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GHC DevX biweekly update 2023-02-09 (#30)
* Starts next biweekly update * Fix date * biweekly: jeff: add CI discussion * jeff: add eDSL and release notes blurbs * jeff: add work on HoH * Update 2023-02-09-ghc-update-2023-02-09.md * Update 2023-02-09-ghc-update-2023-02-09.md * Update 2023-02-09-ghc-update-2023-02-09.md * Update 2023-02-09-ghc-update-2023-02-09.md * Update 2023-02-09-ghc-update-2023-02-09.md * add item about GHCi bytecode size limits * Update 2023-02-09-ghc-update-2023-02-09.md * Update 2023-02-09-ghc-update-2023-02-09.md * Update 2023-02-09-ghc-update-2023-02-09.md * Update 2023-02-09-ghc-update-2023-02-09.md * Update 2023-02-09-ghc-update-2023-02-09.md * Update 2023-02-09-ghc-update-2023-02-09.md * Update 2023-02-09-ghc-update-2023-02-09.md * add section about exceptions and improve the ones about ghci bytecode and temp files * Update 2023-02-09-ghc-update-2023-02-09.md * Update 2023-02-09-ghc-update-2023-02-09.md * Update 2023-02-09-ghc-update-2023-02-09.md * Update 2023-02-09-ghc-update-2023-02-09.md --------- Co-authored-by: doyougnu <jeffrey.young@iohk.io> Co-authored-by: Luite Stegeman <stegeman@gmail.com> Co-authored-by: Josh Meredith <joshmeredith2008@gmail.com>
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| --- | ||
| slug: 2023-02-09-ghc-update | ||
| title: "IOG GHC Update #3 (2023-02-09)" | ||
| authors: [sylvain,doyougnu,luite,josh] | ||
| tags: [ghc,ghc-update] | ||
| --- | ||
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| Biweekly update from the GHC DevX team at IOG. | ||
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| Previous updates can be found [here](https://engineering.iog.io/tags/ghc-update). | ||
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| ## JavaScript backend | ||
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| ### Template Haskell | ||
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| Luite: fixed the support for one-shot mode (GHC's `-c` command-line flag) | ||
| in the TH JS linker. | ||
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| Luite: Investigated a warning about the temporary directory not being removed | ||
| after running Template Haskell with the JavaScript backend. It turned out | ||
| that GHC's `GHC.Utils.TmpFs.newTempDir`, which is used by the Template Haskell | ||
| linker, does not allow the newly created directory to be removed | ||
| (see [#22952](https://gitlab.haskell.org/ghc/ghc/-/issues/22952)). | ||
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| Sylvain: cleaned up the [merge request](https://gitlab.haskell.org/ghc/ghc/-/merge_requests/9779), | ||
| removing unnecessary changes and adding documentation. | ||
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| ### JavaScript backend CI | ||
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| Jeff: JavaScript backend CI was [finally | ||
| merged](https://gitlab.haskell.org/ghc/ghc/-/merge_requests/9552)! Now that we | ||
| have CI we are unblocked on several fronts, such as, implementing [faster | ||
| arithmetic](https://gitlab.haskell.org/ghc/ghc/-/merge_requests/9825) and fixing | ||
| some [async | ||
| exceptions](https://gitlab.haskell.org/ghc/ghc/-/merge_requests/9879). In | ||
| general, we can now have confidence that our work is progressing the JavaScript | ||
| backend to a better state. | ||
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| Sylvain: fixed a spurious failure on JS CI due to some test _passing_ on fast runners | ||
| while it was expected to fail (see [!9934](https://gitlab.haskell.org/ghc/ghc/-/merge_requests/9934)). | ||
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| Josh: fixed some inaccurate test predicates [!9939](https://gitlab.haskell.org/ghc/ghc/-/merge_requests/9939) | ||
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| ### FileStat | ||
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| Josh: rebased and merged [!9755](https://gitlab.haskell.org/ghc/ghc/-/merge_requests/9755). | ||
| This patch changes the representation of the JavaScript equivalent of the C `struct stat` | ||
| to make its field offsets match the C ones: some Haskell codes directly access fields of | ||
| this structure using `hsc2hs` to get the field offsets from the C headers. | ||
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| This patch also adds fields to the JavaScript file stat that were previously not | ||
| included, such as modification and access times. | ||
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| ### JavaScript RTS refactor | ||
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| Josh: rebased [!9794](https://gitlab.haskell.org/ghc/ghc/-/merge_requests/9794) which consists | ||
| in the refactor of the module generating some part of the RTS. The new JS CI job found a bug in the patch that | ||
| caused ~50 tests to time out, so waiting for CI to be set up before merging this MR was judicious. | ||
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| This MR also became an opportunity to revisit some arbitrary cache sizes in the RTS code generator. | ||
| This is still ongoing work. | ||
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| ### Warnings | ||
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| Luite: We accidently removed the check for the "javascript" calling convention on | ||
| foreign imports, allowing this convention to be wrongly used on native platform. | ||
| Fixed in [!9880](https://gitlab.haskell.org/ghc/ghc/-/merge_requests/9880). | ||
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| ### Fix for asynchronous exceptions | ||
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| Luite: Fixed an issue in the garbage collector for the JavaScript backend: | ||
| A thread that posts an asynchronous exception (`throwTo`) to another thread | ||
| is temporarily suspended until the exception has been delivered. The | ||
| garbage collector did not correctly follow the list of threads suspended in | ||
| this way, potentially considering them unreachable and cleaning up data | ||
| referenced by them. See [#22836](https://gitlab.haskell.org/ghc/ghc/-/issues/22836) and | ||
| [!9879](https://gitlab.haskell.org/ghc/ghc/-/merge_requests/9879). | ||
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| ### Integer performance | ||
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| Evaluating the following expression is very slow in general but especially with | ||
| the JS backend: | ||
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| ```haskell | ||
| 1 `shiftL` (1 `shiftL` 20) :: Integer | ||
| ``` | ||
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| We've had to mark a test computing this as broken on CI because it triggers a | ||
| timeout error. Luckily the identification of slow operations is easy with | ||
| JavaScript profiling tools (see graphs in | ||
| https://gitlab.haskell.org/ghc/ghc/-/issues/22835) and we know that `Word32` | ||
| primops are the culprit in this case. | ||
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| Sylvain started replacing the uses of JavaScript's `BigInt` in the | ||
| implementation of these primops with usual JavaScript `numbers`. | ||
| See [!9825](https://gitlab.haskell.org/ghc/ghc/-/merge_requests/9825). | ||
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| ### JavaScript EDSL | ||
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| Jeff: JavaScript eDSL based on | ||
| [sunroof](https://github.com/ku-fpg/sunroof-compiler) close to MR, see | ||
| [#22736](https://gitlab.haskell.org/ghc/ghc/-/issues/22736) for background. | ||
| Compiler is complete. Major items left are: the interpreter to translate to | ||
| `JStat`, filling in documentation, and testing now that CI has been merged. | ||
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| ### Documentation | ||
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| Jeff: Wrote the JavaScript backend release notes. Notes pending | ||
| [approval](https://gitlab.haskell.org/ghc/ghc/-/merge_requests/9828). We cite | ||
| the [JavaScript | ||
| backend](https://gitlab.haskell.org/ghc/ghc/-/wikis/javascript-backend) wiki | ||
| page in the release notes. So Jeff and Sylvain heavily edited the wiki pages to | ||
| make them suitable for external customers. | ||
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| ### Change from `js` to `javascript` architecture | ||
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| In [#22740](https://gitlab.haskell.org/ghc/ghc/-/issues/22740) it was noticed that | ||
| `hackage-server` would prevent the upload of the upcoming `base` package bundled with GHC 9.6. | ||
| The reason is that `hackage-server` relies on the `cabal --check` feature which filters | ||
| out perfectly valid packages (it happened before, for example with [ghc-api-compat](https://gitlab.haskell.org/haskell/ghc-api-compat/-/issues/1)). | ||
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| In our case, the package was rejected because the `js` architecture wasn't recognized | ||
| as a built-in one, but luckily we could fall back to the existing `javascript` built-in | ||
| architecture defined for GHCJS (if it wasn't a JS backend, we would have had to fix Cabal, | ||
| update `hackage-server` dependencies, and redeploy `hackage-server`...). | ||
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| Sylvain completed Ben's MR in [!9814](https://gitlab.haskell.org/ghc/ghc/-/merge_requests/9814). | ||
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| For early users of the JS backend the change means that from now on: | ||
| - `configure` command is: `emconfigure ./configure --target=javascript-unknown-ghcjs` | ||
| - Cabal condition is: `arch(javascript)` | ||
| - CPP condition is: `#if defined(javascript_HOST_ARCH)` | ||
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| ## Compiler performance | ||
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| ### More-strict `break` | ||
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| Josh: opened an MR to add a strict `break'` version to `base` - however, this would require a CLC proposal. | ||
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| https://gitlab.haskell.org/ghc/ghc/-/issues/22865 | ||
| https://gitlab.haskell.org/ghc/ghc/-/merge_requests/9868 | ||
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| Further analysis was done using ticky profiles on a simple test program that benchmarks GHC's startup code. | ||
| This has found an example where a more-strict break is an improvement in GHC, which will provide motivation | ||
| for the CLC proposal. | ||
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| ### Unboxed CodeBuffers | ||
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| Josh: implemented changes to GHC's text encoding buffers to use unboxed tuples on handle encoders/decoders. | ||
| The buffers pass around and repeatedly pack/unpack a tuple in an `IO` inner loop, which causes a significant | ||
| number of unnecessary allocations. By replacing this with an unboxed tuple, and replacing the `IO` with | ||
| manually passing around a `State# RealWorld` in the same tuple, we're able to reduce allocations by nearly 50% | ||
| in a pathological example (non-allocating loop printing characters). | ||
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| https://gitlab.haskell.org/ghc/ghc/-/issues/22946 | ||
| https://gitlab.haskell.org/ghc/ghc/-/merge_requests/9948 | ||
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| ### Optimization Handbook | ||
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| Jeff: opened IT ticket to move the [Optimization | ||
| Handbook](https://github.com/input-output-hk/hs-opt-handbook.github.io) to Haskell Foundation's repository. | ||
| IT stated they need to check with legal, of course. | ||
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| Jeff: almost finished the [lambda | ||
| lifting](https://input-output-hk.github.io/hs-opt-handbook.github.io/src/Optimizations/GHC_opt/lambda_lifting.html) | ||
| chapter; major remaining items are adding some glossary terms and describing the | ||
| interaction between lambda lifting and calling conventions. | ||
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| ### Constant folding for division operations | ||
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| While looking into his old merge requests still opened, Sylvain nerd-snipped | ||
| himself into fixing constant folding rules for division operations (see | ||
| [#22152](https://gitlab.haskell.org/ghc/ghc/-/issues/22152)). | ||
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| MR [!8956](https://gitlab.haskell.org/ghc/ghc/-/merge_requests/8956) adds the | ||
| following rewrite rules: | ||
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| ``` | ||
| case quotRemInt# x y of | ||
| (# q, _ #) -> body | ||
| ====> | ||
| case quotInt# x y of | ||
| q -> body | ||
| case quotRemInt# x y of | ||
| (# _, r #) -> body | ||
| ====> | ||
| case remInt# x y of | ||
| r -> body | ||
| For all primitive numerical types: | ||
| (x `quot` l1) `quot` l2 | ||
| | l1 /= 0 | ||
| | l2 /= 0 | ||
| | l1*l2 doesn't overflow/underflow | ||
| ====> | ||
| x `quot` (l1 * l2) | ||
| ``` | ||
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| It also makes some division primops (Word64/Int64 Quot/Rem, WordQuotRem2Op) | ||
| ok-for-speculation when the divisor is known to be non-zero, similarly to other | ||
| division primops. Otherwise the last rule wasn't firing in the added test | ||
| because we got the following Core (simplified for the presentation): | ||
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| ```haskell | ||
| case quotWord64# x# 10#Word64 of | ||
| ds1 -> case quotWord64# ds1 20#Word64 of | ||
| ds2 -> ... | ||
| ``` | ||
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| and not: | ||
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| ```haskell | ||
| case quotWord64# (quotWord64# x# 10#Word64) 20#Word64 of | ||
| ds2 -> ... | ||
| ``` | ||
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| ## GHCi | ||
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| Luite: A test run of GHC 9.6 with the `-fbyte-code-and-object-code` flag | ||
| on [head.hackage](https://ghc.gitlab.haskell.org/head.hackage/) revealed | ||
| ([issue #22888](https://gitlab.haskell.org/ghc/ghc/-/issues/22888)) with bytecode | ||
| size limits. Many bytecode instructions have `Word16` operands, which | ||
| is not always enough to run programs generated from optimized core. The solution | ||
| is to enable large operands for all the bytecode instructions that deal with | ||
| stack offsets. | ||
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| https://gitlab.haskell.org/ghc/ghc/-/issues/22888 | ||
| https://gitlab.haskell.org/ghc/ghc/-/merge_requests/9957 |