Date: Mon, 11 Nov 2002 13:55:17 +0100 From: Poul-Henning Kamp <phk@critter.freebsd.dk> To: TOMITA Yoshinori <yoshint@flab.fujitsu.co.jp> Cc: FreeBSD-current@FreeBSD.ORG Subject: Re: gcc 3.2.1 optimization bug ? Message-ID: <53679.1037019317@critter.freebsd.dk> In-Reply-To: Your message of "Mon, 11 Nov 2002 21:41:42 %2B0900." <ywlvg34jn21.fsf@cerberus.proc.flab.fujitsu.co.jp>
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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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