#15019: Fix performance regressions from #14737 -------------------------------------+------------------------------------- Reporter: tdammers | Owner: tdammers Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: #14737 | Differential Rev(s): phab:D4568 Wiki Page: | -------------------------------------+------------------------------------- Comment (by tdammers): Replying to [comment:5 simonpj]:
Eh? The code you give doesn't look like a coercion. It looks like a type, or a pair of types. I must be misunderstanding.
Sorry about that, didn't realize that this was misleading without the relevant context. So we have this code here: {{{ pushCoTyArg co ty | isReflCo co = Just (ty, Nothing) -- The following is inefficient - don't do `eqType` here, the coercion -- optimizer will take care of it. See Trac #14737. | tyL `eqType` tyR = pprTrace "eqType fired: " (ppr (tyL, tyR)) $ -- <- this is where we're doing the trace logging Just (ty, Nothing) | isForAllTy tyL = ASSERT2( isForAllTy tyR, ppr co $$ ppr ty ) Just (ty `mkCastTy` mkSymCo co1, Just co2) | otherwise = Nothing where Pair tyL tyR = coercionKind co -- co :: tyL ~ tyR -- tyL = forall (a1 :: k1). ty1 -- tyR = forall (a2 :: k2). ty2 }}} So the two types we see in this log are `tyL` and `tyR`, calculated using `coercionKind co`. And our goal is to find a drop-in replacement for `isReflCo` that detects most of the coercions that hit the `eqType` guard right now, but is significantly cheaper than `eqType` (and `coercionKind` / `coercionKindRole`). It may become less confusing if we dump the actual coercion too: {{{ pushCoTyArg co ty | isReflCo co = Just (ty, Nothing) -- The following is inefficient - don't do `eqType` here, the coercion -- optimizer will take care of it. See Trac #14737. | tyL `eqType` tyR = pprTrace "eqType fired: " (ppr (co, tyL, tyR)) $ Just (ty, Nothing) }}} This gives us output like the following: {{{ eqType fired: (Sym (N:GQuu[0] <K1 R Bool>_N) ; N:GQuu[0] <K1 R Bool>_N, forall a. K1 R Bool a -> [String] -> [String], forall a. K1 R Bool a -> [String] -> [String]) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15019#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler