Avoid wasteful allocations in mkTyConAppCo

sheafrequested to merge
sheaf/ghc:T27648 into master

Ticket: #27648 (closed)

The idiom traverse isReflCo_maybe followed by map fst used in GHC.Core.Coercion.mkTyConAppCo was allocating a lot of waste. This MR uses GHC.Data.Unboxed.traverseMaybeUB to avoid all these unnecessary intermediate allocations.

In a quick microbenchmark for mkTyConAppCo, this change resulted in:

  • refl case (all argument coercions are reflexive):
    • -60% runtime
    • -80% allocations
  • non-refl case:
    • from 0% to -12% runtime (depending on which argument is non-refl)
    • from 0% to -70% allocations ( -- '' -- )

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