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Date: Wed, 14 Aug 2002 13:58:41 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
Organization: Zytor Communications
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To: Willy Tarreau <willy@w.ods.org>
CC: Rogier Wolff <R.E.Wolff@BitWizard.nl>, linux-kernel@vger.kernel.org
Subject: Re: [patch 4/21] fix ARCH_HAS_PREFETCH
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Willy Tarreau wrote:
> 
> There would be a solution to tell gcc not to optimize things, which may
> not require too much work from gcc people. Basically, we would need to
> implement a __builtin_nop() function that would respect dependencies but
> not generate any code. This way, we could have :
> 
> 	for (i=0; i<N, i++);
> 
> optimized as i=N
> and
> 	for (i=0; i<N; i++)
> 		__builtin_nop();
> or even
> 	for (i=0; i<N; __builtin_nop(i++));
> do the real work.
> 
> This way, some loops could be optimized, and the developpers could explicitely
> tell the compiler when they need to prevent any optimization.
> 

#define __nop() asm volatile("")

Since some processors now have "busy wait delay" instructions, this
would also make it possible to do:

#if defined(__i386__) || defined(__x86_64__)

#define __busy_wait() asm volatile("rep;nop")

#else

#define __busy_wait() asm volatile("")

#endif

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