W dniu 29.09.2021 o 20:33, Lucas De Marchi pisze:
> Like the IS_ENABLED() counterpart, we can make IS_CONFIG_NONZERO() to
> return the right thing when the config is not defined rather than a
> build error, with the limitation that it can't be used on preprocessor
> context.
>
> The trick here is that macro names can't start with a number or dash, so
> we stringify the argument and check that the first char is a number != 0
> (or starting with a dash to cover negative numbers). Except for -O0
> builds the strings are all eliminated.
>
> Taking CONFIG_DRM_I915_REQUEST_TIMEOUT in
> drivers/gpu/drm/i915/gem/i915_gem_context.c as example, we have the
> following output of the preprocessor:
>
> old:
> if (((20000) != 0) &&
> new:
> if (( ("20000"[0] > '0' && "20000"[0] < '9') || "20000"[0] == '-' ) &&
>
> New one looks worse, but is also eliminated from the object:
>
> $ size drivers/gpu/drm/i915/gem/i915_gem_context.o.*
> text data bss dec hex filename
> 52021 1070 232 53323 d04b drivers/gpu/drm/i915/gem/i915_gem_context.o.new
> 52021 1070 232 53323 d04b drivers/gpu/drm/i915/gem/i915_gem_context.o.old
>
> Signed-off-by: Lucas De Marchi <lucas.demarchi@xxxxxxxxx>
> ---
> drivers/gpu/drm/i915/i915_utils.h | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_utils.h b/drivers/gpu/drm/i915/i915_utils.h
> index 02bbfa4d68d3..436ce612c46a 100644
> --- a/drivers/gpu/drm/i915/i915_utils.h
> +++ b/drivers/gpu/drm/i915/i915_utils.h
> @@ -28,6 +28,7 @@
> #include <linux/list.h>
> #include <linux/overflow.h>
> #include <linux/sched.h>
> +#include <linux/stringify.h>
> #include <linux/types.h>
> #include <linux/workqueue.h>
>
> @@ -469,6 +470,9 @@ static inline bool timer_expired(const struct timer_list *t)
> *
> * Returns 0 if @config is 0, 1 if set to any value.
> */
> -#define IS_CONFIG_NONZERO(config) ((config) != 0)
> +#define IS_CONFIG_NONZERO(config) ( \
> + (__stringify_1(config)[0] > '0' && __stringify_1(config)[0] < '9') || \
> + __stringify_1(config)[0] == '-' \
> +)
Quite clever trick, but I see two issues:
- gcc < 8.1 treats expressions with string indices (ex. "abc"[0]) as
non-constant expressions, so they cannot be used everywhere, for example
in global variable initializations,
- it does not work with hex (0x1) or octal values (01)
It is probably OK for private macro, but it can hurt in kconfig.h,
especially the 2nd issue
Regards
Andrzej
>
> #endif /* !__I915_UTILS_H */
