#2209: MagicHash extraction is wrong on x86_64 with -fasm -O2
-------------------------------+--------------------------------------------
Reporter: quark | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 6.8.2 | Severity: major
Keywords: | Testcase:
Architecture: x86_64 (amd64) | Os: Linux
-------------------------------+--------------------------------------------
I am trying to directly get at the underlying IEEE representation of a
Double with this code:
{{{
word64ToDouble :: Word64 -> Double
word64ToDouble w@(W64# x) = D# (unsafeCoerce# x)
doubleToWord64 :: Double -> Word64
doubleToWord64 d@(D# x) = W64# (unsafeCoerce# x)
}}}
I have, of course, checked "isEEE x" and "(sizeOf x) == 8" etc, to know
that it is OK to do this.
This works on various x86 computers but fails on x86_64, when compiling
with -fasm and -O2 (which I embedded with the OPTIONS pragma in the
attached example).
The example shows that for a Double "2.0", the Word64 is wrong. And for a
Word64 which should represent Nan, "isNaN" fails. Here is the output of
the program on "x86_64" (the buggy system):
{{{
False: NaN
5697656
expect: 4611686018427387904
}}}
Here is the expected output as seen on x86 systems:
{{{
True
4611686018427387904
expect: 4611686018427387904
}}}
Note that if you insert a "trace" call to display the Integer, Word64, or
Double, the example gives the right output! But when you take out the
trace, it fails.
If a workaround for this problem can be identified, we would really
appreciate it, as we are hitting this bug in code that we need to use but
cannot workaround. If a fix is identified, it would be great if the fix
made it into 6.8.3 (as we are happy to upgrade to that, rather than wait
for a next stable release).
In our system, we do have a workaround for the "doubleToWord64" bug, but I
cannot reproduce it in the small example. What we do is define the
function like this:
{{{
doubleToWord64 :: Double -> Word64
doubleToWord64 d@(D# x) =
if (d == d)
then W64# (unsafeCoerce# x)
else err ("doubleToWord64 " ++ show d)
}}}
This works in out system, but not in the attached example. And, again, we
have no workaround for "word64ToDouble".
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2209>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler_______________________________________________
Glasgow-haskell-bugs mailing list
[email protected]
http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs
