compiler: Add fallthrough() helper

https://bugzilla.nasm.us/show_bug.cgi?id=3392465

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>

aclocal.m4

@@ -199,3 +199,29 @@ inline int foo(int x)
     [Define to 1 if your compiler supports C99 extern inline])],
  [AC_MSG_RESULT([no])
   PA_ADD_CFLAGS([-fgnu89-inline])])])
+
+dnl --------------------------------------------------------------------------
+dnl PA_CHECK_FALLTHROUGH_ATTRIBUTE
+dnl --------------------------------------------------------------------------
+AC_DEFUN(PA_CHECK_FALLTHROUGH_ATTRIBUTE,
+[AC_MSG_CHECKING([if $CC supports fallthrough attribute])
+ AC_COMPILE_IFELSE([AC_LANG_SOURCE([
+AC_INCLUDES_DEFAULT
+int main(int argc, char **argv)
+{
+	switch (argc) {
+	case 1:
+		(void)argc;
+		__attribute__((fallthrough));
+	default:
+		(void)argv;
+		break;
+	}
+	return argc;
+}
+ ])],
+ [AC_MSG_RESULT([yes])
+  AC_DEFINE([HAVE_FALLTHROUGH_ATTRIBUTE], 1,
+    [Define to 1 if your compiler supports __attribute__((fallthrough))])],
+ [AC_MSG_RESULT([no])])
+])

configure.ac

@@ -294,6 +294,11 @@ dnl options.
 dnl
 PA_CHECK_BAD_STDC_INLINE
 
+dnl Figure out if we can use fallthrough attribute or
+dnl should provide simple comment instead.
+dnl
+PA_CHECK_FALLTHROUGH_ATTRIBUTE
+
 dnl
 dnl support ccache
 dnl

include/compiler.h

@@ -240,6 +240,15 @@ size_t strnlen(const char *s, size_t maxlen);
 # define inline_prototypes
 #endif
 
+/*
+ * Fallthrough handling.
+ */
+#ifdef HAVE_FALLTHROUGH_ATTRIBUTE
+# define fallthrough() __attribute__((fallthrough))
+#else
+# define fallthrough() /* fall through */
+#endif
+
 /*
  * Hints to the compiler that a particular branch of code is more or
  * less likely to be taken.