<stacktrace> support

[eternalcomet]

c++23 has introduced a new header , I can use it with g++ -std=c++23 -lstdc++_libbacktrace stacktrace.cpp -o test on Ubuntu gcc (13.2.0). However, mingw gcc doesn't define macro __cpp_lib_stacktrace, which will cause error on compilation.

How can I use on mingw gcc on Windows?

[skeeto]

Poking around, it seems GCC searches for libbacktrace at configure time, which sets HAVE_STACKTRACE and ultimately enables C++23 stacktrace. So it would need to be pulled down as a new dependency, then built before GCC is cross compiled. So maybe after /winpthreads, before /gcc in Dockerfile. If someone would like to take a crack at it, go ahead. Unfortunately there's never been a formal release and no stable release artifacts.

[Peter0x44]

There's a copy of libbacktrace in gcc itself, so I doubt it's required to build and provide it separately
./configure --help in libstdc++v3 shows:

  --enable-libstdcxx-backtrace
                          turns on libbacktrace support [default=auto]

Perhaps it's enough to just pass it explicitly? If I look at how configure seems to be checking whether auto should enable it, I can't see any obvious reason why it would be off.

https://github.com/gcc-mirror/gcc/blob/22847ef193670e761ed205a4a6f0a694b939d4e4/libstdc%2B%2B-v3/acinclude.m4#L5530-L5605

[Peter0x44]

unfortunately, that didn't help.

 $ g++ -v
...
Configured with: /gcc-14.2.0/configure --prefix=/w64devkit --with-sysroot=/w64devkit/x86_64-w64-mingw32 --with-native-system-header-dir=/include --target=x86_64-w64-mingw32 --host=x86_64-w64-mingw32 --enable-static --disable-shared --with-pic --with-gmp-include=/deps/include --with-gmp-lib=/deps/lib --with-mpc-include=/deps/include --with-mpc-lib=/deps/lib --with-mpfr-include=/deps/include --with-mpfr-lib=/deps/lib --enable-languages=c,c++,fortran --enable-libgomp --enable-threads=posix --enable-version-specific-runtime-libs --disable-dependency-tracking --disable-lto --disable-multilib --disable-nls --disable-win32-registry --enable-mingw-wildcard --enable-libstdcxx-backtrace=yes CFLAGS_FOR_TARGET=-Os CXXFLAGS_FOR_TARGET=-Os LDFLAGS_FOR_TARGET=-s CFLAGS=-Os CXXFLAGS=-Os LDFLAGS=-s

$ g++ test.cpp -std=c++26
test.cpp: In function 'int nested_func(int)':
test.cpp:6:23: error: 'std::stacktrace' has not been declared
    6 |     std::cout << std::stacktrace::current() << '\n';
      |                       ^~~~~~~~~~

For completeness, I tried MSYS2 where it works fine...

$ g++ test.cpp -std=c++23 -lstdc++exp

$ g++ -v
...
Configured with: ../gcc-14.2.0/configure --prefix=/ucrt64 --with-local-prefix=/ucrt64/local --build=x86_64-w64-mingw32 --host=x86_64-w64-mingw32 --target=x86_64-w64-mingw32 --with-native-system-header-dir=/ucrt64/include --libexecdir=/ucrt64/lib --enable-bootstrap --enable-checking=release --with-arch=nocona --with-tune=generic --enable-mingw-wildcard --enable-languages=c,lto,c++,fortran,ada,objc,obj-c++,rust,jit --enable-shared --enable-static --enable-libatomic --enable-threads=posix --enable-graphite --enable-fully-dynamic-string --enable-libstdcxx-filesystem-ts --enable-libstdcxx-time --disable-libstdcxx-pch --enable-lto --enable-libgomp --disable-libssp --disable-multilib --disable-rpath --disable-win32-registry --disable-nls --disable-werror --disable-symvers --with-libiconv --with-system-zlib --with-gmp=/ucrt64 --with-mpfr=/ucrt64 --with-mpc=/ucrt64 --with-isl=/ucrt64 --with-pkgversion='Rev3, Built by MSYS2 project' --with-bugurl=https://github.com/msys2/MINGW-packages/issues --with-gnu-as --with-gnu-ld --disable-libstdcxx-debug --enable-plugin --with-boot-ldflags=-static-libstdc++ --with-stage1-ldflags=-static-libstdc++

I cannot see anything special about how gcc is configured here that would make a difference...

[skeeto]

Good catch, @Peter0x44. That's how it's supposed to work, and "auto" means it should already be enabled. However, the configure script is broken. It uses the filetype.awk script from libbacktrace incorrectly, applying it to an EXE instead of an object file, and so fails to detect the target object file format. It works by accident on ELF hosts because objects and images have the same format. This overrides --enable-libstdcxx-backtrace, so it has no effect. Then it builds libbacktrace anyway, but doesn't use it! It seems libbacktrace integration is cursed. I found a workaround (0bbc0d5), and @eternalcomet's test works when I use it. Unless you think there's a better way, Peter0x44, I'll merge it. This bug should probably be reported upstream.

[skeeto]

On further investigation, the script *is* applied to an object file, but filetype.awk cannot detect the -mbig-obj object format, the default since c9c9c07. So it was working before, but then broke. The awk script should be updated upstream in libbacktrace. My testing already confirmed that libbacktrace itself works fine with -mbig-obj. My local workaround is simpler than fixing the script here: We already know the answer.

[skeeto]

I don't know if this will be merged or not, nor when it would make its way into GCC, but here it is:
ianlancetaylor/libbacktrace#150