[Bug target/20227] New: [m68k] long double -> double cast fails with -0.0

["jifl-bugzilla at jifvik dot org" <gcc-bugzilla at gcc dot gnu dot org>]

I found a problem because the FP emulation of long doubles in the m68k port
doesn't treat -0.0 correctly. A cast from a long double -0.0 to a double results
in a double with the value -2.0.

The relevant function is __truncxfdf2 in gcc/config/m68k/fpgnulib.c. There are
two issues here. 

First of all the short-cut test for zero only tested for +0.0, not -0.0. 

However that by itself shouldn't have shown up a problem as the more lengthy
code underneath should have still come up with the right answer. The problem is
that it doesn't treat a long double exponent of 0 as special. Without this
fixed, subnormal numbers continue to be broken when converted to doubles.

I believe this is the correct fix, but I'm not sure:
--- fpgnulib.c~	Mon Jan 12 03:55:42 2004
+++ fpgnulib.c	Sun Feb 27 04:05:13 2005
@@ -315,8 +315,14 @@
   ldl.ld = ld;
   /*printf ("xfdf in: %s\n", dumpxf (ld));*/
 
-  if (!ldl.l.upper && !ldl.l.middle && !ldl.l.lower)
-    return 0;
+  /* Match +/- 0 and subnormals, which get flushed to 0 */
+  if ( EXPX(ldl) == 0 )
+    {
+      /* Preserve sign */
+      dl.l.upper = SIGNX (ldl);
+      dl.l.lower = 0;
+      return dl.d;
+    }
 
   exp = EXPX (ldl) - EXCESSX + EXCESSD;
   /* ??? quick and dirty: keep `exp' sane */

In the above case, both +/- 0 and subnormals cause a correctly signed double 0.0
to be returned. Is this the right fix?

In case it is:
2005-02-27  Jonathan Larmour  <jifl@eCosCentric.com>

	* config/m68k/fpgnulib.c (__truncxfdf2): Match subnormals as well as
	-0.0 in special case test, and preserve sign.

-- 
           Summary: [m68k] long double -> double cast fails with -0.0
           Product: gcc
           Version: 3.4.3
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: jifl-bugzilla at jifvik dot org
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: m68k-elf


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20227