[Bug 277783] libc fma() doesn't not return the correct zero sign

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=277783

--- Comment #4 from Steve Kargl <kargl@FreeBSD.org> ---
Ok, with the given input to fma, one arrives at line 271

                return (xy.hi + vzs + ldexp(xy.lo, spread));

(gdb) p vzs
$12 = -0.5
(gdb) p xy.hi
$13 = 0.5
(gdb) p xy.lo
$14 = -3.944304526105059e-31
(gdb) p spread
$15 = -999

Turns out that ldexp(3.944304526105059e-31, -999) = -0x0p+0.  So, you have

0.5 - 0.5 - 0. = 0. - 0. = +0

So, a pessimestic patch is

Index: src/s_fma.c
===================================================================
--- src/s_fma.c (revision 2834)
+++ src/s_fma.c (working copy)
@@ -267,7 +267,9 @@
                 */
                fesetround(oround);
                volatile double vzs = zs; /* XXX gcc CSE bug workaround */
-               return (xy.hi + vzs + ldexp(xy.lo, spread));
+               xs = ldexp(xy.lo, spread);
+               xy.hi += vzs;
+               return (xy.hi == 0 ? xs : xy.hi + xs);
        }

        if (oround != FE_TONEAREST) {

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