linux-kernel - Re: [GIT PULL] x86 LTO changes for v3.15

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Date:	Tue, 01 Apr 2014 22:49:51 +0200
From:	Paul Bolle <pebolle@...cali.nl>
To:	"H. Peter Anvin" <hpa@...ux.intel.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>, ak@...ux.intel.com,
	gleb@...hat.com, hpa@...or.com, hubicka@....cz,
	linux-kernel@...r.kernel.org, mingo@...nel.org,
	pbonzini@...hat.com, perex@...ex.cz, peterz@...radead.org,
	rostedt@...dmis.org, rusty@...tcorp.com.au, tglx@...utronix.de
Subject: Re: [GIT PULL] x86 LTO changes for v3.15

On Mon, 2014-03-31 at 14:05 -0700, H. Peter Anvin wrote:
> --- a/include/linux/init.h
> +++ b/include/linux/init.h
> @@ -163,6 +163,23 @@ extern bool initcall_debug;
>  
>  #ifndef __ASSEMBLY__
>  
> +#ifdef CONFIG_LTO

I can't find a Kconfig symbol LTO nor a preprocessor define for
CONFIG_LTO. (I only checked master of Linus's tree and linux-next.)

Why is this check for CONFIG_LTO needed?

> +/* Work around a LTO gcc problem: when there is no reference to a variable
> + * in a module it will be moved to the end of the program. This causes
> + * reordering of initcalls which the kernel does not like.
> + * Add a dummy reference function to avoid this. The function is
> + * deleted by the linker.
> + */
> +#define LTO_REFERENCE_INITCALL(x) \
> +	; /* yes this is needed */			\
> +	static __used __exit void *reference_##x(void)	\
> +	{						\
> +		return &x;				\
> +	}
> +#else
> +#define LTO_REFERENCE_INITCALL(x)
> +#endif


Paul Bolle

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