examples/c: prevent uprobe_add/uprobe_sub inlining

Depending on compiler, uprobe_add/uprobe_sub can still be inlined. Use
no-op volatile asm and noinline attribute to prevent compiler from
inlining. Just global function is not enough.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

Author: anakryiko

examples/c/uprobe.c

@@ -12,14 +12,19 @@ static int libbpf_print_fn(enum libbpf_print_level level, const char *format, va
 	return vfprintf(stderr, format, args);
 }
 
-/* It's a global function to make sure compiler doesn't inline it. */
-int uprobed_add(int a, int b)
+/* It's a global function to make sure compiler doesn't inline it. To be extra
+ * sure we also use "asm volatile" and noinline attributes to prevent
+ * compiler from local inlining.
+ */
+__attribute__((noinline)) int uprobed_add(int a, int b)
 {
+	asm volatile ("");
 	return a + b;
 }
 
-int uprobed_sub(int a, int b)
+__attribute__((noinline)) int uprobed_sub(int a, int b)
 {
+	asm volatile ("");
 	return a - b;
 }