Handling of NaN

[Jan Stolarek]

> The rule can test for NaNs, but behave as before for non-NaNs.  That might be best, no?
I was thinking about that, but then I thought about such code:

f :: Bool
f = go 1 == go 2
  where nan = 0.0 / 0.0 :: Double
        go n = if not (isPrime (n * n - n + 41))
               then nan
               else go (n + 1)

The compiler would not be able to tell whether 'go' reduces to NaN or not (perhaps not the best 
possible example because the alternative value is _|_). It would be possible to test for NaNs in 
some trivial cases where one of the operands really is a NaN, but in general I believe it is 
impossible to test whether the expression reduces to NaN or not. And the rules need to be correct 
*always* not *sometimes*. Am I missing something? The only thing that comes to my mind is writing 
a rule that works only on literals, because for literals we can be sure they are not NaNs (on the 
other hand I doubt this rule would trigger often).

Janek

>
> Simon
>
> | -----Original Message-----
> | From: Jan Stolarek [mailto:jan.stolarek at p.lodz.pl]
> | Sent: 22 April 2013 15:51
> | To: Simon Peyton-Jones
> | Cc: ghc-devs at haskell.org
> | Subject: Re: Handling of NaN
> |
> | > Same happens in HEAD, so nothing to do with your changes.
> |
> | I didn't notice that, I was comparing against 7.6.2 :/
> |
> | >  Better define mkFloatingRelOpRule instead, which doesn't have the
> |
> | equal-args thing.
> | That's what I did initially, but I wasn't sure if that's acceptable
> | because some optimisations will be gone, e.g. ==# 3.0 3.0 will not
> | rewrite to #1 (perhaps this isn't that bad, because comparing floating
> | point numbers for equality isn't a good idea anyway).
> |
> | Janek
> |
> | > Simon
> | >
> | > | -----Original Message-----
> | > | From: ghc-devs-bounces at haskell.org
> | > | [mailto:ghc-devs-bounces at haskell.org]
> | > | On Behalf Of Jan Stolarek
> | > | Sent: 22 April 2013 13:48
> | > | To: ghc-devs at haskell.org
> | > | Subject: Handling of NaN
> | > |
> | > | I need some help with my work on ticket #6135. Consider this
> |
> | program:
> | > | {-# LANGUAGE BangPatterns, MagicHash #-} module Main where
> | > |
> | > | import GHC.Exts
> | > |
> | > | main = print $ nan## ==## nan##
> | > |   where !(D# nan##) = 0.0 / 0.0
> | > |
> | > | This prints False, which is a correct implementation of IEEE754
> | > | standard. However when I compile this with my modified compiler that
> | > | uses new comparison primops (they return Int# instead of
> | > | Bool) I get True, whcih obviously is incorrect. I belive that the
> | > | problem lies in this piece of code from prelude/PrelRules.hs:
> | > |
> | > | mkRelOpRule :: Name -> (forall a . Ord a => a -> a -> Bool)
> | > |             -> [RuleM CoreExpr] -> Maybe CoreRule mkRelOpRule nm cmp
> | > | extra
> | > |   = mkPrimOpRule nm 2 $ rules ++ extra
> | > |   where
> | > |     rules = [ binaryLit (\_ -> cmpOp cmp)
> | > |             , equalArgs >>
> | > |               -- x `cmp` x does not depend on x, so
> | > |               -- compute it for the arbitrary value 'True'
> | > |               -- and use that result
> | > |               return (if cmp True True
> | > |                         then trueVal
> | > |                         else falseVal) ]
> | > |
> | > | It looks that equalArgs suddenly started to return True, whereas it
> | > | previously returned False. On the other hand in GHCi I get correct
> | > | result (False). Can anyone give me a hint why is this happening?
> | > |
> | > | Janek
> | > |
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