Handling of NaN

[Simon Peyton-Jones]

Just so.  You could make a float rule that constant-folded 
	lit1 == lit2
to True if lit1 and lit2 were the same, and were not NaNs.

As you point out, being syntactically equal expressions isn't enough.

Simon

| -----Original Message-----
| From: Jan Stolarek [mailto:jan.stolarek at p.lodz.pl]
| Sent: 23 April 2013 09:46
| To: Simon Peyton-Jones
| Cc: ghc-devs at haskell.org
| Subject: Re: Handling of NaN
| 
| > 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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| > | > | ghc-devs at haskell.org
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|