#4814: Nasty bug in RULE matching
---------------------------------+------------------------------------------
Reporter: simonpj | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.0.1
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
---------------------------------+------------------------------------------
Roman discovered a long-standing bug in RULE matching. . Suppose we have
this rule:
{{{
forall x y. f (g x) (h y) = i x y
}}}
which we match against this term:
{{{
f (let a = 1 in g a) (let b = False in h b)
}}}
This is the result of the rewrite:
{{{
let a = 1 in let b = False in (\x y -> i x y) a b
}}}
Fine so far. But suppose a and b have the same name (or, more precisely,
unique). Now the original term looks like this:
{{{
f (let a = 1 in g a) (let a = False in h a)
}}}
and gets rewritten to this:
{{{
let a = 1 in let a = False in (\x y -> i x y) a a
}}}
Disaster!
Here is a concrete test case:
{{{
module RuleLetFloat where
foo :: Int -> Int
{-# INLINE foo #-}
foo x = g (bar (x,x))
bar :: (Int,Int) -> Int
{-# NOINLINE bar #-}
bar (x,y) = x
baz :: Int -> Int
{-# NOINLINE baz #-}
baz x = x
f :: Int -> Int -> Int
{-# NOINLINE f #-}
f x y = x+y
g :: Int -> Int
{-# NOINLINE g #-}
g x = x
{-# RULES
"f/g" [1] forall x y. f (g x) (g y) = x + y
#-}
main = print $ f (foo (baz 1)) (foo (baz 2))
-- Should print 3
-- Bug means that it prints 4
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4814>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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