linux-kernel - Re: [patch 02/12] Immediate Values

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Message-ID: <4AC12B6E.7010805@zytor.com>
Date:	Mon, 28 Sep 2009 14:32:30 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Jason Baron <jbaron@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Adrian Bunk <bunk@...sta.de>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [patch 02/12] Immediate Values - Architecture Independent Code

On 09/28/2009 01:37 PM, Arjan van de Ven wrote:
> 
> this makes me wonder what happens when a variable is used in multiple
> places... that makes the icache overhead multiply right?
> 

On x86, the icache overhead can often be zero or close to zero -- or
even negative in a fairly common subcase[1] -- simply because you are
dropping a displacement used to fetch a global variable with an
immediate in the code itself.

For 8- or 16-bit data items this is even more of a win in terms of
icache space; for 64-bit data it is always a lose.

It is also worth noting that the way this is implemented as a graft-on
rather than with compiler support means that the full instruction set
cannot exploited -- x86 can often use a memory operand or immediate as
part of an operation.  This adds icache pressure.

	-hpa

[1] Common subcase:

	movl global, %reg	; 6 bytes (unless reg is eax on 32 bits)
	movl $immed, %reg	; 5 bytes

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