Handling of NaN

[Jan Stolarek]

> 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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