[GHC] #14414: Profiled program runs 2.5x faster than non-profiled

[GHC]

#14414: Profiled program runs 2.5x faster than non-profiled
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           Reporter:  Fuuzetsu       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I was looking at benchmarks game (attached as fasta.ghc-2.hs). I have
 found that with the flags give there, this program on GHC 8.2.1 runs in
 about a second with `-prof -fprof-auto` and 2.5 seconds without!

 To run without profiling:
 {{{
 ghc --make -Wall -fforce-recomp -fllvm -O2 -XBangPatterns -threaded
 -rtsopts -XOverloadedStrings fasta.ghc-2.hs -o fasta.ghc-2.ghc_run &&
 ./fasta.ghc-2.ghc_run +RTS -N4 -s -RTS 250000 > /dev/null
 }}}

 Same program with profiling
 {{{
  ghc --make -fforce-recomp -fllvm -prof -fprof-auto -O2 -XBangPatterns
 -threaded -rtsopts -XOverloadedStrings fasta.ghc-2.hs -o
 fasta.ghc-2.ghc_run && ./fasta.ghc-2.ghc_run +RTS -N4 -p -s -RTS 250000 >
 /dev/null
 }}}

 I also attach Core outputs for both profiled and unprofiled version.

 To me this seems very strange: profiled version is somehow faster. Perhaps
 what's worse is that this means that there's some optimisation GHC is
 performing when profiling is not on that makes the program a lot slower
 than it could be!

 This program is not minimised.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14414>
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