#13253: Exponential compilation time with RWST & ReaderT stack with `-02` -------------------------------------+------------------------------------- Reporter: phadej | Owner: dfeuer Type: bug | Status: new Priority: normal | Milestone: 8.4.1 Component: Compiler | Version: 8.0.1 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 mpickering): Looking at the n=2 case for simplicity. At one stage of the compilation we have some core which looks like {{{ ($c<*>_a4dL (case x01_a2dZ of { FormMissing -> FormMissing; FormFailure errs_a2dS -> FormFailure errs_a2dS; FormSuccess a_a2dU -> FormSuccess (\ dt_a2M6 -> case a_a2dU of dt_X2M8 { PS ipv_s5JI ipv_s5JJ ipv_s5JK ipv_s5JL -> case dt_a2M6 of dt_X2Ma { PS ipv_s5JO ipv_s5JP ipv_s5JQ ipv_s5JR -> HugeStruct dt_X2M8 dt_X2Ma } }) }) x02_a2e0 }}} Next thing we know, the expression has been commuted and the call to $c<*> has been pushed into the branches !? {{{ (case x01_a2dZ of { FormMissing -> $c<*>_a4dL FormMissing x02_a2e0; FormFailure errs_a2dS -> $c<*>_a4dL (FormFailure errs_a2dS) x02_a2e0; FormSuccess a_a2dU -> $c<*>_a4dL (FormSuccess (\ dt_a2M6 -> case a_a2dU of dt_X2M8 { PS ipv_s5JI ipv_s5JJ ipv_s5JK ipv_s5JL -> case dt_a2M6 of dt_X2Ma { PS ipv_s5JO ipv_s5JP ipv_s5JQ ipv_s5JR -> HugeStruct a_a2dU dt_a2M6 } })) x02_a2e0 }}} and then `$c<*>_a4dL` is inlined individually in each branch rather than once at the top level. I don't know where in the compiler performs this transformation, perhaps you know where it happens David? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13253#comment:15 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler