#16029: Inadequate absence analysis
-------------------------------------+-------------------------------------
Reporter: simonpj | Owner: (none)
Type: bug | Status: new
Priority: normal | Milestone: 8.6.3
Component: Compiler | Version: 8.6.2
Keywords: | Operating System: Unknown/Multiple
Architecture: | Type of failure: None/Unknown
Unknown/Multiple |
Test Case: | Blocked By:
Blocking: | Related Tickets:
Differential Rev(s): | Wiki Page:
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Consider
{{{
data S = MkS Int Int
g1 :: S -> Int -> Int
g1 (MkS x y) 0 = 0
g1 (MkS x y) n = g1 (MkS y x) (n-1)
}}}
With GHC 8.6 we get
{{{
$wg1 :: S -> GHC.Prim.Int# -> GHC.Prim.Int#
[Str=]
$wg1 = \ (w_s2oH :: S)
(ww_s2oL :: GHC.Prim.Int#) ->
case w_s2oH of { MkS x_s2pz y_s2pA ->
case ww_s2oL of ds_X2nb {
__DEFAULT ->
Foo.$wg1 (Foo.MkS y_s2pA x_s2pz) (GHC.Prim.-# ds_X2nb 1#);
0# -> 0# }}
g1 :: S -> Int -> Int
[Str=m]
g1 = \ (w_s2oH :: S) (w1_s2oI :: Int) ->
case w_s2oH of w2_X2pG { MkS ipv_s2p2 ipv1_s2p3 ->
case w1_s2oI of { GHC.Types.I# ww1_s2oL ->
case Foo.$wg1 w2_X2pG ww1_s2oL of ww2_s2oP { __DEFAULT ->
GHC.Types.I# ww2_s2oP }}}
}}}
What terrible code! We evaluate the S argument in the wrapper,
and box and unbox it every time around the loop, even though it is never
ultimately used.
Here's what happens if the arguments are banged:
{{{
data T = MkT !Int !Int
g2 :: T -> Int -> Int
g2 (MkT x y) 0 = 0
g2 (MkT x y) n = g2 (MkT y x) (n-1)
}}}
We get
{{{
$wg2 GHC.Prim.Int# -> GHC.Prim.Int# -> GHC.Prim.Int# -> GHC.Prim.Int#
[Str=]
Foo.$wg2 = \ (ww_s2ow :: GHC.Prim.Int#)
(ww1_s2ox :: GHC.Prim.Int#)
(ww2_s2oB :: GHC.Prim.Int#) ->
case ww2_s2oB of ds_X2n0 {
__DEFAULT -> Foo.$wg2 ww1_s2ox ww_s2ow (GHC.Prim.-# ds_X2n0
1#);
0# -> 0# }
g2 :: T -> Int -> Int
[Str=m ]
g2 = \ (w_s2os :: T) (w1_s2ot :: Int) ->
case w_s2os of { MkT ww1_s2ow ww2_s2ox ->
case w1_s2ot of { GHC.Types.I# ww4_s2oB ->
case Foo.$wg2 ww1_s2ow ww2_s2ox ww4_s2oB of ww5_s2oF {
__DEFAULT -> GHC.Types.I# ww5_s2oF }}}
}}}
Still terrible. We pass the two components around the loop before
discarding them
at the end.
--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16029
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler
