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Ticket #18304 showed that we need to be very careful when exploring the demand (esp usage demand) on recursive product types. This patch solves the problem by trimming the demand on such types -- in effect, a form of "widening". See the Note [Trimming a demand to a type] in DmdAnal, which explains how I did this by piggy-backing on an existing mechansim for trimming demands becuase of GADTs. The significant payload of this patch is very small indeed: * Make GHC.Core.Opt.WorkWrap.Utils.typeShape use RecTcChecker to avoid looking through recursive types. But on the way * I found that ae_rec_tc was entirely inoperative and did nothing. So I removed it altogether from DmdAnal. * I moved some code around in DmdAnal and Demand. (There are no actual changes in dmdFix.) * I changed the API of DmsAnal.dmdAnalRhsLetDown to return a StrictSig rather than a decorated Id * I removed the dead function peelTsFuns from Demand Performance effects: Nofib: 0.0% changes. Not surprising, because they don't use recursive products Perf tests T12227: 1% increase in compiler allocation, becuase $cto gets w/w'd. It did not w/w before because it takes a deeply nested argument, so the worker gets too many args, so we abandon w/w altogether (see GHC.Core.Opt.WorkWrap.Utils.isWorkerSmallEnough) With this patch we trim the demands. That is not strictly necessary (since these Generic type constructors are like tuples -- they can't cause a loop) but the net result is that we now w/w $cto which is fine. UniqLoop: 16% decrease in /runtime/ allocation. The UniqSupply is a recursive product, so currently we abandon all strictness on 'churn'. With this patch 'churn' gets useful strictness, and we w/w it. Hooray Metric Decrease: UniqLoop Metric Increase: T1222742953902
