April 2025 - ghc-devs

Questions regarding the RISCV native codegen & performance
by Daniel Trujillo Viedma 15 Apr '25

15 Apr '25

Hello, ghc-devs! My name is Daniel Trujillo, I'm a Haskell enthusiast from Spain and I'm trying to make my Master's thesis about accelerating Haskell programs with a custom ISA extension. Right now, my focus is in executing software written in Haskell within QEMU in order to get traces that tells me, basically, how many times each block (not exactly basic blocks, but sort of) of assembly code has been executed, with the hope of finding some patterns of RISCV instructions that I could implement together into 1 instruction. As you can see, my method is a bit crude, and I was wondering if the people involved with any of the different internal representations (STG, Cmm...) and/or native code generators (particularly RISCV) could provide me hints about assembly instructions that would have made the work easier, by removing the need of "massaging" the Cmm code to make CodeGen easier, or the need of particular optimizations, or in general, dirty tricks because of lacking of proper support of the standard RISCV ISA. And of course, I would also appreciate very much other hints from people involved in general performance (as oppossed to, for example, libraries for SIMD and parallel execution, or Haskell wrappers to lower-level code for performance reasons). P.D. I'm sorry if I broke any netiquette rule, but I'm very new to the email list, and haven't received yet any email from it. Looking forward to hear from you! Cheers, Dani.

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GitLab maintenance this evening
by Ben Gamari 14 Apr '25

14 Apr '25

Hello all, Tonight GitLab will be down for maintenance between 17:00 EDT and 18:00 EDT as part of the on-going infrastructure migration. Cheers, - Ben

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Testsuite tests
by Simon Peyton Jones 08 Apr '25

08 Apr '25

Friends In CI for !10479 I'm getting test failures for some call-stack tests e.g. `callstack001`. But if I try to run this in my build tree hadrian/build -j8 test --skip=_build/stage1/rts/** --skip=_build/stage1/** --skip=_build/stage0/** --docs=none --only=callstack001 I get SUMMARY for test run started at Fri Apr 4 22:22:47 2025 0:00:02.413942 spent to go through 1 total tests, which gave rise to 8 test cases, of which 8 were skipped 0 had missing libraries 0 expected passes 0 expected failures 0 caused framework failures 0 caused framework warnings 0 unexpected passes 0 unexpected failures 0 unexpected stat failures 0 fragile tests So I can't reproduce the failure. What am i doing wrong? Can anyone help? Thanks Simon

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Next steps forward for a stable Template Haskell
by Teo Camarasu 08 Apr '25

08 Apr '25

Hi folks, At Tuesday's GHC devs meeting, we talked about having a discussion about template-haskell stability. I've jotted down some notes to help focus our future discussion. It's mostly just a list of what I take to be the next most important issues to tackle, and some details about each. https://docs.google.com/document/d/1gui1YXOvrNihR3NpXuMVCKV3lTkKPL8y_HDRjbZ… Cheers, Teo

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Extended deadline - CFP - Haskell Implementors' Workshop 2025
by Andreas Herrmann 07 Apr '25

07 Apr '25

Extended deadline to April 18, 2025 Call for proposals for the Haskell Implementors' Workshop https://haskell.foundation/events/2025-haskell-implementors-workshop.html June 6, 2025 Organized by the Haskell Community Co-located with ZuriHac 2025 and Haskell Ecosystem Workshop 2025 Hosted by the Haskell Foundation at Eastern Switzerland University of Applied Sciences (OST) https://www.ost.ch/en/university-of-applied-sciences/campus/rapperswil-jona… ## Overview * Extended Deadline: April 18, 2025 * Notification: May 5, 2025 * Workshop: June 6, 2025 The 17th Haskell Implementors' Workshop is to be held alongside ZuriHac 2025 this year near Zurich. It is a forum for people involved in the design and development of Haskell implementations, tools, libraries, and supporting infrastructure to share their work and to discuss future directions and collaborations with others. Talks and/or demos are proposed by submitting an abstract, and selected by a small program committee. There will be no published proceedings. The workshop will be informal and interactive, with open spaces in the timetable and room for ad-hoc discussion, demos, and lightning talks. In the past the Haskell Implementors’ Workshop was co-located with ICFP (International Conference on Functional Programming). However, in recent years it has become more and more challenging to attract a large enough audience and sufficiently many speakers for an appealing program. ZuriHac and the Haskell Ecosystem Workshop have become an important annual gathering of a large part of the Haskell community. This year the Haskell Implementors’ Workshop will be co-located with these events to be accessible to a broader audience. ## Scope and Target Audience The Haskell Implementors' Workshop is an ideal place to describe a Haskell extension, describe works-in-progress, demo a new Haskell-related tool, or even propose future lines of Haskell development. Members of the wider Haskell community are encouraged to attend the workshop - we need your feedback to keep the Haskell ecosystem thriving. Students working with Haskell are especially encouraged to share their work. The scope covers any of the following topics. There may be some topics that people feel we've missed, so by all means submit a proposal even if it doesn't fit exactly into one of these buckets: * Compilation techniques * Language features and extensions * Type system implementation * Concurrency and parallelism: language design and implementation * Performance, optimization and benchmarking * Virtual machines and run-time systems * Libraries and tools for development or deployment ## Talks We invite proposals from potential speakers for talks and demonstrations. We are aiming for 20-minute talks with 5 minutes for questions and changeovers. We want to hear from people writing compilers, tools, or libraries, people with cool ideas for directions in which we should take the platform, proposals for new features to be implemented, and half-baked crazy ideas. Submissions can be made via the form linked below until April 18, 2025 (anywhere on earth). https://docs.google.com/forms/d/e/1FAIpQLSdczGbxJYGc4eusvPrxwBbZl561PnKeYno… We will also have a lightning talks session. Lightning talks should be ~7mins and are scheduled on the day of the workshop. Suggested topics for lightning talks are to present a single idea, a work-in-progress project, a problem to intrigue and perplex Haskell implementors, or simply to ask for feedback and collaborators. ## Program Committee * Luite Stegeman * Jaro Reinders * Emily Pillmore * Rodrigo Mesquita * Ian-Woo Kim * Andreas Herrmann (chair) ## Contact * Andreas Herrmann

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Late CC profiling
by Simon Peyton Jones 07 Apr '25

07 Apr '25

Hi Matthew, Andreas, Ben In GHC today there is a strange inconsistency: - Bindings for data constructor wrappers and class method selectors are added at the end of CoreTidy. - Bindings for data constructor workers are added at the start of CorePrep This is deeply strange. In !10479 (unary class patch) <https://gitlab.haskell.org/ghc/ghc/-/merge_requests/10479> I had to make some changes in this area and after being baffled I moved all these implicit bindings to one place at the start of CorePrep. That is simpler and neater. There is no reason for them to be separate. BUT I find that one thing happens between Tidy and Prep, namely late-cost-centre injection. That means we will no longer get late-cost-centres for data constructor wrappers and class method selectors. I think that this is what is causing diffs in the output for callstack001 and friends in !10479. I think that this is perfectly fine, and I propose to accept the changes. Another alternative would be to inject all of these bindings at the end of Tidy (instead of the start of Prep), But it's much nicer at the start of Prep -- we don't want any of these bindings to appear in interface files. Can you confirm that the change is ok? Thanks Simon

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GHC memory usage when typechecking from source vs. loading ModIfaces
by Gergő Érdi 03 Apr '25

03 Apr '25

Hi, I’m using the GHC API to typecheck 35,000 modules that form a complicated dependency graph (with multiple top-level modules, i.e. there’s no single “god module” that would transitively depend on everything else), and I noticed that peak memory usage is wildly different when everything is done from scratch vs. when everything is loaded from files containing ModIfaces: 17G vs. 8G. This ratio replicates for smaller samples as well, e.g. 80M vs 33M for 407 modules. I’m aware of https://gitlab.haskell.org/ghc/ghc/-/issues/13586 and so when I finish typechecking a module, I take the resulting ModIface and create the ModDetails that ends up in the HomeUnitGraph from that. My understanding of Matt’s original GHC fix in https://gitlab.haskell.org/ghc/ghc/-/merge_requests/5478 is that it does the same, i.e. it only makes a fresh ModDetails only once per module, after the ModIface is ready. But of course that still means that ModDetails can only keep growing as more and more parts of it are used for typechecking more and more dependants. Could that be the cause? I tried a crude experiment of “putting the toothpaste back in the tube” by replacing all ModDetails with a fresh one in the HUG after each finished typechecking , but that’s a complete disaster for memory usage: even for the small 407 module example, the memory usage shoots up to 1.5G. I can imagine it’s because imported Ids are probably not shared anymore between different importer modules. Any ideas on how I could improve memory usage in the from-scratch case, so that it's more similar to the from-ModIface case? Thanks, Gergo

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can't move to latest version of llvm on ghc 9.12.2
by George Colpitts 02 Apr '25

02 Apr '25

llvm 20 is out but unlike in earlier versions of ghc moving to it means you can no longer use llvm: compiling: ghc -fllvm hello.hs Loaded package environment from /Users/avie/.ghc/aarch64-darwin-9.12.2/environments/default [1 of 2] Compiling Main ( hello.hs, hello.o ) <no location info>: error: [GHC-66599] GHC was not configured with a supported LLVM toolchain Make sure you have installed LLVM between [13 and 20) and reinstall GHC to make -fllvm work from configure: configure: We only support llvm 13 upto 20 (non-inclusive) (found 20.1.1). Can we move to llvm 20 on HEAD and can we revert to the old behavior on ghc 9.12.3? Should I file an ER? Thanks

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