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Date: Sun, 28 Jul 2002 18:16:00 -0000
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From: Makoto Fujiwara <makoto@ki.nu>
Subject: Re: optimization/7427: gcc-3.1.1 -O2 problem for checksum calculation (powerpc)
Reply-To: Makoto Fujiwara <makoto@ki.nu>
X-SW-Source: 2002-07/txt/msg00752.txt.bz2
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The following reply was made to PR optimization/7427; it has been noted by GNATS.

From: Makoto Fujiwara <makoto@ki.nu>
To: Andrew Pinski <pinskia@physics.uc.edu>
Cc: gcc-gnats@gcc.gnu.org
Subject: Re: optimization/7427: gcc-3.1.1 -O2 problem for checksum calculation (powerpc)
Date: Mon, 29 Jul 2002 10:14:59 +0900

 Andrew Pinski> Not a bug in gcc, but a bug in your code because of C aliasing rules.
 Andrew Pinski> > 	unsigned short *w = (void *)&tmp;
 
 Thanks Andrew, for super quick response.
 
 (1) 'gcc -Wall' gave nothing on the line 12. Is this still right ?
 
 (2) I have changed the code little bit and ..
 
 I have modified (a) and modifed (b) for further evaluation.
 (a) got printf in between while (b) does not. Code (a) with printf
 gave me the right sum even with -O2, but (b) gave the wrong sum.
 
 Is this right behavior of optimization ?
 (a)
 low = 0x181
 hi  = 0x8e84
 sum = 0x68ac
 (b) 
 sum = 0x8fb9
 
 ------ (a) ------
 #include <stdio.h>
 
 struct buf {
 	int data;
 };
 
 bug(m)
 	struct buf *m;
 {
 	int sum = 0;
 	struct buf tmp;
 	unsigned short *w = (void *) &tmp;
 	unsigned short low,hi;
 
 	bzero(&tmp, sizeof tmp);
 	tmp.data = m->data;
 
 	//asm volatile ("" ::: "memory");
 	low = w[0];
 	hi  = w[1];
 
 	printf("low = 0x%x\n", low);
 	printf("hi  = 0x%x\n", hi);
 
 	sum += w[0];
 	sum += w[1];
 
 	printf("sum = 0x%x\n", sum);
 
 	return 0;
 }
 
 main()
 {
   struct buf m;
 	m.data = 0x12345678;
 	bug(&m);
 }
 -----  (b) -------
 #include <stdio.h>
 
 struct buf {
 	int data;
 };
 
 bug(m)
 	struct buf *m;
 {
 	int sum = 0;
 	struct buf tmp;
 	unsigned short *w = (void *) &tmp;
 	unsigned short low,hi;
 
 	bzero(&tmp, sizeof tmp);
 	tmp.data = m->data;
 
 	//asm volatile ("" ::: "memory");
 	low = w[0];
 	hi  = w[1];
 
 	//	printf("low = 0x%x\n", low);
 	//	printf("hi  = 0x%x\n", hi);
 
 	sum += w[0];
 	sum += w[1];
 
 	printf("sum = 0x%x\n", sum);
 
 	return 0;
 }
 
 main()
 {
   struct buf m;
 	m.data = 0x12345678;
 	bug(&m);
 }
 ------------------
 
 ---
 Makoto Fujiwara, 
 Chiba, Japan, Narita Airport and Disneyland prefecture.