linux-kernel - Re: [PATCH 7/8] i386: bitops: Kill needless usage of __asm__ __volatile_

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Date:	Mon, 23 Jul 2007 23:30:41 +0200
From:	Andi Kleen <ak@...e.de>
To:	Trent Piepho <xyzzy@...akeasy.org>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Satyam Sharma <ssatyam@....iitk.ac.in>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	David Howells <dhowells@...hat.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 7/8] i386: bitops: Kill needless usage of __asm__ __volatile__

> gcc also tries to count the number of instructions, to guess how large in
> bytes the asm block is, as it could make a difference for near vs short
> jumps, etc.

Are you sure? I doubt it. It would need a full asm parser to do this
properly and then even it could be wrong 
(e.g. when the sections are switched like Linux does extensively)  

gcc doesn't have such a parser.

Also on x86 gcc doesn't need to care about long/short anyways because
the assembler takes care of this and the other instructions who cared
about this (loop) isn't generated anymore.

You're probably confusing it with some other compiler, who sometimes
do this. e.g. the Microsoft inline asm syntax requires assembler parsing
in the compiler.

> I wonder it it also affects the instruction count the inline heuristics
> use?

AFAIK it counts like one operand.

-Andi
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