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Subject: [Bug fortran/55539] [4.8 Regression] -fno-sign-zero may generate output with the wrong sign
Date: Tue, 11 Dec 2012 22:33:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55539

--- Comment #3 from Harald Anlauf <anlauf at gmx dot de> 2012-12-11 22:33:21 UTC ---
(In reply to comment #2)

I played a little bit with the code and found that the
following patch appears to fix my problem:

Index: libgfortran/io/write_float.def
===================================================================
--- libgfortran/io/write_float.def      (revision 194417)
+++ libgfortran/io/write_float.def      (working copy)
@@ -492,7 +492,7 @@
   /* To format properly, we need to know if the rounded result is zero and if
      so, we set the zero_flag which may have been already set for
      actual zero.  */
-  if (i == ndigits)
+  if (i == ndigits && (digits[i] == '0' || digits[i] == '.'))
     {
       zero_flag = true;
       /* The output is zero, so set the sign according to the sign bit unless


Testcase:

  implicit none
  real              :: x = 0.5 + epsilon(0.)
  character(len=80) :: line
  print '(7f5.1)', x, -x
  write (line,'(2f5.1)') x, -x
  print '(A)', line
  if (line /= "  0.5 -0.5") call abort ()
  print '(7f5.0)', x, -x
  write (line,'(2f5.0)') x, -x
  print '(A)', line
  if (line /= "   1.  -1.") call abort ()
end


Strangely enough I needed to add some epsilon to 0.5 so that
the second test passes, because the exact value 0.5 appears
to get rounded down to 0 in formatted output.