https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113664
Bug ID: 113664
Summary: False positive warnings with -fno-strict-overflow
(-Warray-bounds, -Wstringop-overflow)
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: stefan at bytereef dot org
Target Milestone: ---
These false positives only occur in combination with fno-strict-overflow:
================
-Warray-bounds
================
foo.c
=========================================================
#include <stdio.h>
static char *
f(char *s, int n, char *dot)
{
switch(n) {
case 1:
if (s == dot) {
*s++ = '.';
}
*s++ = '0'; /* fall-through (yes, really!) */
default:
if (s == dot) {
*s++ = '.';
}
}
*s = '\0';
return s;
}
char *
g(char *s)
{
return f(s, 1, NULL);
}
=========================================================
$ /home/skrah/gcc/bin/gcc -Wall -O3 -c foo.c
$ /home/skrah/gcc/bin/gcc -Wall -O3 -fno-strict-overflow -c foo.c
In function ‘f’,
inlined from ‘g’ at foo.c:25:10:
foo.c:11:10: warning: array subscript 0 is outside array bounds of ‘char[0]’
[-Warray-bounds=]
11 | *s++ = '0'; /* fall-through (yes, really!) */
| ~~~~~^~~~~
In function ‘g’:
cc1: note: source object is likely at address zero
=====================
-Wstringop-overflow
=====================
bar.c
=========================================================
#include <stdio.h>
static char *
f(char *s, int n, char *dot)
{
switch(n) {
case 1:
if (s == dot) {
*s++ = '.';
}
*s++ = '0'; /* fall-through (yes, really!) */
default:
if (s == dot) {
*s++ = '.';
}
}
*s = '\0';
return s;
}
char *
g(char *s)
{
char sign = '+';
*s++ = sign;
return f(s, 1, NULL);
}
=========================================================
$ /home/skrah/gcc/bin/gcc -Wall -O3 -c bar.c
$ /home/skrah/gcc/bin/gcc -Wall -O3 -fno-strict-overflow -c bar.c
In function ‘f’,
inlined from ‘g’ at bar.c:28:12:
bar.c:11:10: warning: writing 1 byte into a region of size 0
[-Wstringop-overflow=]
11 | *s++ = '0'; /* fall-through (yes, really!) */
| ~~~~~^~~~~
In function ‘g’:
cc1: note: destination object is likely at address zero
Note that a very small change gives a very different warning.
