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

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Date:	Mon, 23 Jul 2007 14:48:28 -0700
From:	Nicholas Miell <nmiell@...cast.net>
To:	Andi Kleen <ak@...e.de>
Cc:	Trent Piepho <xyzzy@...akeasy.org>,
	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__

On Mon, 2007-07-23 at 23:30 +0200, Andi Kleen wrote:
> > 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

GCC counts newlines and semicolons and uses that number as the likely
instruction count.

See asm_insn_count() in gcc/gcc/final.c

-- 
Nicholas Miell <nmiell@...cast.net>

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