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

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Message-ID: <20090928220500.GC27947@Krystal>
Date:	Mon, 28 Sep 2009 18:05:00 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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

* H. Peter Anvin (hpa@...or.com) wrote:
> 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.

Indeed, these cases could make good use of compiler support to let
immediate values be added to a wider range of operations. Currently,
being limited to "mov" is somewhat limiting on x86. We could definitely
do better.

Mathieu

> 
> 	-hpa
> 
> [1] Common subcase:
> 
> 	movl global, %reg	; 6 bytes (unless reg is eax on 32 bits)
> 	movl $immed, %reg	; 5 bytes
> 

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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