ghc-lib-parser causes segfaults when used together with the ghc library on ghc-9.10

The FastString table built into GHC changed recently.

Since this table is shared by all versions of the GHC source code via an RTS symbol this causes segfaults when ghc-lib-parser-9.12 and ghc-9.10 are used together since they use different memory layouts for this table.

The fix for this is either to change GHC.Data.FastString such that the sharedCAF mechanism is not used. However that means FastStrings created by ghc-lib-parser and ghc are never allowed to be mixed.

The other alternative is to use CPP to adjust the memory layout of the faststring table depending on the GHC version used.

This is the change in question: https://gitlab.haskell.org/ghc/ghc/-/commit/f2cc1107790d42fee1a11d5b16bc282d31ea6f78

The simplest option of course would be to mark ghc-lib-parser-9.12 unbuildable on ghc-9.10, but perhaps also a bit of a shame.

Related HLS bug where this was discovered: haskell/haskell-language-server#4674

The other alternative is to use CPP to adjust the memory layout of the faststring table depending on the GHC version used.

what would that cpp look like and where would it go? FastString.hs? we can have ghc-lib generate it. care to offer a patch?

I don't know where to put this as a PR, but I'm guessing a patch would look something like this:

diff --git a/compiler/GHC/Data/FastString.hs b/compiler/GHC/Data/FastString.hs
index 017a8d3926a..c72a24cf16b 100644
--- a/compiler/GHC/Data/FastString.hs
+++ b/compiler/GHC/Data/FastString.hs
@@ -310,6 +310,7 @@ data FastStringTable = FastStringTable
   {-# UNPACK #-} !FastMutInt
   -- ^ The unique ID counter shared with all buckets
   --
+#if MIN_VERSION_ghc(9,12,0)
   -- We unpack the 'FastMutInt' counter as it is always consumed strictly.
   {-# NOUNPACK #-} !FastMutInt
   -- ^ Number of computed z-encodings for all buckets.
@@ -318,6 +319,9 @@ data FastStringTable = FastStringTable
   -- in 'mkFastStringWith' and needs to be boxed any way.
   -- If this is unpacked, then we box this single 'FastMutInt' once for each
   -- allocated FastString.
+#else
+  {-# UNPACK #-} !FastMutInt -- number of computed z-encodings for all buckets
+#endif
   (Array# (IORef FastStringTableSegment)) -- ^  concurrent segments

 data FastStringTableSegment = FastStringTableSegment

@dylant-da

cc @dylant-da

Seen this just now - I'll take a look tomorrow to see what I can understand of the issue and the proposed fix.

@vaibhavsagar

What @vaibhavsagar posted seems right to me.

What is the status of fixing this?

Apologies, this fell off my radar - taking a look at incorporating this change later today.

Gonna need a bit longer to make the time for this.

Were you able to make any progress on this?

@dylant-da

@dylant-da Are you still planning to address this?

@paulbrauner-da

@paulbrauner-da Are you looking for new maintainers for this library?

Navigation Menu

Repository navigation

ghc-lib-parser causes segfaults when used together with the `ghc` library on ghc-9.10 #620

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

ghc-lib-parser causes segfaults when used together with the ghc library on ghc-9.10 #620

Description

Activity

AndreasPK commented on Dec 2, 2025

shayne-fletcher commented on Dec 7, 2025

vaibhavsagar commented on Dec 19, 2025

shayne-fletcher commented on Dec 21, 2025

dylant-da commented on Dec 22, 2025

AndreasPK commented on Dec 22, 2025

hasufell commented on Jan 6, 2026

dylant-da commented on Jan 12, 2026

dylant-da commented on Jan 13, 2026

vaibhavsagar commented on Jul 6, 2026

ysangkok commented on Sep 12, 2026

ysangkok commented on Oct 20, 2026

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions

ghc-lib-parser causes segfaults when used together with the `ghc` library on ghc-9.10 #620