#5775: Inconsistency in demand analysis
------------------------------+---------------------------------------------
Reporter: rl | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.5 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: None/Unknown | Testcase:
Blockedby: | Blocking:
Related: |
------------------------------+---------------------------------------------
A small program:
{{{
{-# LANGUAGE MagicHash, UnboxedTuples #-}
module U where
import GHC.Prim
import GHC.Types
idx :: Addr# -> Int -> Int
{-# INLINE idx #-}
idx a (I# i) = case readIntOffAddr# a i realWorld# of (# _, y #) -> I# y
f :: Int -> Int -> Int
{-# INLINE f #-}
f x y = y + x
foo :: Addr# -> Int -> Int
foo a n = n `seq` loop (idx a 0) 1
where
loop x i = case i >= n of
False -> loop (f x (idx a i)) (i+1)
True -> x
}}}
GHC infers the demand `LU(L)` for `loop`, only unboxes the second
argument, ultimately generates a loop of type `Int -> Int# -> Int` and
always allocates a thunk for the first argument:
{{{
$wloop_si9 [Occ=LoopBreaker] :: Int -> Int# -> Int
[LclId, Arity=2, Str=DmdType LL]
$wloop_si9 =
\ (w1_shU :: Int) (ww1_shX :: Int#) ->
case >=# ww1_shX ww_si5 of _ {
False ->
$wloop_si9
(case readIntOffAddr# @ RealWorld w_si2 ww1_shX realWorld#
of _ { (# _, y_a9S #) ->
case w1_shU of _ { I# y1_ahb -> I# (+# y_a9S y1_ahb) }
})
(+# ww1_shX 1);
True -> w1_shU
}; }
}}}
But if I change the pragma on `f` from `INLINE` to `NOINLINE`, `loop` gets
the demand `U(L)U(L)m` and GHC manages to unbox both arguments as well as
the result, producing a nice tight loop:
{{{
$wloop_sii [Occ=LoopBreaker] :: Int# -> Int# -> Int#
[LclId, Arity=2, Str=DmdType LL]
$wloop_sii =
\ (ww1_shW :: Int#) (ww2_si0 :: Int#) ->
case >=# ww2_si0 ww_sib of _ {
False ->
case readIntOffAddr# @ RealWorld w_si8 ww2_si0 realWorld#
of _ { (# _, y1_Xac #) ->
case f (I# ww1_shW) (I# y1_Xac) of _ { I# ww3_Xin ->
$wloop_sii ww3_Xin (+# ww2_si0 1)
}
};
True -> ww1_shW
}; }
}}}
It would be nice if this happened in both cases.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5775>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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