Exponential runtime performance regression in GHC 8.2 + Data.Text.Lazy + Text.RE.TDFA
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/tree/07b7bb32c6e90e8f2d2eada4b59943f37e632d53 .
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,
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!