Re: [patch 02/12] Immediate Values - Architecture Independent Code

From: "H. Peter Anvin" <hpa@zytor.com>
To: Arjan van de Ven <arjan@infradead.org>
Cc: Andi Kleen <andi@firstfloor.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
	Ingo Molnar <mingo@elte.hu>,
	linux-kernel@vger.kernel.org, Jason Baron <jbaron@redhat.com>,
	Rusty Russell <rusty@rustcorp.com.au>,
	Adrian Bunk <bunk@stusta.de>,
	Christoph Hellwig <hch@infradead.org>
Subject: Re: [patch 02/12] Immediate Values - Architecture Independent Code
Date: Mon, 28 Sep 2009 14:32:30 -0700	[thread overview]
Message-ID: <4AC12B6E.7010805@zytor.com> (raw)
In-Reply-To: <20090928223735.74267e3e@infradead.org>

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