Seeking performance-minded reviewers on !2965

Ryan Scott at
Mon Oct 12 09:51:19 UTC 2020

!2965 [1] seeks to improve the performance of derived Generic(1) instances
by heuristically annotating implementations of the from(1) and to(1) class
methods with INLINE[1] pragmas. For the most part, this results in
performance wins, as the generic intermediate representations of data types
can be optimized away in more situations.

However, I'm unclear on one aspect of the MR: it actually leads to a
compile-time performance increase (specifically, in bytes allocated) in one
test case: T12227:

                          Test    Metric         value     New value Change
                T12227(normal) ghc/alloc   518417756.0   526479616.0  +1.6%

The MR author provides an explanation for why this happens here [2].
However, as I am not really a GHC performance guru, it's difficult for me
to accurately judge whether this difference is within acceptable limits or

For those of you who care about compile-time performance, would you be
willing to give !2965 a look over and judge whether the difference is
acceptable? I think this is the last remaining hurdle to clear before we
can land !2965, so your input would be greatly appreciated. Thanks!

Ryan S.
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