[LLVMbugs] [Bug 216] [llvmgcc] floating-point unary minus is incorrect for +0.0

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.cs.uiuc.edu/bugs/show_bug.cgi?id=216

sabre at nondot.org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED
            Summary|floating-point unary minus  |[llvmgcc] floating-point
                   |is incorrect                |unary minus is incorrect for
                   |                            |+0.0
   Target Milestone|---                         |1.2



------- Additional Comments From sabre at nondot.org  2004-02-02 11:45 -------
I surreptitiously started a thread on the GCC list about this topic, and got the
following response from Geoff Keating:

"""I looked this up.  '-0.0 - X' is the same as '-0.0 + (-X)'.  When -X
!= 0.0, the result will be -X, so that's OK.  Suppose X is +/- 0.0.
Then the following rules apply:

- If both operands have the same sign, the sign of the result is the
  same as the sign of the operands.  So if X is +0.0, you'll get -0.0,
  the same as -X.

- If the sum of two operands with opposite sign is exactly zero, the
  sign is positive in all rounding modes except round towards -Inf, in
  which case the sign is negative.  So, if X is -0.0, you'll get
  +0.0, the same as -X, *unless* we're rounding towards -Inf.

Thus, the answer to your question is "no".  The case that differs is
when X is -0.0 and the rounding mode is towards -Inf."""


Which I interpret to mean "yes", it's the same in all cases except when the
rounding mode is tweaked to be -Inf, which is not the default.  As this is the
case, I've commited the following patch to the CFE:
http://mail.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20040202/011060.html

Note that will still will eventually need "strict" vs "non-strict" floating
point operators, particularly to support languages like Fortran and Java well.

-Chris



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