#12150: Compile time performance degradation on code that uses undefined/error with CallStacks -------------------------------------+------------------------------------- Reporter: thomie | Owner: (none) Type: bug | Status: new Priority: high | Milestone: 8.2.2 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: #10844 | Differential Rev(s): Phab:D3753 Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj):
For some reason, we're not eliminating the ...
I assume this is just in the ''first'' simplifier run, when `case-of-case` is off? In the second simplifier run, the `case` gets eliminated, correct? I know why this is. In `Simplify` we have: {{{ simplExprF1 env (Case scrut bndr _ alts) cont | sm_case_case (getMode env) = simplExprF env scrut (Select { sc_dup = NoDup, sc_bndr = bndr , sc_alts = alts , sc_env = env, sc_cont = cont }) | otherwise = do { (env', scrut') <- simplExprF (zapFloats env) scrut $ mkBoringStop (substTy env (idType bndr)) ; let scrut'' = wrapJoinFloats (seJoinFloats env') scrut' env'' = env `addLetFloats` env' ; rebuildCase env'' scrut'' bndr alts cont } }}} That is, if `case-of-case` is off, we simplify the scrutinee in a trivial continuation, and then wrap the alternatives around it. So disabling case-of-case has also disabled case-of-bottom. I did this fairly recently, which accounts for the change. Let me think about a better way. Meanwhile you are working on that `botSig` stuff, making it work with functions too, right? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12150#comment:19 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler