#10108: Dramatic slowdown with -O2 bytestream and list streams combined. -------------------------------------+------------------------------------- Reporter: Truman | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.10.2 Component: Compiler | Version: 7.8.4 Resolution: | Keywords: Operating System: Linux | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | break_in.hs Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by Truman): Sorry about not including the library information. I should have known better. Here is a log showing compiling non-optimized, running, compiling optimized, and running again, that shows the runtime difference. I shortened the input file (described above) to 5000 lines to speed things up, but the problem is still clear. The versions of the two seemingly relevant imports are: bytestring-0.10.4.0 stream-fusion 0.1.2.5 The log files detail lots of other information. I'm running GHC 7.8.4 under Arch Linux 3.19.2-1-ARCH, and I have tested and confirmed this on both 32 and 64 bit machines. I will attach the two verbose compilation log files. CPC~/ghc_prob> touch break_ins.hs CPC~/ghc_prob> ghc -v5 break_ins.hs >ghc_nopt.log 2>&1 CPC~/ghc_prob> time break_ins bi_nopt.out real 0m0.077s user 0m0.077s sys 0m0.003s CPC~/ghc_prob> touch break_ins.hs CPC~/ghc_prob> ghc -v5 -O2 break_ins.hs >ghc_opt.log 2>&1 CPC~/ghc_prob> time break_ins bi_opt.out real 0m3.110s user 0m3.090s sys 0m0.017s CPC~/ghc_prob> -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10108#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#10108: Dramatic slowdown with -O2 bytestream and list streams combined. -------------------------------------+------------------------------------- Reporter: Truman | Owner: Type: bug | Status: closed Priority: normal | Milestone: 7.10.2 Component: Compiler | Version: 7.8.4 Resolution: invalid | Keywords: Operating System: Linux | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | break_in.hs Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Changes (by rwbarton): * status: new => closed * resolution: => invalid Comment: It's a bug or limitation in the stream-fusion package. This simpler program has the same problem: {{{ import qualified Data.List.Stream as SL bad_sum :: [Int] -> Int bad_sum xs = if null xs then 0 else head xs + bad_sum (SL.tail xs) main = print $ bad_sum [1..20000] }}} The reason is this rewrite rule for `SL.tail`: {{{ {-# RULES "tail -> fusible" [~1] forall xs. tail xs = unstream (Stream.tail (stream xs)) --"tail -> unfused" [1] forall xs. -- unstream (Stream.tail (stream xs)) = tail xs #-} }}} Note that the second rule is commented out! So each recursive call adds a layer of something similar to `foldr (:) []` to the remainder of the list, and the total time is quadratic. You might submit a bug report on stream-fusion, though I don't know whether it is still maintained. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10108#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#10108: Dramatic slowdown with -O2 bytestream and list streams combined. -------------------------------------+------------------------------------- Reporter: Truman | Owner: Type: bug | Status: closed Priority: normal | Milestone: 7.10.2 Component: Compiler | Version: 7.8.4 Resolution: invalid | Keywords: Operating System: Linux | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | break_in.hs Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by simonpj): Thanks for investigating, Reid. Very helpful. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10108#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler