Compile time performance degradation on code that uses undefined/error with CallStacks

GHC 8 has a lot of trouble compiling the following program:

module Serialize where

data Result a = Success a | Error String

{- 80 guards

   ghc-7.10.3 -O :  0.3s
   ghc-8.0.1 -O  :  1.8s 
-}

instance Functor Result where
    {-# INLINE fmap #-}
    fmap | bool = f
         | bool = f
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         | bool = f
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         | bool = f
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         | bool = f
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         | bool = f
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         | bool = f
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         | bool = f
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         | bool = f
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         | bool = f
         | bool = f
         | bool = f
         | bool = f
         | bool = f
         | bool = f

      where
        bool = undefined
        f = undefined

Here are some timing results, depending on the number of | bool = f clauses:

* ghc-8.0.1

N clauses : time (s)
10        : 0.61
20        : 0.78
40        : 1.03
80        : 1.64
160       : 2.83
320       : 5.16
640       : 10.37
1280      : 21.16

* ghc-7.10.3

N clauses : time (s)
10        : 0.33
20        : 0.29
40        : 0.34
80        : 0.30
160       : 0.32
320       : 0.35
640       : 0.48
1280      : 0.80

I think this compile time difference is caused by the CallStack changes introduced in GHC 8.0. When I use a version of undefined that doesn't have a CallStack, there is no difference in compile time when using GHC 7.10 or GHC 8.0.

This is my implementation of undefined without CallStack:

import GHC.Exception (errorCallException)
import GHC.Prim (raise#)
import Prelude (Char)

error :: [Char] -> a
error s = raise# (errorCallException s)

undefined :: a
undefined = error "undefined without callstack"

This is the quick and dirty Python script I used to generate those timing results (ghc version is hardcoded):

import os
import tempfile
import time
import subprocess

def src(n):
    return '''
module Test where

data Result a = Success a | Error String

instance Functor Result where
    {{-# INLINE fmap #-}}
    fmap {0}
      where
        bool = undefined
        f = undefined

'''.format('\n        | bool = f' * n)


tempfile = tempfile.mktemp('.hs')
print('tempfile = {0}'.format(tempfile))

print('N clauses : time (s)')
for i in range(8):
    n = 10 * 2 ** i
    with open(tempfile, 'w') as f:
        f.write(src(n))
        f.flush()

        t0 = time.time()
        subprocess.call(['ghc-8.0.1', '-fforce-recomp', '-v0', '-O', tempfile])
        t1 = time.time()

        print(str(n).ljust(10) + ': %.2f' % (t1 - t0))

os.remove(tempfile)