| Message ID | 1393261707-30565-2-git-send-email-joshc@codeaurora.org (mailing list archive) |
|---|---|
| State | RFC, archived |
| Headers | show |
On Mon, Feb 24, 2014 at 11:08:25AM -0600, Josh Cartwright wrote: > The assign_if() and assign_if_enable() macros are intended to be used > in static initializers for function pointers, where the pointer is > expected to be NULL when a compile-time condition does not hold. > > These macros allow for implementing this behavior, without requiring the > functions be wrapped in #ifdef conditionals, and while providing > typesafety regardless of the value of the conditional. > > For example, the following pattern is common: > > #ifdef CONFIG_FOO > static void foo_callback(void) > { > } > #else > #define foo_callback NULL > #endif > > static struct foo_object foo_obj = { > .callback = foo_callback, > }; > > Usage of assign_if_enabled() allows for achieving the same effect > without the preprocessor conditional, and in addition, allowing the > compiler to typecheck the function regardless of CONFIG_FOO. > > static void foo_callback(void) > { > } > > static struct foo_object foo_obj = { > .callback = assign_if_enabled(CONFIG_FOO, foo_callback), > }; > > Cc: Andrew Morton <akpm@linux-foundation.org> > Signed-off-by: Josh Cartwright <joshc@codeaurora.org> > --- > include/linux/typecheck.h | 18 ++++++++++++++++++ > 1 file changed, 18 insertions(+) > > diff --git a/include/linux/typecheck.h b/include/linux/typecheck.h > index eb5b74a..04134c7 100644 > --- a/include/linux/typecheck.h > +++ b/include/linux/typecheck.h > @@ -21,4 +21,22 @@ > (void)__tmp; \ > }) > > +/* > + * Intended for use in static object initializers, > + * assign_if(const_expr, function) evaluates to 'function' if 'const_expr', > + * otherwise NULL. > + * > + * The type of the assign_if() expression is typeof(function), and therefore > + * can provide typechecking regardless of 'const_expr'. > + * > + * gcc considers 'function' to be used and will not generate a 'defined but not > + * used' warning when not 'const_expr', however, gcc is smart enough to > + * eliminate 'function' if assign_if() is the only reference. > + */ What version of gcc started doing this? Does llvm also do this? thanks, greg k-h -- To unsubscribe from this list: send the line "unsubscribe linux-pm" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Thu, Feb 27, 2014 at 11:00:32AM -0800, Greg Kroah-Hartman wrote: > On Mon, Feb 24, 2014 at 11:08:25AM -0600, Josh Cartwright wrote: > > +/* > > + * Intended for use in static object initializers, > > + * assign_if(const_expr, function) evaluates to 'function' if 'const_expr', > > + * otherwise NULL. > > + * > > + * The type of the assign_if() expression is typeof(function), and therefore > > + * can provide typechecking regardless of 'const_expr'. > > + * > > + * gcc considers 'function' to be used and will not generate a 'defined but not > > + * used' warning when not 'const_expr', however, gcc is smart enough to > > + * eliminate 'function' if assign_if() is the only reference. > > + */ > > What version of gcc started doing this? Does llvm also do this? I'll need to dig up some old gcc's to give this a more thorough testing; testing with clang 3.4, and it appears to have the same behavior, at least when I throw a trivial usecase at it. $ clang --version clang version 3.4 (tags/RELEASE_34/final) Target: x86_64-unknown-linux-gnu Thread model: posix $ cat test.c static void foo(void) { extern void BROKEN(void); BROKEN(); } void (*callback)(void) = 0 ? foo : 0; $ clang -Wall -Werror -c test.c $ size test.o text data bss dec hex filename 0 0 8 8 8 /tmp/test.o $ nm test.o 0000000000000000 B callback Although, given the feedback on the other patches, assign_if() may become just be a solution in search of a problem :). Thanks, Josh
diff --git a/include/linux/typecheck.h b/include/linux/typecheck.h index eb5b74a..04134c7 100644 --- a/include/linux/typecheck.h +++ b/include/linux/typecheck.h @@ -21,4 +21,22 @@ (void)__tmp; \ }) +/* + * Intended for use in static object initializers, + * assign_if(const_expr, function) evaluates to 'function' if 'const_expr', + * otherwise NULL. + * + * The type of the assign_if() expression is typeof(function), and therefore + * can provide typechecking regardless of 'const_expr'. + * + * gcc considers 'function' to be used and will not generate a 'defined but not + * used' warning when not 'const_expr', however, gcc is smart enough to + * eliminate 'function' if assign_if() is the only reference. + */ +#define assign_if(const_expr,function) \ + ((const_expr) ? function : NULL) + +#define assign_if_enabled(option,function) \ + assign_if(IS_ENABLED(option), function) + #endif /* TYPECHECK_H_INCLUDED */
The assign_if() and assign_if_enable() macros are intended to be used in static initializers for function pointers, where the pointer is expected to be NULL when a compile-time condition does not hold. These macros allow for implementing this behavior, without requiring the functions be wrapped in #ifdef conditionals, and while providing typesafety regardless of the value of the conditional. For example, the following pattern is common: #ifdef CONFIG_FOO static void foo_callback(void) { } #else #define foo_callback NULL #endif static struct foo_object foo_obj = { .callback = foo_callback, }; Usage of assign_if_enabled() allows for achieving the same effect without the preprocessor conditional, and in addition, allowing the compiler to typecheck the function regardless of CONFIG_FOO. static void foo_callback(void) { } static struct foo_object foo_obj = { .callback = assign_if_enabled(CONFIG_FOO, foo_callback), }; Cc: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Josh Cartwright <joshc@codeaurora.org> --- include/linux/typecheck.h | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+)