On Saturday 22 May 2010 15:00:25, Thomas Schilling wrote:
Actually, in this case it would be safe to do CSS. Because
a) the function is strict in both arguments so GHC creates a worker which only uses unboxed types b) this cannot cause any space leaks (it contains no pointers)
The generated Core does look pretty weird, though:
$wnewton = \ (ww_s115 :: Double#) (ww1_s119 :: Int#) -> case ww1_s119 of ds_Xr8 { __DEFAULT -> case ^_r11D (case $wnewton ww_s115 (-# ds_Xr8 1) of ww2_s11d { __DEFAULT -> D# ww2_s11d -- box the result of $wnewton }) lvl_r11B of _ { D# x_avk -> -- unbox it again
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?
case $wnewton ww_s115 (-# ds_Xr8 1) of ww2_s11d { __DEFAULT -> /## (+## x_avk ww_s115) (*## 2.0 ww2_s11d) } }; 0 -> ww_s115 }