SF bug #477221: abs and divmod act oddly with -0.0

Try to ensure that divmod(-0.0, 1.0) -> (-0.0, +0.0) across platforms.
It always did on Windows, and still does.  It didn't on Linux.  Alas,
there's no platform-independent way to write a test case for this.
Bugfix candidate.

diff --git a/Objects/floatobject.c b/Objects/floatobject.c
index 1efef4b02e22bd69d11ffb7221d3ce02872b542b..7e12a09885eaf2e4398faf3a8efe6a1d8a69e138 100644 (file)
--- a/Objects/floatobject.c
+++ b/Objects/floatobject.c
@@ -464,17 +464,34 @@ float_divmod(PyObject *v, PyObject *w)
           it will always be very close to one.
        */
        div = (vx - mod) / wx;
-       /* note: checking mod*wx < 0 is incorrect -- underflows to
-          0 if wx < sqrt(smallest nonzero double) */
-       if (mod && ((wx < 0) != (mod < 0))) {
-               mod += wx;
-               div -= 1.0;
+       if (mod) {
+               /* ensure the remainder has the same sign as the denominator */
+               if ((wx < 0) != (mod < 0)) {
+                       mod += wx;
+                       div -= 1.0;
+               }
+       }
+       else {
+               /* the remainder is zero, and in the presence of signed zeroes
+                  fmod returns different results across platforms; ensure
+                  it has the same sign as the denominator; we'd like to do
+                  "mod = wx * 0.0", but that may get optimized away */
+               mod = 0.0;
+               if (wx < 0.0)
+                       mod = -mod;
        }
        /* snap quotient to nearest integral value */
-       floordiv = floor(div);
-       if (div - floordiv > 0.5)
-               floordiv += 1.0;
-       PyFPE_END_PROTECT(div)
+       if (div) {
+               floordiv = floor(div);
+               if (div - floordiv > 0.5)
+                       floordiv += 1.0;
+       }
+       else {
+               /* div is zero - get the same sign as the true quotient */
+               div *= div;     /* hide "div = +0" from optimizers */
+               floordiv = div * vx / wx; /* zero w/ sign of vx/wx */
+       }
+       PyFPE_END_PROTECT(floordiv)
        return Py_BuildValue("(dd)", floordiv, mod);
 }