On 22 May 2010 16:06, Daniel Fischer
On Saturday 22 May 2010 16:48:27, Daniel Fischer wrote:
The boxing is due to the use of (^). If you write x*x instead of x^2, it can use the primop *## and needn't box it. As a side effect, the original time leak probably wouldn't have occured with x*x instead of x^2 because one would've made it let x = newton a (n-1) in (x*x +a) / (2*x) instead of writing out newton a (n-1) thrice anyway, wouldn't one?
Even if. With
newton :: Double -> Int -> Double newton a 0 = a newton a n = (((newton a (n-1)) * (newton a (n-1)) ) + a)/(2*(newton a (n-1)))
(and optimisations of course), GHC does share newton a (n-1).
Lesson: Writing x^2 is a baad thing.
Interesting. Clearly GHC needs a better partial evaluator! :) (^) is not inlined because it's recursive (or rather it's worker is) and there also is no SPECIALISE pragma for Double -> Integer -> Double. Yes, it's Integer, not Int, because the literal "2" defaults to Integer. It doesn't seem to be possible to add SPECIALISE pragmas for non-local functions. If I copy over the definition of (^) no pragma is needed. GHC creates an worker for Double# -> Integer -> Double# and that seems to be sufficient to make CSE work. -- Push the envelope. Watch it bend.