[PATCH v2 1/2] kconfig.h: Add IF_ENABLED() macro
From: Paul Cercueil
Date: Sun Dec 13 2020 - 18:59:36 EST
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 <paul@xxxxxxxxxxxxxxx>
---
Notes:
v2: Only add IF_ENABLED() and don't verify that the argument is a
pointer; since the second argument of the ?: ternary operator
is NULL, the compiler should issue an error if the other
argument is not a pointer.
include/linux/kconfig.h | 6 ++++++
1 file changed, 6 insertions(+)
diff --git a/include/linux/kconfig.h b/include/linux/kconfig.h
index 9d12c970f18f..e78e17a76dc9 100644
--- a/include/linux/kconfig.h
+++ b/include/linux/kconfig.h
@@ -72,4 +72,10 @@
*/
#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 */
--
2.29.2