#7088: Float-in bug
---------------------------------+------------------------------------------
Reporter: simonpj | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.4.2
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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Consider
{{{
let m = e
foo = \(s:State) (x:Int). body
in ...(foo s x)...
}}}
Then `loatIn.noFloatIntoRhs` will say `False`, because the top-level
lambda is oneshot
But the attempt to float into the lambda-group will fail, leaving
{{{
foo = let m = e in \s x. body
}}}
That changes foo’s arity, which is bad.
Moreover, m might have been marked “demanded” in the original program, by
the clever strictness analyser. Then the Simplifier will not float the
m-binding back out of ‘foo’, because it thinks that might make it lazier.
So then the Simplifier leaves the m-binding in foo’s RHS, and then eta-
expands it, giving
{{{
foo = \s. let m = e in \x. body
}}}
which is very bad:
{{{
*** Core Lint errors : in result of Simplifier ***
{-# LINE 11 "Float.hs #-}: Warning:
[RHS of f_snv :: GHC.Prim.State# GHC.Prim.RealWorld
-> GHC.Types.Int
-> (# GHC.Types.Int, GHC.Prim.State#
GHC.Prim.RealWorld #)]
Demand type has 2 arguments, rhs has 1 arguments, f_snv
Binder's strictness signature: DmdType LL
*** Offending Program ***
}}}
It's hard to trigger this bug but Geoff managed to, and this code does it:
{{{
{-# LANGUAGE MagicHash, UnboxedTuples #-}
module Float where
import GHC.Prim
foo vs
= let w = if length (reverse vs) > 10 then Just (length vs) else Nothing
f :: State# RealWorld -> Int -> (# Int, State# RealWorld #)
f s x | Just 0 <- w = case f s (x+1) of
(# r, s' #) -> (# r, s' #)
| otherwise = (# x, s #)
in f realWorld# 1
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7088>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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