math-util: introduce iszero_safe() and fp_equal()

src/basic/math-util.h

@@ -0,0 +1,14 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include <math.h>
+
+#include "macro.h"
+
+/* On some optimization level, iszero(x) is converted to (x == 0.0), and emits warning -Wfloat-equal.
+ * The argument must be a floating point, i.e. one of float, double, or long double. */
+#define iszero_safe(x) (fpclassify(x) == FP_ZERO)
+
+/* To avoid x == y and triggering compile warning -Wfloat-equal. This retuns false if one of the argument is
+ * NaN or infinity. One of the argument must be a floating point. */
+#define fp_equal(x, y) iszero_safe((x) - (y))

src/basic/meson.build

@@ -129,6 +129,7 @@ basic_sources = files(
         'login-util.c',
         'login-util.h',
         'macro.h',
+        'math-util.h',
         'memfd-util.c',
         'memfd-util.h',
         'memory-util.c',