Compute the union of imp_finsts on the side

Authored by niteria on Feb 2 2018, 8:55 AM.

Description

Compute the union of imp_finsts on the side

I've explained most of the rationale in a new Note.
I'd happily add a test for this, but the difference is only
visible in run time, allocations remain more or less the same.

FWIW running generateModules from Trac #14693 with DEPTH=16, WIDTH=30
finishes in 23s before, and 11s after.

Test Plan: ./validate

Reviewers: simonpj, simonmar, bgamari

Reviewed By: simonpj

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: Trac #14693

Differential Revision: https://phabricator.haskell.org/D4369