When 9.2.1 was [released], I apparently was confused by the wording. The
NCG (-fasm) codegen backend for aarch64 not only works on
aarch64-darwin, but also aarch64-linux. `useLLVM` being enabled on
aarch64-linux had no adverse effect, as GHC used -fasm anyways, but it
did inflate closure size unnecessarily which we can rectify now.
[released]: https://www.haskell.org/ghc/blog/20211029-ghc-9.2.1-released.html
This was already applied to GHC 9.2.x, but was not copied to GHC 9.4.x.
I have had issues with this locally.
The same patch works for both Cabal 3.6 and 3.8, so we can just reuse it.
The main problem was GHC exceeding the Hydra output limit with profiling
libs on aarch64-linux which made us disable the feature. Nowadays the
limit is 3GB, the GHC output is a bit over 2GB, so easily under the
limit.
aarch64-darwin uses a different codegen backend and was never really
affected by the problem: Its output with profiling enabled is around
1.6GB.
Consequently we can enable profiling for all platforms again, as we have
no output size issues for those we build on Hydra.
Thanks to flokli for helping me track down these up to date numbers.
Due to link time dead code elimination not working on aarch64-darwin,
some unused store path references in Paths_* modules are retained. This
causes reference cycles when a separate `bin` output is used.
To prevent this, add a patch to Cabal as shipped by GHC which infers
based on the installation layout (which is influenced by
enableSeparateBinOutput, enableSeparateDataOutput etc. in a Nix build)
which references can be retained without causing a reference cycle. This
ensures that packages that were fine with a bin output will also work on
aarch64-darwin. Packages that cause a reference cycle anyways (by
actually using references that do cause one) fail due to a missing
symbol – here we are trading the overall benefit for a more confusing
error message.
For details, refer to the explanation comment in the patch.
