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Date: Wed, 14 Aug 2002 22:45:56 +0200
From: Willy Tarreau <willy@w.ods.org>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Rogier Wolff <R.E.Wolff@BitWizard.nl>, linux-kernel@vger.kernel.org
Subject: Re: [patch 4/21] fix ARCH_HAS_PREFETCH
Message-ID: <20020814204556.GA7440@alpha.home.local>
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On Wed, Aug 14, 2002 at 12:41:04PM -0700, H. Peter Anvin wrote:
> Rogier Wolff wrote:
> >>
> >>Bullsh*t.  It can legitimately transform it into:
> >>
> >>	   i = N;
> >
> >
> >Right! But people are confusing "practise", "published interface", and 
> >"spec" again. 
> >
> >Published interface in this case is that gcc will not optimize an empty
> >loop away, as it is often used to generate a timing loop. 
> >
> 
> Yes.  This is a gcc-specific wart, a bad idea from the start, and 
> apparently one which has caught up with them to the point that they've 
> had to abandon it.

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.

Cheers,
Willy

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