#6047: GHC retains unnecessary binding
---------------------------------+------------------------------------------
Reporter: simonmar | Owner: simonpj
Type: bug | Status: new
Priority: normal | Milestone: 7.6.1
Component: Compiler | Version: 7.4.1
Keywords: | Os: Unknown/Multiple
Architecture: Unknown/Multiple | Failure: None/Unknown
Difficulty: Unknown | Testcase:
Blockedby: | Blocking:
Related: |
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milan posted this example on #6042, I'm making a separate ticket for it.
He says:
----------
BTW, when I preparing for the advanced functional programming course, I
found out that the following code
{{{
module Test where
factorial :: Int -> Int
factorial n | n > 0 = f n 1
where f 0 acc = acc
f n acc = f (n-1) (n * acc)
}}}
produces the following STG, on both GHC 7.0.4 and 7.4.1, with unused
method `factorial_f`:
{{{
Test.factorial2 =
\u srt:(0,*bitmap*) []
Control.Exception.Base.patError
"a.hs:(4,1)-(6,35)|function factorial";
SRT(Test.factorial2): [Control.Exception.Base.patError]
Test.$wf =
\r [ww_srk ww1_sro]
case ww_srk of wild_srm {
__DEFAULT ->
case *# [wild_srm ww1_sro] of sat_srK {
__DEFAULT ->
case -# [wild_srm 1] of sat_srL {
__DEFAULT -> Test.$wf sat_srL sat_srK;
};
};
0 -> ww1_sro;
};
SRT(Test.$wf): []
Test.factorial_f =
\r [w_srs w1_srv]
case w_srs of w2_srN {
GHC.Types.I# ww_sry ->
case w1_srv of w3_srM {
GHC.Types.I# ww1_srz ->
case Test.$wf ww_sry ww1_srz of ww2_srB {
__DEFAULT -> GHC.Types.I# [ww2_srB];
};
};
};
SRT(Test.factorial_f): []
Test.factorial1 = NO_CCS GHC.Types.I#! [1];
SRT(Test.factorial1): []
Test.factorial =
\r srt:(0,*bitmap*) [n_srD]
case n_srD of wild_srP {
GHC.Types.I# x_srG ->
case ># [x_srG 0] of wild1_srO {
GHC.Types.False -> Test.factorial2;
GHC.Types.True ->
case Test.$wf x_srG 1 of ww_srJ {
__DEFAULT -> GHC.Types.I# [ww_srJ];
};
};
};
SRT(Test.factorial): [Test.factorial2]
}}}
The `Test.factorial_f` appears also in asm and in the object file.
----------
simonpj explained to me why this is happening: `factorial_f` is being
retained because it is referenced in an INLINE pragma for `factorial`. In
fact it will never be used, because `factorial_f` itself is also INLINE.
We're going to investigate possible workarounds/solutions.
Note it's only a code size issue, performance is not affected (compile
time is affected a little though). We're not sure how much it happens.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6047>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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