kconfig.h: Add IF_ENABLED() macro

IF_ENABLED(CONFIG_FOO, ptr) evaluates to (ptr) if CONFIG_FOO is set to 'y'
or 'm', NULL otherwise. The (ptr) argument must be a pointer.

The IF_ENABLED() macro can be very useful to help GCC drop dead code.

For instance, consider the following:

#ifdef CONFIG_FOO_SUSPEND
static int foo_suspend(struct device *dev)
{
...
}
#endif

static struct pm_ops foo_ops = {
#ifdef CONFIG_FOO_SUSPEND
.suspend = foo_suspend,
#endif
};

While this works, the foo_suspend() macro is compiled conditionally,
only when CONFIG_FOO_SUSPEND is set. This is problematic, as there could
be a build bug in this function, we wouldn't have a way to know unless
the config option is set.

An alternative is to declare foo_suspend() always, but mark it as maybe
unused:

static int __maybe_unused foo_suspend(struct device *dev)
{
...
}

static struct pm_ops foo_ops = {
#ifdef CONFIG_FOO_SUSPEND
.suspend = foo_suspend,
#endif
};

Again, this works, but the __maybe_unused attribute is required to
instruct the compiler that the function may not be referenced anywhere,
and is safe to remove without making a fuss about it. This makes the
programmer responsible for tagging the functions that can be
garbage-collected.

With this patch, it is now possible to write the following:

static int foo_suspend(struct device *dev)
{
...
}

static struct pm_ops foo_ops = {
.suspend = IF_ENABLED(CONFIG_FOO_SUSPEND, foo_suspend),
};

The foo_suspend() function will now be automatically dropped by the
compiler, and it does not require any specific attribute.

Signed-off-by: Paul Cercueil <[e-mail]>
Acked-by: Arnd Bergmann <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Linus Walleij <[e-mail]>

include/linux/kconfig.h

···
  */
 #define IS_ENABLED(option) __or(IS_BUILTIN(option), IS_MODULE(option))
 
+/*
+ * IF_ENABLED(CONFIG_FOO, ptr) evaluates to (ptr) if CONFIG_FOO is set to 'y'
+ * or 'm', NULL otherwise.
+ */
+#define IF_ENABLED(option, ptr) (IS_ENABLED(option) ? (ptr) : NULL)
+
 #endif /* __LINUX_KCONFIG_H */