linux-kernel - Re: [patch 4/7] Immediate Values

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Date:	Thu, 20 Sep 2007 11:24:12 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Andi Kleen <ak@....de>, "H. Peter Anvin" <hpa@...or.com>,
	Chuck Ebbert <cebbert@...hat.com>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [patch 4/7] Immediate Values - i386 Optimization

On Tuesday 18 September 2007 22:07, Mathieu Desnoyers wrote:
> i386 optimization of the immediate values which uses a movl with code patching
> to set/unset the value used to populate the register used as variable source.
> 
> Changelog:
> - Use text_poke_early with cr0 WP save/restore to patch the bypass. We are doing
>   non atomic writes to a code region only touched by us (nobody can execute it
>   since we are protected by the immediate_mutex).
> - Put immediate_set and _immediate_set in the architecture independent header.

> +struct __immediate {
> +	long var;		/* Pointer to the identifier variable of the
> +				 * immediate value
> +				 */
> +	long immediate;		/*
> +				 * Pointer to the memory location of the
> +				 * immediate value within the instruction.
> +				 */
> +	long size;		/* Type size. */
> +};


> +		case 2:							\
> +			asm (	".section __immediate, \"a\", @progbits;\n\t" \
> +					".long %1, (0f)+2, 2;\n\t"	\
> +					".previous;\n\t"		\
> +					"1:\n\t"			\
> +					".align 2;\n\t"			\
> +					"0:\n\t"			\
> +					"mov %2,%0;\n\t"		\
> +				: "=r" (value)				\
> +				: "m" (name##__immediate),		\
> +				  "i" (0));				\

Instead of letting gcc use whatever instruction it sees fit best
for accessing the variable (like add/cmp/test...)
now we force it to use mov imm,reg first. Maybe with preceding nop
due to "align 2".

And then we use 12 more bytes in __immediate section
*for each* place where you read the variable.

Do you plan to use the same approach on x86_64?
I mean, longs there are twice as long.

Can this be made conditional, on CONFIG_CC_OPTIMIZE_FOR_SIZE perhaps?
--
vda
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