#14519: Exponential runtime performance regression in GHC 8.2 + Data.Text.Lazy + Text.RE.TDFA -------------------------------------+------------------------------------- Reporter: ntc2 | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Runtime Unknown/Multiple | performance bug Test Case: | Blocked By: https://github.com/ntc2/ghc-8.2.1 | -regex-lazy-text-bug | Blocking: | Related Tickets: 13745 Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- When using `Text.RE.TDFA` from the regex-tdfa package on GHC 8.2 to match trivial regexes against `Data.Text.Lazy` strings, I see **exponential runtime**. On GHC 8.0, or using `Data.Text` strict strings or `String` strings, the runtime is as expected. Here's my repo with simple test code illustrating the regression and timing stats: https://github.com/ntc2/ghc-8.2.1-regex-lazy-text-bug. In summary, for the problematic combination of GHC version and libraries, the run times are 3s, 10s, 22s, and 40s for counting regex matches in files with 10000, 20000, 30000, and 40000 lines, respectively. For all of the unproblematic combinations -- i.e. GHC 8.0.2, `String`, strict `Data.Text`, or building with profiling -- the run time is always about 1s! And **this is a Heisenbug**, in that the performance problem goes away if I build with profiling support! There is the separate Issue #13745 about **compile-time regressions** in GHC 8.2 + the regex-tdfa package, that [https://ghc.haskell.org/trac/ghc/ticket/13745#comment:18 I commented on]. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14519 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler