Parser performance: 10% regression in 8.8

[Vladislav Zavialov]

Hello ghc-devs,

This February I did some changes to the parser that require higher rank types support in ‘happy’. Unfortunately, as I discovered, happy’s --coerce option is severely broken in the presence of higher rank types, so I had to disable it. My benchmarks have shown a 10% slowdown from disabling --coerce (https://gist.github.com/int-index/38af0c5dd801088dc1de59eca4e55df4 <https://gist.github.com/int-index/38af0c5dd801088dc1de59eca4e55df4>).

Alongside my changes I submitted a pull request to happy which fixes the issue (https://github.com/simonmar/happy/pull/134 <https://github.com/simonmar/happy/pull/134>), in the hope that it would get merged, released, and I could re-enable --coerce in GHC ‘happy' configuration.

Unfortunately, my patch has been ignored to this day (for 3 months now), and the performance regression reached 8.8-alpha. We need to act swiftly if we want to avoid a performance regression in the actual release. Here’s what needs to be done:

1. Merge https://github.com/simonmar/happy/pull/134 <https://github.com/simonmar/happy/pull/134>
2. Release a new ‘happy’
3. (Optional) Specify in GHC’s build system that it builds only with the latest 'happy' release
4. Restore the --coerce option in GHC’s build system ‘happy’ configuration
5. Backport it to the ghc-8.8 branch

I have no access to do 1 & 2, I believe Simon Marlow does. I’d appreciate if someone took care of 3, currently the build system does not install ‘happy’ and assumes a system-wide installation without checking its version. This means that users of all but the newly released version will encounter obscure error messages. We need a version check. Then I will do 4, as planned, and create a merge request for 5.

All the best,
- Vladislav
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