gh-131032: Fix math.fma(x, y, z) zero sign

Fix result sign when z is zero.

Co-Authored-by: Sergey B Kirpichev <[email protected]>

Author: vstinner

Lib/test/test_math.py

@@ -2765,12 +2765,6 @@ def test_fma_infinities(self):
                 self.assertEqual(math.fma(-b, -math.inf, c), math.inf)
                 self.assertEqual(math.fma(-b, math.inf, c), -math.inf)
 
-    # gh-73468: On some platforms, libc fma() doesn't implement IEE 754-2008
-    # properly: it doesn't use the right sign when the result is zero.
-    @unittest.skipIf(
-        sys.platform.startswith(("freebsd", "wasi", "netbsd", "emscripten"))
-        or (sys.platform == "android" and platform.machine() == "x86_64"),
-        f"this platform doesn't implement IEE 754-2008 properly")
     def test_fma_zero_result(self):
         nonnegative_finites = [0.0, 1e-300, 2.3, 1e300]
 

Modules/mathmodule.c

@@ -2344,7 +2344,22 @@ static PyObject *
 math_fma_impl(PyObject *module, double x, double y, double z)
 /*[clinic end generated code: output=4fc8626dbc278d17 input=e3ad1f4a4c89626e]*/
 {
-    double r = fma(x, y, z);
+    double r;
+    if (z) {
+        r = fma(x, y, z);
+    }
+    else {
+        // gh-73468, gh-131032: On some platforms (ex: WASI, NetBSD,
+        // Emscripten, musl C library), libc fma() doesn't implement
+        // IEEE 754-2008 properly: it doesn't use the right sign when the
+        // result is zero.
+        if (x && y) {
+            r = x * y;
+        }
+        else {
+            r = copysign(1, z) == 1 ? x*y + z : x*y;
+        }
+    }
 
     /* Fast path: if we got a finite result, we're done. */
     if (isfinite(r)) {