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To: linux-kernel@vger.kernel.org
From: "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [patch 4/21] fix ARCH_HAS_PREFETCH
Date: 13 Aug 2002 15:12:53 -0700
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References: <3D56B13A.D3F741D1@zip.com.au> <Pine.NEB.4.44.0208132322340.1351-100000@mimas.fachschaften.tu-muenchen.de>
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Followup to:  <Pine.NEB.4.44.0208132322340.1351-100000@mimas.fachschaften.tu-muenchen.de>
By author:    Adrian Bunk <bunk@fs.tum.de>
In newsgroup: linux.dev.kernel
> >
> > Because the compiler sees:
> >
> > 	for (i = 0; i < N; i++)
> > 		;
> >
> > and it says "ah ha.  A busy wait delay loop" and leaves it alone.
> >
> > It's actually a special-case inside the compiler to not optimise
> > away such constructs.
> 
> Why is this a special case? As long as a compiler can't prove that the
> computed value of i isn't used later it mustn't optimize it away.
> 

Bullsh*t.  It can legitimately transform it into:

	   i = N;

> Kernighan/Ritchie (German translation of the second edition) contains the
> following example program that shows why the compiler mustn't optimize it
> away:
> 
> <--  snip  -->
> 
> #include <stdio.h>
> 
> main()
> {
>   double nc;
> 
>   for (nc = 0; getchar() != EOF; ++nc)
> 	;
>   printf("%.0f\n", nc);
> 
> }
> 

getchar() has side effects.

	-hpa
-- 
<hpa@transmeta.com> at work, <hpa@zytor.com> in private!
"Unix gives you enough rope to shoot yourself in the foot."
http://www.zytor.com/~hpa/puzzle.txt	<amsp@zytor.com>
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