Handling of NaN

[Jan Stolarek]

Is there a way of measuring how often such a rule is triggered? I think no programmer will write a 
program which explicitly compares two floating point literals, but I'd like to know how often 
such cases result from program transformation.

Janek

Dnia wtorek, 23 kwietnia 2013, Simon Peyton-Jones napisał:
> 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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