[GHC] #16004: Vector performance regression in GHC 8.6

[GHC]

#16004: Vector performance regression in GHC 8.6
-------------------------------------+-------------------------------------
           Reporter:  guibou         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.3
          Component:  Compiler       |           Version:  8.6.2
           Keywords:                 |  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:
-------------------------------------+-------------------------------------
 Hello.

 With the following code, I can observe a performance regression between
 ghc 8.4 and 8.6:


 {{{#!haskell
 {-# LANGUAGE ScopedTypeVariables #-}
 module Main where
 import qualified Data.Vector.Unboxed.Mutable as Vector
 import qualified Data.Vector.Unboxed as VectorU

 import Data.Foldable (for_)

 main :: IO ()
 main = do
   let n = 1000

   let vUnmutable :: VectorU.Vector Double = VectorU.generate (n * n) (\i
 -> fromIntegral i)
   v :: Vector.IOVector Double <- VectorU.unsafeThaw vUnmutable

   for_ [0..(n - 1)] $ \k -> do
     for_ [0..(n - 1)] $ \i -> do
       for_ [0..(n - 1)] $ \j -> do
         a <- Vector.unsafeRead v (i * n + k)
         b <- Vector.unsafeRead v (k * n + j)
         c <- Vector.unsafeRead v (i * n + j)
         Vector.unsafeWrite v (i * n + j) (min (a + b) c)
 }}}

 Built with `-O2` and with / without `-fllvm`. I'm using `vector-0.12.0.1`.
 Here are the timing results:

 GHC 8.2.2
    no llvm: 1.7s
    llvm: 1.0s
 GHC 8.4.4
    no llvm: 1.6s
    llvm: 0.9s
 GHC 8.6.2
    no llvm: 4.8s
    llvm: 4.3s

 I'm using the following bash + nix script to gather theses timings:

 {{{#!bash
 nix-shell -p 'haskell.packages.ghc822.ghcWithPackages(p: [p.vector])'
 --run "ghc-pkg list | grep vector; ghc -O2 FloydBench.hs -Wall -fforce-
 recomp; time ./FloydBench"
 nix-shell -p 'haskell.packages.ghc822.ghcWithPackages(p: [p.vector])' -p
 llvm_39 --run "ghc-pkg list | grep vector; ghc -O2 FloydBench.hs -Wall
 -fforce-recomp -fllvm; time ./FloydBench"
 nix-shell -p 'haskell.packages.ghc844.ghcWithPackages(p: [p.vector])'
 --run "ghc-pkg list | grep vector; ghc -O2 FloydBench.hs -Wall -fforce-
 recomp; time ./FloydBench"
 nix-shell -p 'haskell.packages.ghc844.ghcWithPackages(p: [p.vector])' -p
 llvm_5 --run "ghc-pkg list | grep vector; ghc -O2 FloydBench.hs -Wall
 -fforce-recomp -fllvm; time ./FloydBench"
 nix-shell -p 'haskell.packages.ghc862.ghcWithPackages(p: [p.vector])'
 --run "ghc-pkg list | grep vector; ghc -O2 FloydBench.hs -Wall -fforce-
 recomp; time ./FloydBench"
 nix-shell -p 'haskell.packages.ghc862.ghcWithPackages(p: [p.vector])' -p
 llvm_6 --run "ghc-pkg list | grep vector; ghc -O2 FloydBench.hs -Wall
 -fforce-recomp -fllvm; time ./FloydBench"
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16004>
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