[Bug middle-end/111696] New: [11/12/13/14 Regression] Spurious -Wstringop-overflow

["stilor at att dot net" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111696

            Bug ID: 111696
           Summary: [11/12/13/14 Regression] Spurious -Wstringop-overflow
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: stilor at att dot net
  Target Milestone: ---

Created attachment 56052
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=56052&action=edit
test case

The attached testcase started producing a warning beginning with commit
8d57bdadd2d9c2e5c95515ca7a583d7b407b55c4 when compiled with `-O3` (or `-O2
-ftree-slp-vectorize`). Starting with commit
2b8453c401b699ed93c085d0413ab4b5030bcdb8, vectorization is also enabled at
`-O2` and the testcase starting failing at `-O2` as well. The warning can be
reproduced at current HEAD (96557ee6a0a234821af865800d22621efa6e7390) as well.

Failure message:

$ /usr/gcc-test/bin/gcc -c -O3 -Wall -Werror stringop-overflow-bug-gcc.c 
stringop-overflow-bug-gcc.c: In function ‘update’:
stringop-overflow-bug-gcc.c:25:3: error: writing 24 bytes into a region of size
16 [-Werror=stringop-overflow=]
   25 |   memcpy(&last->s, &cur.s, sizeof(struct nested));
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
stringop-overflow-bug-gcc.c:10:17: note: destination object ‘a’ of size 16
   10 |   unsigned long a[2];
      |                 ^
cc1: all warnings being treated as errors
$ /usr/gcc-test/bin/gcc -v
Using built-in specs.
COLLECT_GCC=/usr/gcc-test/bin/gcc
COLLECT_LTO_WRAPPER=/usr/gcc-test/libexec/gcc/x86_64-pc-linux-gnu/11.0.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc/configure --enable-languages=c --prefix=/usr/gcc-test
--disable-bootstrap
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.0.1 20210303 (experimental) (GCC) 


The message is bogus:
- the "note" points to a member inside a structure and references that member's
size while memcpy has the whole structure as a destination.
- commenting out any of the first two statements in the update() function
removes the warning even though it has no effect on the addresses passed to
memcpy().