Handling of NaN

[Simon Peyton-Jones]

Use -ddump-rule-firings to display rule firings

Simon

| -----Original Message-----
| From: Jan Stolarek [mailto:jan.stolarek at p.lodz.pl]
| Sent: 24 April 2013 07:53
| To: Simon Peyton-Jones
| Cc: ghc-devs at haskell.org
| Subject: Re: Handling of NaN
| 
| 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
| > | > | > |
| > | > | > | _______________________________________________
| > | > | > | ghc-devs mailing list
| > | > | > | ghc-devs at haskell.org
| > | > | > | http://www.haskell.org/mailman/listinfo/ghc-devs
|