Re: [GHC] #14980: Runtime performance regression with binary operations on vectors

21 Jun 2018

      #14980: Runtime performance regression with binary operations on vectors
-------------------------------------+-------------------------------------
        Reporter:  ttylec            |                Owner:  bgamari
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:  vector
                                     |  bitwise operations
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by tdammers):

 Hmm, I spent all day trying to reproduce this, but I'm getting exactly the
 results you'd want. I built the package with cabal, once using ghc 8.0.2,
 and once using 8.2.2 (both installed from linux-x86_64 release bundles
 downloaded directly from haskell.org), and I get the same behavior across
 both compilers, with performance metrics similar to what you're seeing in
 the 8.0.2 case.

 Output from `performance-bug-pair1`, compiled with GHC 8.2.2:

 {{{
 tobias@zoidberg:~/well-typed/devel/ghc-T14980/ghc-bug/ > performance-bug-
 pair-1
 "Generated"
 benchmarking 256 columns/raw unbox vectors
 time                 478.6 μs   (477.8 μs .. 479.9 μs)
                      1.000 R²   (1.000 R² .. 1.000 R²)
 mean                 476.4 μs   (476.0 μs .. 477.0 μs)
 std dev              1.501 μs   (1.076 μs .. 2.501 μs)

 benchmarking 256 columns/binary packed
 time                 275.2 μs   (271.7 μs .. 278.6 μs)
                      0.999 R²   (0.998 R² .. 1.000 R²)
 mean                 280.2 μs   (276.6 μs .. 283.2 μs)
 std dev              6.706 μs   (5.090 μs .. 9.040 μs)
 variance introduced by outliers: 13% (moderately inflated)

 }}}

 And with 8.0.2:

 {{{
 "Generated"
 benchmarking 256 columns/raw unbox vectors
 time                 476.9 μs   (474.4 μs .. 480.5 μs)
                      1.000 R²   (1.000 R² .. 1.000 R²)
 mean                 473.5 μs   (472.5 μs .. 475.1 μs)
 std dev              4.097 μs   (2.212 μs .. 7.591 μs)

 benchmarking 256 columns/binary packed
 time                 291.3 μs   (286.8 μs .. 295.4 μs)
                      0.999 R²   (0.998 R² .. 1.000 R²)
 mean                 297.4 μs   (293.6 μs .. 300.1 μs)
 std dev              6.918 μs   (4.971 μs .. 9.873 μs)
 variance introduced by outliers: 12% (moderately inflated)

 }}}

 So there must be something about my setup that makes the bug disappear.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14980#comment:6
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