https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106904
Bug ID: 106904 Summary: Incorrect -Wstringop-overflow with partial memcpy() into a nested structure Product: gcc Version: 12.2.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c Assignee: unassigned at gcc dot gnu.org Reporter: zfigura at codeweavers dot com Target Milestone: --- Created attachment 53562 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53562&action=edit minimal test case I encountered a warning while trying to compile 32-bit wine 7.17 with gcc 12.2, specificially at this line here: https://source.winehq.org/git/wine.git/blob/wine-7.17:/dlls/win32u/message.c#l359 The relevant code copies a smaller structure into a larger one of a different type. (This may be a violation of aliasing rules, but adding -fno-strict-aliasing doesn't change anything.) I was able to reproduce this with a minimal test case. This is a very weird set of conditions, but I couldn't seem to reduce this test case any further. Changing the type of "ps" to "struct packed_windowpos" makes the error go away; so does changing the first argument of the memcpy to "ps". leslie@terabithia:~$ gcc --version gcc (Debian 12.2.0-1) 12.2.0 Copyright (C) 2022 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. leslie@terabithia:~$ gcc -m32 test.c -c -o test.o -Wall -O2 test.c: In function ‘func’: test.c:26:5: warning: writing 8 bytes into a region of size 4 [-Wstringop-overflow=] 26 | __builtin_memcpy(&ps->wp, &wp, sizeof(wp)); | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ test.c:9:9: note: destination object ‘hwnd’ of size 4 9 | int hwnd; | ^~~~