#14737: Improve performance of Simplify.simplCast -------------------------------------+------------------------------------- Reporter: tdammers | Owner: (none) Type: bug | Status: patch Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: #11735 #14683 | Differential Rev(s): Phab:D4385 Wiki Page: | -------------------------------------+------------------------------------- Comment (by tdammers): Replying to [comment:13 simonpj]:
On to `mkInstCo`. I see that it does an inefficient single-variable substitution. So if we have {{{ mkInstCo (mkInstCo (mkInstCo co (Refl t1)) (Refl t2)) (Refl t3) }}} we will traverse `co` three times (and `t1` twice etc). Bad bad.
Idea: leave that to the coercion optimiser. Try simply removing the first equation for `mkInstCo` leaving {{{ mkInstCo = InstCo }}}
That doesn't seem to work well at all - I don't know how long exactly it takes, but it's been compiling Grammar.hs for about half an hour now, so I think it's safe to say that it's much slower now than the 8 seconds we got previously. Unfortunately it's still running, so I can't say where the time is going yet. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14737#comment:14 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler