[Haskell-cafe] Code runs 7x FASTER with profiling

[Brandon Allbery]

Bugs, usually. I think there was one such in 8.2.1 and if so it may not
have been completely fixed (trying to read around the edges of some dev
discussion, since I'm kinda far from expert when it comes to ghc internals).

On Thu, Dec 7, 2017 at 4:25 PM, Neil Mayhew <
neil_mayhew at users.sourceforge.net> wrote:

> On 2017-12-07 12:49 AM, Jonas Scholl wrote:
>
> Looking at the produced core of both versions reveals that in the profiled
> build a closure of type Regex is floated to top level. The non-profiled
> build doesn’t do this, thus it recompiles the regex for every iteration.
> This is most likely the source of the slowdown of the non-profiled build.
>
> Thanks, Jonas. This does indeed seem to be the problem. I changed the code
> to use a compiled regex (with makeRegex and match instead of =~) but in
> the non-profiling case the run-time doesn’t improve unless I float the
> compiled regex myself:
>
> parseFilename :: String -> Either String (String, String)parseFilename fn = case (pattern `match` fn :: [[String]]) of
>     [[_, full, _, time]] -> Right $ (full, time)
>     _ -> Left fn
> pattern :: Regexpattern = makeRegex
>     "^\\./duplicity-(full|inc|new)(-signatures)?\\.\
>     \([0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9]T[0-9][0-9][0-9][0-9][0-9][0-9]Z)\\."
>
> Then it runs 2-3x faster than the profiled code.
>
> The question remains, however: why doesn’t the ghc optimizer spot this
> fairly obvious loop-invariant in the non-profiled build when it does manage
> to spot it in the profiled one? In other words, when I make pattern a
> local definition of parseFilename, why isn’t it treated as a CAF that’s
> evaluated only once (‘floated to the top level’)? Enabling profiling
> shouldn’t change the meaning of a program.
>
> I remember back in the day having to be careful with regexes in Python to
> make sure they were always precompiled outside of loops and functions, but
> one of the nice things about Haskell is that one can usually let the
> compiler take care of this. (Nowadays Python gets around this by caching
> compiled regexes, but I prefer Haskell’s statically-optimized approach.)
> ​
>
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-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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