$ g++ -v Using built-in specs. COLLECT_GCC=g++ COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-pc-linux-gnu/9.2.0/lto-wrapper Target: x86_64-pc-linux-gnu Configured with: /var/tmp/portage/sys-devel/gcc-9.2.0-r2/work/gcc-9.2.0/configure --host=x86_64-pc-linux-gnu --build=x86_64-pc-linux-gnu --prefix=/usr --bindir=/usr/x86_64-pc-linux-gnu/gcc-bin/9.2.0 --includedir=/usr/lib/gcc/x86_64-pc-linux-gnu/9.2.0/include --datadir=/usr/share/gcc-data/x86_64-pc-linux-gnu/9.2.0 --mandir=/usr/share/gcc-data/x86_64-pc-linux-gnu/9.2.0/man --infodir=/usr/share/gcc-data/x86_64-pc-linux-gnu/9.2.0/info --with-gxx-include-dir=/usr/lib/gcc/x86_64-pc-linux-gnu/9.2.0/include/g++-v9 --with-python-dir=/share/gcc-data/x86_64-pc-linux-gnu/9.2.0/python --enable-languages=c,c++,fortran --enable-obsolete --enable-secureplt --disable-werror --with-system-zlib --enable-nls --without-included-gettext --enable-checking=release --with-bugurl=https://bugs.gentoo.org/ --with-pkgversion='Gentoo 9.2.0-r2 p3' --disable-esp --enable-libstdcxx-time --enable-shared --enable-threads=posix --enable-__cxa_atexit --enable-clocale=gnu --enable-multilib --with-multilib-list=m32,m64 --disable-altivec --disable-fixed-point --enable-targets=all --enable-libgomp --disable-libmudflap --disable-libssp --disable-systemtap --enable-vtable-verify --enable-lto --without-isl --enable-default-pie --enable-default-ssp Thread model: posix gcc version 9.2.0 (Gentoo 9.2.0-r2 p3) --- Discovered on StackOverflow: https://stackoverflow.com/q/59076583/1806289 The following code produces an unexpected result in g++ (Gentoo 9.2.0-r2 p3) 9.2.0: #include <cstdio> const char *strArr[2][1] = { {"foo"}, {""}}; int main(void) { printf("%p\t%p\n", strArr[0][0], strArr[1][0]); return 0; } Compiling with `g++ -std=c++17 -pedantic test.cpp` and executing results in: $ ./a.out 0x55c61494d004 (nil) The expected result is for `strArr[1][0]` to hold a proper address and not a null pointer. This behavior is not present in the previous g++ 8.3.0 version, so I believe it is a regression in the current version. Reproducible: Always
At a glance looks like a gcc bug.
Probably slightly more evident bug: $ cat a.cc /* * https://bugs.gentoo.org/701364 * * $ g++ a.cc -o a && ./a * a1[0] = 0x557f4d60e004 * a2[0][0] = (nil) * * Here (nil) is entirely unexpected. */ #include <cstdio> const char *a1[1] = {""}; const char *a2[1][1] = { {""} }; int main(void) { printf("a1[0] = %p\n", a1[0]); printf("a2[0][0] = %p\n", a2[0][0]); return 0; } $ g++-9.2.0 a.cc -o a && ./a a1[0] = 0x5587a1b80004 a2[0][0] = (nil) $ clang++-9 a.cc -o a && ./a a1[0] = 0x402012 a2[0][0] = 0x402012
Vanilla gcc from git master seems to work fine. Let's see if I can bisect it down to a fix.
Bisected master down to a https://gcc.gnu.org/PR90947 fix: 36154c6c9d44a8f4e8329c96f436ea47e89a18bc is the first bad commit commit 36154c6c9d44a8f4e8329c96f436ea47e89a18bc Author: msebor <msebor@138bc75d-0d04-0410-961f-82ee72b054a4> Date: Thu Aug 1 23:45:36 2019 +0000 PR c++/90947 - Simple lookup table of array of strings is miscompiled gcc/cp/ChangeLog: PR c++/90947 * decl.c (reshape_init_array_1): Avoid truncating initializer lists containing string literals. gcc/testsuite/ChangeLog: PR c++/90947 * c-c++-common/array-1.c: New test. * g++.dg/abi/mangle73.C: New test. * g++.dg/cpp2a/nontype-class23.C: New test. * g++.dg/init/array53.C: New test. gcc/ChangeLog: PR c++/90947 * tree.c (type_initializer_zero_p): Define. * tree.h (type_initializer_zero_p): New function. git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@273989 138bc75d-0d04-0410-961f-82ee72b054a4 gcc/ChangeLog | 6 + gcc/cp/ChangeLog | 6 + gcc/cp/decl.c | 5 +- gcc/testsuite/ChangeLog | 8 + gcc/testsuite/c-c++-common/array-1.c | 247 +++++++++++++++++++++++++++ gcc/testsuite/g++.dg/abi/mangle73.C | 96 +++++++++++ gcc/testsuite/g++.dg/cpp2a/nontype-class23.C | 102 +++++++++++ gcc/testsuite/g++.dg/init/array53.C | 33 ++++ gcc/tree.c | 67 ++++++++ gcc/tree.h | 6 + 10 files changed, 574 insertions(+), 2 deletions(-) create mode 100644 gcc/testsuite/c-c++-common/array-1.c create mode 100644 gcc/testsuite/g++.dg/abi/mangle73.C create mode 100644 gcc/testsuite/g++.dg/cpp2a/nontype-class23.C create mode 100644 gcc/testsuite/g++.dg/init/array53.C
I suggest pulling the fix along with future gcc-9.3 release.
gcc-9.3.0 is in tree.
