https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101525
Bug ID: 101525
Summary: "out of the bounds" warning for an Innocuous memset
call with LTO
Product: gcc
Version: 11.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: Hi-Angel at yandex dot ru
Target Milestone: ---
Created attachment 51176
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51176&action=edit
preprocessed file that triggers the warnings
Given this memset call
static bool
append_space_for_newline (struct it *it, bool default_face_p) {
[…]
memset (&it->position, 0, sizeof it->position);
GCC complains:
xdisp.c: In function ‘append_space_for_newline’:
xdisp.c:21906:7: warning: ‘memset’ offset [2352, 2359] from the object at
‘it_122(D)’ is out of the bounds of referenced subobject ‘charpos’ with type
‘long int’ at offset 2344 [-Warray-bounds]
21906 | memset (&it->position, 0, sizeof it->position);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from composite.h:29,
from xdisp.c:441:
dispextern.h:214:13: note: subobject ‘charpos’ declared here
214 | ptrdiff_t charpos;
| ^~~~~~~
I can't see any overflow on this line. Either way, further experiments show
that removing the function content below the offending line makes warnings
disappear. Since it's impossible to make overflow disappear by removing a code
below the overflow, the warning disappearance suggests the warning is a GCC bug
and should not be there.
----------------------------------------
I couldn't reduce it down to a minimal testcase because removing irrelevant
code makes warning go away, so instead in steps-to-reproduce I use a
preprocessed source. I had to compress it to be able to attach. The original is
a `src/xdisp.c` file in Emacs project at commit `6ebe8b03d80`.
# Steps to reproduce
1. Download `xdisp.preprocessed.c.zst`
2. Unpack with: zstd -d xdisp.preprocessed.c.zst
3. Compile with: gcc -c -Warray-bounds=2 -flto=2 -O3 -ffat-lto-objects
xdisp.preprocessed.c
## Expected
No warnings for `append_space_for_newline` function
## Actual
There is a warning:
xdisp.c: In function ‘append_space_for_newline’:
xdisp.c:21906:7: warning: ‘memset’ offset [2352, 2359] from the object at
‘it_140(D)’ is out of the bounds of referenced subobject ‘charpos’ with type
‘long int’ at offset 2344 [-Warray-bounds]
In file included from composite.h:29,
from xdisp.c:441:
dispextern.h:214:13: note: subobject ‘charpos’ declared here
