[Bug c/118212] New: False positive -Wstringop-truncation despite [[gnu::nonstring]]

[lukeshu at lukeshu dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118212

            Bug ID: 118212
           Summary: False positive -Wstringop-truncation despite
                    [[gnu::nonstring]]
           Product: gcc
           Version: 14.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lukeshu at lukeshu dot com
  Target Milestone: ---

System / GCC version / GCC options:

    $ gcc -v
    Using built-in specs.
    COLLECT_GCC=/usr/bin/gcc
    COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/14.2.1/lto-wrapper
    Target: x86_64-pc-linux-gnu
    Configured with: /build/gcc/src/gcc/configure
--enable-languages=ada,c,c++,d,fortran,go,lto,m2,objc,obj-c++,rust
--enable-bootstrap --prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib
--mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=https://gitlab.archlinux.org/archlinux/packaging/packages/gcc/-/issues
--with-build-config=bootstrap-lto --with-linker-hash-style=gnu
--with-system-zlib --enable-__cxa_atexit --enable-cet=auto
--enable-checking=release --enable-clocale=gnu --enable-default-pie
--enable-default-ssp --enable-gnu-indirect-function --enable-gnu-unique-object
--enable-libstdcxx-backtrace --enable-link-serialization=1
--enable-linker-build-id --enable-lto --enable-multilib --enable-plugin
--enable-shared --enable-threads=posix --disable-libssp --disable-libstdcxx-pch
--disable-werror
    Thread model: posix
    Supported LTO compression algorithms: zlib zstd
    gcc version 14.2.1 20240910 (GCC)

Reproducer source code:

    static struct {
      [[gnu::nonstring]] char name[16];
    } table[1] = {0};

    void set_name(unsigned int rownum, const char *name) {
      __builtin_strncpy(table[rownum].name, name, sizeof(table[0].name));
    }

Reproducer command and output:

    $ gcc -O2 -Wstringop-truncation -Werror -c f.c
    f.c: In function ‘set_name’:
    f.c:6:3: error: ‘__builtin_strncpy’ specified bound 16 equals destination
size [-Werror=stringop-truncation]
        6 |   __builtin_strncpy(table[rownum].name, name,
sizeof(table[0].name));
          |  
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    cc1: all warnings being treated as errors

Notes:

    This goes away if the size of table is increased beyond `1`, but
    comes back if rownum is set to the table size minus 1:

        static struct {
          [[gnu::nonstring]] char name[16];
        } table[3] = {0};

        void set_name(const char *name) {
          unsigned int rownum = 3;
          __builtin_strncpy(table[rownum-1].name, name, sizeof(table[0].name));
        }