[GHC] #13426: compile-time memory-usage regression for DynFlags between GHC 8.0 and GHC 8.2

[GHC]

#13426: compile-time memory-usage regression for DynFlags between GHC 8.0 and GHC
8.2
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        Reporter:  hvr               |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Very useful to know that the stable unfoldings alone are enough.

 I have an idea.  Consider `Simplify` line 1005
 {{{
 simplExprF1 env (Type ty)      cont = ASSERT( contIsRhsOrArg cont )
                                       rebuild env (Type (substTy env ty))
 cont
 }}}
 Yikes!  That `(substTy env ty)` is a thunk that the simplifier may not
 force; and that'll hold onto `env` which is disater.

 Replace the RHS by
 {{{
   = do { ty' <- simplType env ty
        ; retbuild env (Type ty') cont }
 }}}
 You'll see that `simplType` is careful to `seq` on substituted type; and
 `TyCoRep.substTy` has the property (I think, though it is sadly not
 documented) that `seq`ing on the result is enough to push the substitution
 right through the type (see the `$!` calls in `subst_ty`.

 It is very unsatisfactory that this kind of leak is SO hard to find.

 But finding it will help ALL programs!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13426#comment:10>
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