linux-kernel - Re: Immediate values

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Message-ID: <4ABBA3DD.2040403@zytor.com>
Date:	Thu, 24 Sep 2009 09:52:45 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jason Baron <jbaron@...hat.com>
CC:	Ingo Molnar <mingo@...e.hu>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>, Andi Kleen <ak@...e.de>,
	linux-kernel@...r.kernel.org
Subject: Re: Immediate values

Jason Baron wrote:
> 
> that's right. The optimal solution doesn't require the the NOP5 at all,
> and I've been playing around with an implementation that doesn't have
> it. The problem I've been running into is that sometimes the compiler
> will put in a short jump - '0xeb', with a 1 byte offset, but the jump
> target is further away. Thus, I need to either ensure the target is
> close, or somehow force a longer jump '0xe9' into the code so I always
> have the space. The other advantage of not including the nop is easier
> support for all x86 implementations, since I'm not sure a 5 byte atomic
> nop is always available, whereas a jump is always atomic. I'm pretty
> sure we can come up with a patch that avoids the nop...I'll keep working
> on it.
> 

Unfortunately gas doesn't have any equivalent of the NASM "strict" 
operand modifier, which would be ideal here.  The following *seems* to 
work on binutils-2.18.50.0.9-8.fc10.x86_64 at least:

	.byte 0xe9
	.long %0-1f
1:

	-hpa

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