#7378: Identical alts/bad divInt# code
-------------------------------------+--------------------------------------
Reporter: daniel.is.fischer | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.6.1 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: Runtime performance bug | Testcase:
Blockedby: | Blocking:
Related: 7360 |
-------------------------------------+--------------------------------------
[http://stackoverflow.com/questions/13139875/removing-case-with-duplicate-
branches-from-haskells-core This] may be the same as #7360, but it may
also be a problem in the code generation for `divInt#`. I've reduced it to
the following minimal test case:
When compiling
{{{
fun :: Int -> Int
fun i = i `div` 2
}}}
with `ghc -O2 -funfolding-use-threshold=150`, the generated core becomes
{{{
Div2.fun =
\ (i_aeH :: GHC.Types.Int) ->
case i_aeH of _ { GHC.Types.I# ww_aiV ->
case GHC.Prim.># ww_aiV 0 of _ {
GHC.Types.False ->
case GHC.Prim.<# ww_aiV 0 of _ {
GHC.Types.False -> GHC.Types.I# (GHC.Prim.quotInt# ww_aiV 2);
GHC.Types.True ->
GHC.Types.I#
(GHC.Prim.-# (GHC.Prim.quotInt# (GHC.Prim.+# ww_aiV 1) 2) 1)
};
GHC.Types.True ->
case GHC.Prim.<# ww_aiV 0 of _ {
GHC.Types.False -> GHC.Types.I# (GHC.Prim.quotInt# ww_aiV 2);
GHC.Types.True ->
GHC.Types.I#
(GHC.Prim.-# (GHC.Prim.quotInt# (GHC.Prim.+# ww_aiV 1) 2) 1)
}
}
}
}}}
with two identical `case` alternatives, and in the `True` case, the next
comparison is guaranteed to return `False`.
The generated core is good if the divisor is negative, though, so that
speaks for taking a look at `divInt#`.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7378>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
_______________________________________________
Glasgow-haskell-bugs mailing list
Glasgow-haskell-bugs@haskell.org
http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
