linux-kernel - Re: [PATCH] debug: fix BUILD_BUG_ON() for non-constant expressions

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Message-ID: <48AC1A41.60907@panasas.com>
Date:	Wed, 20 Aug 2008 16:21:05 +0300
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Alexey Dobriyan <adobriyan@...il.com>,
	torvalds@...uxfoundation.org, akpm@...uxfoundation.org,
	linux-kernel@...r.kernel.org, Sam Ravnborg <sam@...nborg.org>
Subject: Re: [PATCH] debug: fix BUILD_BUG_ON() for non-constant expressions

Ingo Molnar wrote:
> From: Ingo Molnar <mingo@...e.hu>
> Date: Sun, 17 Aug 2008 12:18:01 +0200
> Subject: [PATCH] debug, x86: move BUILD_BUG_ON() and __FUNCTION__
> 
> move BUILD_BUG_ON variants and the __FUNCTION__ definition from
> kernel.h to compiler.h.
> 
> Besides being the correct location for such trivial wrappers around
> compiler functionality, this also allows the removal of a duplicate
> (and now slighly incompatible) definition of BUILD_BUG_ON from
> arch/x86/boot/boot.h.
> 
> [ boot.h cannot just include kernel.h to pick up the new definition of
>   BUILD_BUG_ON(), as it is also built into user-space utilities on the
>   host system. ]
> 
> Signed-off-by: Ingo Molnar <mingo@...e.hu>
> ---
>  arch/x86/boot/boot.h     |    3 ---
>  include/linux/compiler.h |   30 ++++++++++++++++++++++++++++++
>  include/linux/kernel.h   |   26 --------------------------
>  3 files changed, 30 insertions(+), 29 deletions(-)
> 
> diff --git a/arch/x86/boot/boot.h b/arch/x86/boot/boot.h
> index 616b804..f09b79a 100644
> --- a/arch/x86/boot/boot.h
> +++ b/arch/x86/boot/boot.h
> @@ -27,9 +27,6 @@
>  #include "bitops.h"
>  #include <asm/cpufeature.h>
>  
> -/* Useful macros */
> -#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
> -
>  extern struct setup_header hdr;
>  extern struct boot_params boot_params;
>  
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index c8bd2da..727862f 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -194,4 +194,34 @@ extern void __chk_io_ptr(const volatile void __iomem *);
>   */
>  #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
>  
> +/*
> + * Force a compilation error if condition is true [array index becomes
> + * negative], and a linker error if condition is not constant [non-defined
> + * variable is used as an array index]:
> + *
> + * ( The linker trick relies on gcc optimizing out a multiplication with
> + *   constant zero - which should be reasonable enough. )
> + */
> +#ifndef __ASSEMBLY__
> +extern unsigned int __BUILD_BUG_ON_non_constant;
> +#endif
> +
> +#define BUILD_BUG_ON(condition)					\
> +do {								\
> +	(void)sizeof(char[1 - 2*!!(condition)]);		\
> +	if (!__builtin_constant_p(condition))			\
> +		__BUILD_BUG_ON_non_constant++;			\
> +} while (0)
> +
> +/*
> + * Force a compilation error if condition is true, but also produce a
> + * result (of value 0 and type size_t), so the expression can be used
> + * e.g. in a structure initializer (or where-ever else comma expressions
> + * aren't permitted):
> + */
> +#define BUILD_BUG_ON_ZERO(e) (sizeof(char[1 - 2 * !!(e)]) - 1)
> +
> +/* Trap pasters of __FUNCTION__ at compile-time */
> +#define __FUNCTION__ (__func__)
> +
>  #endif /* __LINUX_COMPILER_H */
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 36c841e..1ceafa4 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -467,32 +467,6 @@ struct sysinfo {
>  	char _f[20-2*sizeof(long)-sizeof(int)];	/* Padding: libc5 uses this.. */
>  };
>  
> -/*
> - * Force a compilation error if condition is true [array index becomes
> - * negative], and a linker error if condition is not constant [non-defined
> - * variable is used as an array index]:
> - *
> - * ( The linker trick relies on gcc optimizing out a multiplication with
> - *   constant zero - which should be reasonable enough. )
> - */
> -extern unsigned int __BUILD_BUG_ON_non_constant;
> -
> -#define BUILD_BUG_ON(condition)					\
> -do {								\
> -	(void)sizeof(char[1 - 2*!!(condition)]);		\
> -	if (!__builtin_constant_p(condition))			\
> -		__BUILD_BUG_ON_non_constant++;			\
> -} while (0)
> -
> -/* Force a compilation error if condition is true, but also produce a
> -   result (of value 0 and type size_t), so the expression can be used
> -   e.g. in a structure initializer (or where-ever else comma expressions
> -   aren't permitted). */
> -#define BUILD_BUG_ON_ZERO(e) (sizeof(char[1 - 2 * !!(e)]) - 1)
> -
> -/* Trap pasters of __FUNCTION__ at compile-time */
> -#define __FUNCTION__ (__func__)
> -
>  /* This helps us to avoid #ifdef CONFIG_NUMA */
>  #ifdef CONFIG_NUMA
>  #define NUMA_BUILD 1
> --
Ingo Hi!

I got bitten by this duplication too. Please submit an equivalent patch
for today's code. That could be nice.

Then later we can take care of BUILD_BUG_ON which ever variant.

Thanks
Boaz
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