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I discovered that the dramatic imprvoement in perf/should_run/T9339 with the introduction of join points was really rather a fluke, and very fragile. The real problem (see Note [Making SpecConstr keener]) is that SpecConstr wasn't specialising a function even though it was applied to a freshly-allocated constructor. The paper describes plausible reasons for this, but I think it may well be better to be a bit more aggressive. So this patch add -fspec-constr-keen, which makes SpecConstr a bit keener to specialise, by ignoring whether or not the argument corresponding to a call pattern is scrutinised in the function body. Now the gains in T9339 should be robust; and it might even be a better default. I'd be interested in what happens if we switched on -fspec-constr-keen with -O2. Reviewers: austin, bgamari Reviewed By: bgamari Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D3186
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