[Openmp-commits] [flang] [llvm] [openmp] [Flang-RT] Fix GCC 15.1 Fortran Runtime libstdc++ Undefined Symbols (PR #157385)

[Michael Kruse via Openmp-commits]

================
@@ -231,6 +231,12 @@ function (add_flangrt_library name)
       target_compile_options(${tgtname} PRIVATE
           $<$<COMPILE_LANGUAGE:CXX>:-fno-exceptions -fno-rtti -funwind-tables -fno-asynchronous-unwind-tables>
         )
+
+      # Define our own _GLIBCXX_THROW_OR_ABORT because libstdc++ headers
+      # reference std::exception in its definition, and we do not want
+      # to link against std::exception since doing that would link us to
+      # the C++ runtime.
----------------
Meinersbur wrote:

The comment is somewhat incorrect: libstdc++ does NOT use `std::exception` in its definition, (in the sense of `#define _GLIBCXX_THROW_OR_ABORT(...) std::exception`), but uses the `_EXC` argument which is typically is an exception and thus must not be used.

Please also add the following details:
 * libstdc++ uses `(void)_EXC` to silence a warning
 * Some compilers may not always optimize away that unreachable code (unreachable because it occurs after `__builtin_abort()`)
 * The intention is to only remove that `(void)_EXC`, and nothing else 
 * Redefining _GLIBCXX_THROW_OR_ABORT is not officially supported by libstdc++

Since this use of _GLIBCXX_THROW_OR_ABORT is unsupported/undocumented, describing (correctly) what and why we are doing this is REALLY important.

https://github.com/llvm/llvm-project/pull/157385