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

[Michael Kruse via flang-commits]

================
@@ -231,6 +231,26 @@ function (add_flangrt_library name)
       target_compile_options(${tgtname} PRIVATE
           $<$<COMPILE_LANGUAGE:CXX>:-fno-exceptions -fno-rtti -funwind-tables -fno-asynchronous-unwind-tables>
         )
+
+      # We define our own _GLIBCXX_THROW_OR_ABORT here because, as of
+      # GCC 15.1, libstdc++ uses (void)_EXC in its definition in order
+      # to silence a warning.
+      #
+      # This is a problem for us because some compilers, specifically
+      # clang, do not always optimize away that (void)_EXC even though
+      # it is unreachable since it occurs after a call to
+      # _builtin_abort().  Because _EXC is typically an object derived
+      # from std::exception, (void)_EXC, when not optimized away,
+      # causes a link reference to std::exception, and we do not want
+      # to link against std::exception since doing that would link us
+      # against the C++ runtime library, and we do not want to link
+      # the Fortran runtime against the C++ runtime.
----------------
Meinersbur wrote:

Reads like "causes A, and we do not want to do A since doing that would do B, and we do not want to do B" with A = "link to std::exception" and B = "link against C++ standard library". Can shorten and state the reason instead.

```suggestion
      # calls std::exception methods in the libstdc++ static or dynamic library. We cannot link libstdc++ because it may conflict with a C++ runtime of a hybrid Fortran/C++ application.
```


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