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* optimization/5159: ptr[i] = ptr[++i]; // works differently with malloc() and -O
@ 2001-12-18 14:56 robc
0 siblings, 0 replies; 2+ messages in thread
From: robc @ 2001-12-18 14:56 UTC (permalink / raw)
To: gcc-gnats
>Number: 5159
>Category: optimization
>Synopsis: ptr[i] = ptr[++i]; // works differently with malloc() and -O
>Confidential: no
>Severity: non-critical
>Priority: low
>Responsible: unassigned
>State: open
>Class: wrong-code
>Submitter-Id: net
>Arrival-Date: Tue Dec 18 14:56:00 PST 2001
>Closed-Date:
>Last-Modified:
>Originator: RobC
>Release: gcc version 2.95.2 19991024 (release)
>Organization:
>Environment:
Linux 2.2.19, Pentium II
>Description:
ptr[i] = ptr[++i];
when ptr is a pointer and compiled with -O then it behaves differently.
>How-To-Repeat:
compile with "gcc -O"
>Fix:
ptr[i] = ptr[i+1];
i++;
>Release-Note:
>Audit-Trail:
>Unformatted:
----gnatsweb-attachment----
Content-Type: text/plain; name="test.c"
Content-Disposition: inline; filename="test.c"
#include <stdio.h>
#include <stdlib.h>
int main()
{
int i=0;
int *pos = malloc(sizeof(int) * 2);
pos[0] = 0;
pos[1] = 1;
pos[i] = pos[++i];
printf("%i %i\n", pos[0], pos[1]);
return(0);
}
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: optimization/5159: ptr[i] = ptr[++i]; // works differently with malloc() and -O
@ 2001-12-18 15:22 neil
0 siblings, 0 replies; 2+ messages in thread
From: neil @ 2001-12-18 15:22 UTC (permalink / raw)
To: gcc-bugs, gcc-prs, nobody, robc
Synopsis: ptr[i] = ptr[++i]; // works differently with malloc() and -O
State-Changed-From-To: open->closed
State-Changed-By: neil
State-Changed-When: Tue Dec 18 15:22:36 2001
State-Changed-Why:
Not a bug - you should read up on sequence points. Basically, multiple reads of a variable (i) modified between sequence points is undefined. Your code falls into this category. So,
ptr[i] = ptr[++i];
is fundamentally always meaningless.
http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=5159
^ permalink raw reply [flat|nested] 2+ messages in thread
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2001-12-18 14:56 optimization/5159: ptr[i] = ptr[++i]; // works differently with malloc() and -O robc
2001-12-18 15:22 neil
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