helper: add compiler.h to handle compiler specific workarounds

Not all compilers nor compiler versions supports the attributes
used in OpenOCD code.
Collect in a single file the workaround to handle them.

Change-Id: I92d871337281169134ce8e40b2064591518be71f
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7519
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>

src/helper/compiler.h

@@ -0,0 +1,44 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+
+/*
+ * This file contains compiler specific workarounds to handle different
+ * compilers and different compiler versions.
+ * Inspired by Linux's include/linux/compiler_attributes.h
+ * and file sys/cdefs.h in libc and newlib.
+ */
+
+#ifndef OPENOCD_HELPER_COMPILER_H
+#define OPENOCD_HELPER_COMPILER_H
+
+/*
+ * __has_attribute is supported on gcc >= 5, clang >= 2.9 and icc >= 17.
+ */
+#ifndef __has_attribute
+# define __has_attribute(x) 0
+#endif
+
+/*
+ * The __returns_nonnull function attribute marks the return type of the function
+ * as always being non-null.
+ */
+#ifndef __returns_nonnull
+# if __has_attribute(__returns_nonnull__)
+#  define __returns_nonnull __attribute__((__returns_nonnull__))
+# else
+#  define __returns_nonnull
+# endif
+#endif
+
+/*
+ * The __nonnull function attribute marks pointer parameters that
+ * must not be NULL.
+ */
+#ifndef __nonnull
+# if __has_attribute(__nonnull__)
+#  define __nonnull(params) __attribute__ ((__nonnull__ params))
+# else
+#  define __nonnull(params)
+# endif
+#endif
+
+#endif /* OPENOCD_HELPER_COMPILER_H */