Re: [PATCH] CodingStyle: Add a chapter on conditional compilation

[Martin Kelly <martin@martingkelly.com>]

On 10/29/2014 11:15 AM, Josh Triplett wrote:
> Document several common practices and conventions regarding conditional
> compilation, most notably the preference for ifdefs in headers rather
> than .c files.
> 
> Signed-off-by: Josh Triplett <josh@joshtriplett.org>
> ---
> 
> I found myself explaining a few of these unwritten rules in patch
> feedback, so I figured I'd document them.
> 
>  Documentation/CodingStyle | 43 +++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 43 insertions(+)
> 
> diff --git a/Documentation/CodingStyle b/Documentation/CodingStyle
> index 3171822..9f28b14 100644
> --- a/Documentation/CodingStyle
> +++ b/Documentation/CodingStyle
> @@ -845,6 +845,49 @@ next instruction in the assembly output:
>  	     : /* outputs */ : /* inputs */ : /* clobbers */);
>  
>  
> +		Chapter 20: Conditional Compilation
> +
> +Wherever possible, don't use preprocessor conditionals (#if, #ifdef) in .c
> +files; doing so makes code harder to read and logic harder to follow.  Instead,
> +use such conditionals in a header file defining functions for use in those .c
> +files, providing no-op stub versions in the #else case, and then call those
> +functions unconditionally from .c files.  The compiler will avoid generating
> +any code for the stub calls, producing identical results, but the logic will
> +remain easy to follow.
> +
> +Prefer to compile out entire functions, rather than portions of functions or
> +portions of expressions.  Rather than putting an ifdef in an expression, factor
> +out part or all of the expression into a separate helper function and apply the
> +conditional to that function.
> +
> +If you have a function or variable which may potentially go unused in a
> +particular configuration, and the compiler would warn about its definition
> +going unused, mark the definition as __maybe_unused rather than wrapping it in
> +a preprocessor conditional.  (However, if a function or variable *always* goes
> +unused, delete it.)
> +
> +Within code, where possible, use the IS_ENABLED macro to convert a Kconfig
> +symbol into a C boolean expression, and use it in a normal C conditional:
> +
> +	if (IS_ENABLED(CONFIG_SOMETHING)) {
> +		...
> +	}
> +
> +The compiler will constant-fold the conditional away, and include or exclude
> +the block of code just as with an #ifdef, so this will not add any runtime
> +overhead.  However, this approach still allows the C compiler to see the code
> +inside the block, and check it for correctness (syntax, types, symbol
> +references, etc).  Thus, you still have to use an #ifdef if the code inside the
> +block references symbols that will not exist if the condition is not met.
> +
> +At the end of any non-trivial #if or #ifdef block (more than a few lines),
> +place a comment after the #endif on the same line, noting the conditional
> +expression used.  For instance:
> +
> +#ifdef CONFIG_SOMETHING
> +...
> +#endif /* CONFIG_SOMETHING */
> +
>  
>  		Appendix I: References
>  
> 

I like that you're formalizing this in the official document; thanks!