A colleague of mine pointed out that ghc wasn't performing as he
expected when optimising some code. I wonder if anyone could offer
any insight as to why its not noting this common subexpression:
main = print $ newton 4 24
newton a 0 = a
newton a n = ((newton a (n-1))^2 + a)/(2*(newton a (n-1)))
Compiled with 'ghc -O3 --make perf.hs', results in:
real 0m5.544s
user 0m5.492s
sys 0m0.008s
However if we factor out the repeated call to the newton method:
main = print $ newton2 4 24
newton2 a 0 = a
newton2 a n = (x^2 + a)/(2*x)
where
x = newton2 a (n-1)
real 0m0.004s
user 0m0.004s
sys 0m0.004s
It looks to me like Referential transparency should make this a sound
optimisation to apply, but ghc isn't doing it even on -O3.
regards,
Richard
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