linux-kernel - Re: [PATCH] compiler.h: don't use temporary variable in __compiletime

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Date:	Thu, 8 May 2014 06:18:39 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Johannes Berg <johannes@...solutions.net>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Daniel Santos <daniel.santos@...ox.com>
Subject: Re: [PATCH] compiler.h: don't use temporary variable in
 __compiletime_assert()

On Thu, May 08, 2014 at 08:31:25AM +0200, Johannes Berg wrote:
> From: Johannes Berg <johannes.berg@...el.com>
> 
> Usually, BUG_ON and friends aren't even evaluated in sparse, but
> recently compiletime_assert_atomic_type() was added, and that now
> results in a sparse warning every time it is used.
> 
> The reason turns out to be the temporary variable, after it sparse
> no longer considers the value to be a constant, and results in a
> warning and an error. The error is the more annoying part of this
> as it suppresses any further warnings in the same file, hiding
> other problems.
> 
> Since this is all about compile time and the condition should be
> side-effect free to start with, there's no downside (apart maybe
> from a slight compilation time penalty?) to just duplicating it,
> leaving sparse able to evaluate it at check time, getting rid of
> the warning and error.
> 
> Signed-off-by: Johannes Berg <johannes.berg@...el.com>

Acked-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

> ---
>  include/linux/compiler.h | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 2472740d7ab2..38c0e00ddef8 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -324,11 +324,10 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
> 
>  #define __compiletime_assert(condition, msg, prefix, suffix)		\
>  	do {								\
> -		bool __cond = !(condition);				\
>  		extern void prefix ## suffix(void) __compiletime_error(msg); \
> -		if (__cond)						\
> +		if (!(condition))					\
>  			prefix ## suffix();				\
> -		__compiletime_error_fallback(__cond);			\
> +		__compiletime_error_fallback(!(condition));		\
>  	} while (0)
> 
>  #define _compiletime_assert(condition, msg, prefix, suffix) \
> -- 
> 2.0.0.rc0
> 

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