W dniu 30.09.2021 o 00:54, Lucas De Marchi pisze:
On Wed, Sep 29, 2021 at 11:08:18PM +0200, Andrzej Hajda wrote:
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@intel.com
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,
ugh, that would kill the idea - having the strings and additional runtime checks would not be good. Maybe if we check with __builtin_constant_p() and do the simpler expansion if it's not constant?
I think it is just matter of disallowing such construct in places where compiler expects constant expression.
If accepted, the expression is apparently evaluated in compile time. See [1].
[1]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69960#c18
- it does not work with hex (0x1) or octal values (01)
indeed, but I guess that would be fixable by checking (s[0] == '0' && s[1] == '\0')? However, it seems kconfig doesn't support setting int options to hex or octal.
I've spotted in include/generated/autoconf.h following line:
#define CONFIG_ILLEGAL_POINTER_VALUE 0xdead000000000000
It corresponds to following kconfig entry:
config ILLEGAL_POINTER_VALUE hex default 0 if X86_32 default 0xdead000000000000 if X86_64
Grepping shows more: grep -r --include=Kconfig -3 -P '^\s*hex' .
Anyway do you really need to handle undefined case? If not, the macro can stay simple, w/o hacky constructs.
Regards
Andrzej
If I try an hex value in menuconfig it says "You have made an invalid entry." If I try editing .config or setting via scripts/config --set-val, it just gets reset when trying to generate include/generated/autoconf.h
Lucas De Marchi
It is probably OK for private macro, but it can hurt in kconfig.h, especially the 2nd issue
Regards
Andrzej
#endif /* !__I915_UTILS_H */