https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106762
Bug ID: 106762
Summary: incorrect array bounds warning (-Warray-bounds) at -O2
on memset()
Product: gcc
Version: 11.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: para at tampabay dot rr.com
Target Milestone: ---
Created attachment 53517
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53517&action=edit
Minimized test case to reproduce the issue
This bug also affects GCC 12.0.1.
I have an inline function in a header file that does an array lookup, returning
NULL if the index given is out of bounds in the array. The result is passed to
another function to reset the structure. At -O2 this produces the following
warning:
bug.c:29:2: warning: ‘memset’ offset [0, 7] is out of the bounds [0, 0]
[-Warray-bounds]
29 | memset(&obj->field1, 0xff, sizeof(obj->field1));
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
This does not happen at -O0 or -O1. It appears that some part of the compiler
is incorrectly concluding that `obj` must be NULL. For now I am suppressing the
warning by assigning a structure field before calling memset() to hint that the
memset() is only reachable if the pointer is non-NULL.
One of the necessary conditions for reproducing this bug is curiously that the
field being memset() cannot be the first in the structure.
In the attached code that reproduces the issue I am setting the array index
`idx` to 0. The original code used a loop `for(idx = 0; idx < ary->objcnt;
idx++)` such that the else case cannot be hit.
