Hello.

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

{-# 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:

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"