#13426: compile-time memory-usage regression for DynFlags between GHC 8.0 and GHC 8.2 -------------------------------------+------------------------------------- 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: | -------------------------------------+------------------------------------- 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 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler