From owner-freebsd-current  Mon Nov 11  4:55:44 2002
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To: TOMITA Yoshinori <yoshint@flab.fujitsu.co.jp>
Cc: FreeBSD-current@FreeBSD.ORG
Subject: Re: gcc 3.2.1 optimization bug ? 
In-Reply-To: Your message of "Mon, 11 Nov 2002 21:41:42 +0900."
             <ywlvg34jn21.fsf@cerberus.proc.flab.fujitsu.co.jp> 
Date: Mon, 11 Nov 2002 13:55:17 +0100
Message-ID: <53679.1037019317@critter.freebsd.dk>
From: Poul-Henning Kamp <phk@critter.freebsd.dk>
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In message <ywlvg34jn21.fsf@cerberus.proc.flab.fujitsu.co.jp>, TOMITA Yoshinori
 writes:
>For the source code below, compiling gcc -O2/-O3 seem to produce
>incorrect code.
>
>-----------------------------------
>#include <stdio.h>
>int main(int argc, char* argv[])
>{
>    unsigned int x = 0x12345678;
>    unsigned short tmp;
>    printf("%x\n", x);
>    tmp = ((unsigned short *)&x)[0];
>    ((unsigned short *)&x)[0] = ((unsigned short *)&x)[1];
>    ((unsigned short *)&x)[1] = tmp;
>    printf("%x\n", x);
>    return 0;
>}
>-----------------------------------

>% gcc -O2 a.c
>% ./a.out
>12345678
>12341234

% sed 's/unsigned int/unsigned volatile int/' a.c > b.c
% gcc -O2 b.c
% ./a.out
12345678
56781234

Your code forgot to tell the compiler that you would be messing
with the variables storage directly.

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.

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