#3744: Comparisons against minBound/maxBound not optimised
---------------------------------+------------------------------------------
Reporter: rl | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 7.0.1
Component: Compiler | Version: 6.13
Keywords: | Testcase:
Blockedby: | Difficulty:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: Runtime performance bug
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Changes (by michalt):
* cc: michal.terep...@… (added)
Comment:
I've created a patch with rules for PrelRules.lhs that detect comparisons
with
minBound and maxBound that are always true or always false (like in the
above
examples). And it seems to work just fine:
{{{
foo1, foo2, foo3, foo4 :: Int -> Bool
foo1 n = n < minBound
foo2 n = n > maxBound
foo3 n = n >= minBound
foo4 n = n <= maxBound
}}}
gives now
{{{
Test.foo1 =
\ (n_aaD :: GHC.Types.Int) ->
case n_aaD of _ { GHC.Types.I# _ -> GHC.Bool.False }
Test.foo2 =
\ (n_ag0 :: GHC.Types.Int) ->
case n_ag0 of _ { GHC.Types.I# _ -> GHC.Bool.False }
Test.foo3 =
\ (n_ag1 :: GHC.Types.Int) ->
case n_ag1 of _ { GHC.Types.I# _ -> GHC.Bool.True }
Test.foo4 =
\ (n_ag2 :: GHC.Types.Int) ->
case n_ag2 of _ { GHC.Types.I# _ -> GHC.Bool.True }
}}}
This works fine also for Char, and Word, but interestingly in case of
Int64 only
maxBound is inlined, but not minBound (and so foo2 and foo4 are optimised
but
not foo1 and foo3), whereas in case of Word64 the opposite happens. For
example
by changing the type to Int64 in the above code we have:
{{{
Test.foo1 =
\ (n_aaF :: GHC.Int.Int64) ->
case n_aaF of _ { GHC.Int.I64# a1_ar3 ->
case GHC.Int.$fBoundedInt64_$cminBound
of _ { GHC.Int.I64# b1_ar7 ->
GHC.Prim.<# a1_ar3 b1_ar7
}
}
Test.foo2 =
\ (n_ag2 :: GHC.Int.Int64) ->
case n_ag2 of _ { GHC.Int.I64# _ -> GHC.Bool.False }
Test.foo3 =
\ (n_ag3 :: GHC.Int.Int64) ->
case n_ag3 of _ { GHC.Int.I64# a1_arn ->
case GHC.Int.$fBoundedInt64_$cminBound
of _ { GHC.Int.I64# b1_arr ->
GHC.Prim.>=# a1_arn b1_arr
}
}
Test.foo4 =
\ (n_ag4 :: GHC.Int.Int64) ->
case n_ag4 of _ { GHC.Int.I64# _ -> GHC.Bool.True }
}}}
Anyway, I'll attach the patch in a second. And please let me know what do
you
think about it (especially since I don't really know a lot about GHC
internals).
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3744#comment:5>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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