By the way, speaking of floating-point precision, is there a real reason why
haskell forces us to write :
foreign import ccall unsafe "math.h frexp" c_frexp::CDouble->(Ptr CInt)->IO ()
foreign import ccall unsafe "math.h ldexp" c_ldexp::CDouble->CInt->IO CDouble
ulp::Double->Double
ulp x=unsafePerformIO $ do
expon<-alloca (\e->do
c_frexp (realToFrac x) e
peek e)
(c_ldexp 0.5 $ expon-52) >>= return.realToFrac
To allow us to change the IEEE-754 rounding mode ? Shouldn't it be rather
"roundDown $ x+3*y/z" or "roundUp $ (cos x)**y" ?
Moreover, due to laziness it appears quite difficult (at least to me !) to
implement these roundDown and roundUp functions with C calls to select a
hardware rounding mode, then unsafePerformIO. It would be way faster than an
allocation (on the stack, I agree), then C calls with a memory access (probably
cached, but...) in between !
Does anyone know how this translates to LLVM (maybe in the forthcoming GHC
backend) ?
If anyone has got an answer or a solution...
Cheers,
PE
El 21/05/2010, a las 17:10, Don Stewart escribió:
> dvde:
>> Dear Haskellers,
>>
>> I just want to share an observation. I had to convert a Double to a
>> Float value in an inner loop of an application, and I used somethin like
>> this:
>>
>> xf = (fromRational $ toRational xd) :: Float
>>
>> The program works on windows but it did not on OSX - it was too slow.
>> Now, after big headaches and much frustration, I replaced the code above
>> with this line (why didn't I come up with this earlier?):
>>
>> xf = double2Float xd
>>
>> and now everything works just fine.
>>
>> I am not really surprised by the speed-up (and no-one should be), but I
>> am still surprised how often such kinds of unobvious problems occur
>> while programming in Haskell. So I write this email just to remind me
>> and you to look out for such pitfalls.
>
> There's no rewrite rule for this optimization. There's a ticket though
> with some of the solutions.
>
> -- Don
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