id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	os	architecture	failure	difficulty	testcase	blockedby	blocking	related
2823	Another arity expansion bug	simonpj		"Roman reports: I've finally tracked down one big optimisation problem (at least, I
think it is big). Here is a small example:
{{{
foo :: Eq a => a -> a
{-# NOINLINE foo #-}
foo x = x

bar :: Eq a => a -> a
{-# INLINE [1] bar #-}
bar x = let p = foo (x,x)
             q = foo (p,p) in fst (fst q)
}}}
For some reason, bar's arity is 1 which is wrong. If we replace `(fst (fst q))` by `(fst p)`, it gets the correct arity of 2.

The problem is that because of the arity, `(bar $dEq)` is then floated
out as far as possible which breaks fusion if we have RULES for bar.
In case you are interested, this affects `splitSD` in `dph-prim-par/Data/
Array/Parallel/Unlifted/Distributed/Arrays.hs`. I haven't noticed this
previously because we didn't use segmented arrays as much.

"	bug	new	lowest	7.6.2	Compiler	6.10.1				Unknown/Multiple	Unknown/Multiple	Runtime performance bug	Unknown