Path: utzoo!attcan!uunet!husc6!ukma!tut.cis.ohio-state.edu!osu-cis!att!cbnews!lvc
From: lvc@cbnews.ATT.COM (Lawrence V. Cipriani)
Newsgroups: comp.lang.c
Subject: Does your compiler get this program right?
Keywords: float, increment
Message-ID: <2298@cbnews.ATT.COM>
Date: 23 Nov 88 14:16:58 GMT
Organization: AT&T Bell Laboratories, Columbus
Lines: 69

A friend of mine found a bug in his C compiler.  He found
the bug on a VAX; it also exists in some 3B compilers.  The
compiler on this SVR3 3b2 does it right and is shown at the
end.  The bug was that the increment to the float pointer
was happening twice; see the /* miscompiled line */ below.
I suspect other pcc derived compilers get it wrong too.

Larry Cipriani, AT&T Network Systems, Columbus OH,
Path: att!cbnews!lvc    Domain: lvc@cbnews.ATT.COM


----------------------fbug.c-----------------------

float list1[30], list2[10];

main()
{
	int i;
	float *f, *g;
	
	for (i = 0; i < 10; ++i) {
		list1[i] = (float) i;
		list2[i] = (float) 2 * i;
	}
	printf("List 1 is: ");
	pr_list(list1,10);
	printf("List 2 is: ");
	pr_list(list2,10);
	f = list1;
	g = list2;
	for (i = 0; i < 10; ++i) {
		*f++ += *g++;		/* miscompiled line */
	}
	printf("List 1 is: ");
	pr_list(list1,10);
	printf("List 2 is: ");
	pr_list(list2,10);
}

pr_list(list,num)
float list[];
int num;
{
	int i;

	for (i = 0; i < num; ++i) {
		if (i % 5 == 0)
			printf("\n");
		printf("%7.2f\t",list[i]);
	}
	printf("\n\n");
}

-------------------fbug.out------------------
List 1 is: 
   0.00	   1.00	   2.00	   3.00	   4.00	
   5.00	   6.00	   7.00	   8.00	   9.00	

List 2 is: 
   0.00	   2.00	   4.00	   6.00	   8.00	
  10.00	  12.00	  14.00	  16.00	  18.00	

List 1 is: 
   0.00	   3.00	   6.00	   9.00	  12.00	
  15.00	  18.00	  21.00	  24.00	  27.00	

List 2 is: 
   0.00	   2.00	   4.00	   6.00	   8.00	
  10.00	  12.00	  14.00	  16.00	  18.00