Message-ID: <4ABB7DDF.8010100@zytor.com>
Date: Thu, 24 Sep 2009 07:10:39 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Jason Baron <jbaron@redhat.com>
CC: Ingo Molnar <mingo@elte.hu>,
       Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
       Thomas Gleixner <tglx@linutronix.de>,
       Steven Rostedt <rostedt@goodmis.org>, Andi Kleen <ak@suse.de>,
       linux-kernel@vger.kernel.org
Subject: Re: Immediate values
References: <20090924123154.GA12907@Krystal> <20090924123428.GA27282@elte.hu> <20090924140228.GA2648@redhat.com>
In-Reply-To: <20090924140228.GA2648@redhat.com>
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Jason Baron wrote:
> 
> right we've proposed an alternative to the immediate values, which I've
> been calling 'jump label', here:
> 
> http://marc.info/?l=linux-kernel&m=125200966226921&w=2
> 
> The basic idea is that gcc, 4.5 will have support for an 'asm goto'
> construct which can refer to c code labels. Thus, we can replace a nop
> in the code stream with a 'jmp' instruction to various branch targets.
> 
> In terms of a comparison between the two, IMO, I think that the syntax
> for the immediate variables can be more readable, since it just looks
> like a conditional expression.
> 
> The immediate values do a 'mov', 'test' and then a jump, whereas jump
> label can just do a jump. So in this respect, I believe jump label can
> be more optimal. Additinally, if we want to mark sections 'cold' so they
> don't impact the istruction cache, the jump label already has the labels
> for doing so. Obviously, a performance comparison would be interesting
> as well.
> 

Direct jumps should at least theoretically be able to have better 
performance, but it would still be nice to have measurements of both.

	-hpa

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