linux-kernel - Re: [patch] x86: phase out forced inlining

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Date:	Mon, 3 Mar 2008 15:56:23 +0100
From:	Sam Ravnborg <sam@...nborg.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Adrian Bunk <bunk@...nel.org>,
	Alexey Starikovskiy <aystarik@...il.com>, lenb@...nel.org,
	astarikovskiy@...e.de, linux-acpi@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [patch] x86: phase out forced inlining

Hi Ingo.

> Subject: x86: phase out forced inlining

Any particular reason you made the patch x86 specific?

It is preferred to gain this size decrease for other arch too
and having a single definiton of OPTIMIZE_FOR_SIZE
would help here.

We have this for other gcc options already.

> +config OPTIMIZE_INLINING

Other (not all) config options that deal with gcc behaviour are
named CC_*. But they mostly impact gcc options.
CC_OPTIMIZE_INLINING would match the naming of CC_OPTIMIZE_SIZE,
except in the latter OPTIMIZE refer to the -O option.

CC_DEFAULT_INLINE may give the right associations?

> +	bool "Allow gcc to uninline functions marked 'inline'"
> +	default y
> +	help
> +	  This option determines if the kernel forces gcc to inline the functions
> +	  developers have marked 'inline'. Doing so takes away freedom from gcc to
> +	  do what it thinks is best, which is desirable for the gcc 3.x series of
> +	  compilers. The gcc 4.x series have a rewritten inlining algorithm and
> +	  disabling this option will generate a smaller kernel there. Hopefully
> +	  this algorithm is so good that allowing gcc4 to make the decision can
> +	  become the default in the future, until then this option is there to
> +	  test gcc for this.

Would it be worth here to mention that stuff that really
needs inlining should use __always_inle and not inline?

> + */
> +#ifndef CONFIG_OPTIMIZE_INLINING
> +# define inline		inline		__attribute__((always_inline))
> +# define __inline__	__inline__	__attribute__((always_inline))
> +# define __inline	__inline	__attribute__((always_inline))
> +#endif

A quick google did not tell me the difference between inline, __inline, __inline__.
But it turned out the december 2005 thread where there was a lenghty discussion about
trusting gcc with respect to inlining.
It is not the subject of this patch but I just wondered why we need all these variants.

	Sam
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