAD_SIZE 1 We use this in a few places in the compiler and add at least one word space in the payload. However the comment is actually wrong, forwarding pointers are made by tagging the info ptr field so we don't need a word in the payload for forwarding pointers. I tried updating this as 0 but that caused a lot of test failures (mostly in GHCi). I'm wondering if I'm missing anything or is it just some code assuming min payload size 1 without using this macro. Any ideas? Ömer

3 6

How do I find out which info table a continuation belongs to?
by Ömer Sinan Ağacan 02 Sep '19

02 Sep '19

I'm currently working on a bug and one of the things I often want to know is what's on the stack. The problem is I can't see labels of continuations so the information is really useless. Example: >>> call printStack(((StgTSO*)0x42000e0198)->stackobj) 0x42000c8788: RET_SMALL (0x512d70) 0x42000c8790: RET_SMALL (0x40edf0) stk[5] (0x42000c8798) = 0x7b3938 0x42000c87a0: CATCH_FRAME(0x735a98,0x7d3ff2) 0x42000c87b8: STOP_FRAME(0x7311b8) (I modified the printer to print stack locations when printing stacks) Here I need to know which info table the RET_SMALLs return to. Normally I do this for other kinds of closures: >>> print ((StgClosure*)...)->header.info $15 = (const StgInfoTable *) 0x404dc0 <Main_nats_info> But for continuations that doesn't work: >>> print ((StgClosure*)0x42000c8788)->header.info $11 = (const StgInfoTable *) 0x512d80 >>> info symbol 0x512d80 No symbol matches 0x512d80. Anyone know how to make this work? Can I maybe mark the continuations label in the generated assembly somehow to make those labels available in gdb? Thanks Ömer

2 3

Any ways to test a GHC build against large set of packages (including test suites)?
by Ömer Sinan Ağacan 25 Jul '19

25 Jul '19

Hi, I'd like to test some GHC builds + some compile and runtime flag combinations against a large set of packages by building them and running test suites. For this I need - A set of packages that are known to work with latest GHC - A way to build them and run their test suites (if I could specify compile and runtime flags that'd be even better) I think stackage can serve as (1) but I don't know how to do (2). Can anyone point me to the right direction? I vaguely remember some nix-based solution for this that was being discussed on the IRC channel, but can't recall any details. Thanks, Ömer

9 15

GitHub Mirror is broken
by Artem Pelenitsyn 31 Mar '19

31 Mar '19

Hello devs, This is just to let you know that the latestes commit on GitHub ghc/ghc repo dates back to 22th of January. Personally, I find GitHub mirror quite useful for ocasional searches over the code base. Therefore, I'd appreciated repairing the mirror. -- Best of luck, Artem Pelenitsyn

5 5

Newcomers' Guide to GHC Development
by Tobias Dammers 12 Mar '19

12 Mar '19

Dear all, With the migration of our affairs from Trac to GitLab nearing completion, I would like to ask for a final round of feedback on the new Newcomers' Guide to GHC development. The draft can be found here: https://github.com/tdammers/ghc-wiki/blob/wip/newcomers/newcomers-tutorial.… TL;DR: If you have any kind of input / critique / praise regarding this document, feel free to reply, or, even better, issue a PR on github. Some background: The purpose of this document is to provide potential contributors with a practical, no-nonsense tutorial, guiding them from "I know nothing about GHC development" to their first successful merge request. The document has been compiled using existing wiki content, revised and edited to match the current state of affairs (particularly using Hadrian as the recommended build system), and to tune it to the target audience of first-time contributors. As such, we avoid going off on tangents (e.g., we do not explain how to use the make-based alternative build system), and we only explain what you need to understand in order to get going (e.g., we do not provide a complete run-down of all hadrian options). A few nonlinearities were deemed necessary in order to make the tutorial suitable across target platforms; Windows in particular requires some special attention. Other than that, however, we try to provide as linear an experience as we reasonably can. So with that said; all feedback and suggestions on this are welcome. We have gotten some great responses already, but I'd like to gather one more round of feedback before merging it into the freshly-migrated Haskell Wiki on GitLab. Thank you for your attention! -- Tobias Dammers - tdammers(a)gmail.com

5 6

Gitlab bug?
by Simon Peyton Jones 26 Feb '19

26 Feb '19

Look here: https://gitlab.haskell.org/ghc/ghc/merge_requests/444/diffs#350c4076427c611… Look at the comment "Is this zipping necessary? We haven't zonked the scoped_kvs or the tc_tvs, I think" from Richard Eisenberg, just below line 1925 in TcHsType. Now click on the grey "..." above his comment, to expand more code lines above. Uh oh! * The same comment now occurs twice, just below line 1904, and again just below 1925. * The first time it's on the wrong line of code * But in the second position (the correct one) the "Reply" box is inactive. I can't type into it. Is this is a bug? How to report it? Simon

3 5

RE: --share option for hadrian doesn't work with hs-boot files
by Andrey Mokhov 22 Feb '19

22 Feb '19

Hi Matt, Thanks! Switching branches that add/remove hs-boot files is exactly the kind of scenario which is hard to predict :) This looks like a bug, so please create a ticket. If you manage to reproduce this somehow without switching branches, e.g. by just adding or removing an hs-boot file, that would make it easier to debug. > Could you please write down some advice Andrey about > how to solve issues like this? It seems very fragile making > sure that every case is covered. Shake 0.17.6 has the following two commands that may help to partially clean up the cache in presence of such bugs: --share-list List the shared cache files. --share-remove[=SUBSTRING] Remove the shared cache keys. By running Hadrian with --share-remove=_build/stage0/compiler/build/Var* you should be able to evict the corresponding build rules from the cache and hopefully the build will go through. If this does help, please also mention this in the ticket. Cheers, Andrey From: Matthew Pickering [mailto:matthewtpickering@gmail.com] Sent: 22 February 2019 08:53 To: GHC developers <ghc-devs(a)haskell.org>; Andrey Mokhov <andrey.mokhov(a)newcastle.ac.uk> Subject: --share option for hadrian doesn't work with hs-boot files I have been trying the new `--share` option implemented in hadrian but I haven't actually managed to complete any builds yet with it enabled after the initial one. The current error is ``` <no location info>: error: ‘Var.AnonArgFlag’ is exported by the hs-boot file, but not exported by the module Error when running Shake build system: at action, called at src/Rules.hs:35:19 in main:Rules at need, called at src/Rules.hs:52:5 in main:Rules * Depends on: _build/stage0/bin/ghc at need, called at src/Utilities.hs:71:18 in main:Utilities * Depends on: _build/stage0/compiler/build/libHSghc-8.9.a at need, called at src/Rules/Library.hs:118:5 in main:Rules.Library * Depends on: _build/stage0/compiler/build/Var.o * Raised the exception: user error (Development.Shake.cmd, system command failed ``` I get this after building Simon's `FunTy` patch which does add this flag and the definition to `Var.hs-boot` and then switching back to master with the cache enabled. Could you please write down some advice Andrey about how to solve issues like this? It seems very fragile making sure that every case is covered. Cheers, Matt

1 0