Run time increases by 40% in fractal plotter core loop

I'm writing a toy fractal plotter to teach myself some Haskell. For a while I used GHC 7.4, recently upgraded to 7.10, and found a significant drop in run time performance. Tests with other versions show that the drop dates back to 7.8.

Not sure if this is a good practice, or if this is relevant here, but I used bangs to improve performance by adding strictness: this gives a good speedup with 7.4/7.6, but perf is unchanged with 7.8/7.10.

version	run time (sec) - bangs	no bangs
7.4.2	6.09	18.73
7.6.3	6.10	18.93
7.8.4	8.92	8.88
7.10.1	8.92	9.04

(All builds with ghc -O2.)

Source:

{-# LANGUAGE BangPatterns #-}

iterations :: Int -> Double -> Double -> Double -> Double -> Int -> Int
iterations !maxK !x !y !x0 !y0 !k =
  let !x2 = x * x
      !y2 = y * y
  in
   if k == maxK || x2 + y2 > 4
   then k
   else iterations maxK (x2 - y2 + x0) (2 * x * y + y0) x0 y0 (k + 1)

inSet :: Int -> Int -> (Double, Double) -> Bool
inSet !minK !maxK (!x, !y) = k >= minK && k < maxK
  where !k = iterations maxK x y x y 0

main = do
    let pointSource = repeat $ ((-0.6518209274278592), 0.3549264214581329)
        nPoints =  1000 * 1000
        points = take nPoints pointSource
        selected = filter (inSet 1000 (1000 * 20)) points
    print $ length selected == nPoints

The host C compiler is gcc 4.7.2.

Trac metadata

Trac field	Value
Version	7.8.4
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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