[Bug c/115527] New: -ftrivial-auto-var-init appears to clobber explicit initializer

[nora at norasandler dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115527

            Bug ID: 115527
           Summary: -ftrivial-auto-var-init appears to clobber explicit
                    initializer
           Product: gcc
           Version: 13.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nora at norasandler dot com
  Target Milestone: ---

This program behaves incorrectly when compiled with -ftrivial-auto-var=pattern
or -ftrivial-auto-var=zero:

// Bug in GCC -ftrivial-auto-var
struct inner {
    double a;
    char b;
    long l;
};

struct outer {
    struct inner in_array[3];
    int bar;
};

int main(void) {
    struct inner si = {1., 0, 0};
    struct outer o = {// in_array
                      {
                          si,
                          {0, 0, 0},
                          {0, 0, 0},
                      },
                      // bar
                      0};

    if (o.in_array[0].a != 1.) {
        return 1; // fail
    }
    return 0;
}

Without the -ftrivial-auto-var option, this returns 0, as expected. With
-ftrivial-auto-var=pattern or -ftrivial-auto-var=zero it returns 1:


$ gcc -ftrivial-auto-var-init=pattern auto-var-bug.c
$ ./a.out
$ echo $?
1
$ gcc auto-var-bug.c
$ ./a.out
$ echo $?
0

I saw this behavior in GCC 13.1.0, 13.2.0, and 14.0.1 (I haven't tried it with
other versions).

Full version info for the version of 13.2.0 I tested this with:
$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-linux-gnu/13/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
13.2.0-23ubuntu4' --with-bugurl=file:///usr/share/doc/gcc-13/README.Bugs
--enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-13
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/libexec --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new
--enable-libstdcxx-backtrace --enable-gnu-unique-object --disable-vtable-verify
--enable-plugin --enable-default-pie --with-system-zlib
--enable-libphobos-checking=release --with-target-system-zlib=auto
--enable-objc-gc=auto --enable-multiarch --disable-werror --enable-cet
--with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32
--enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none=/build/gcc-13-uJ7kn6/gcc-13-13.2.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-13-uJ7kn6/gcc-13-13.2.0/debian/tmp-gcn/usr
--enable-offload-defaulted --without-cuda-driver --enable-checking=release
--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 13.2.0 (Ubuntu 13.2.0-23ubuntu4)