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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".

Trac metadata

Trac field	Value
Version	6.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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