Exceptions thrown inside the GCC-compiled libstdc++.so can't be caught by EDG-compiled code (terminate) #445

Issue Summary

With the C-generating back end, exception handling is lowered to EDG's own
runtime (libedgrt, lib_src/throw.c): handlers are a stack of frames rooted
at __curr_eh_stack_entry. libstdc++.so is compiled by GCC and throws with
the Itanium C++ ABI's __cxa_throw, which never looks at EDG's frames. So a
catch in EDG-compiled code never matches an exception thrown inside
libstdc++.so, and the program calls terminate(). The same exception thrown
from EDG-compiled code is caught normally.

Anything that throws from inside libstdc++.so is affected:

• std::stoi and friends (std::invalid_argument)
• std::vector::at and std::string::at, which call std::__throw_out_of_range_fmt in the .so
• iostreams with exceptions enabled (std::__throw_ios_failure)
• std::locale("name") (std::runtime_error)
• presumably also std::filesystem and std::thread errors (not tested individually)

In Boost, this fails Boost.JSON's value, Boost.Regex's regex_regress and
Boost.Charconv's github_issue_122.

Reproducing Source Code

#include <stdexcept>
#include <string>

int main() {
  try {
    (void)std::stoi("x"); /* throws std::invalid_argument from libstdc++.so */
  } catch (const std::invalid_argument &) {
    return 0;
  }
  return 1;
}

Command-line Options

EDG_USE_SYSTEM_HEADERS=1 eccp --c++17 libstdcxx_throw.cpp -o libstdcxx_throw && ./libstdcxx_throw

Type of Issue

[Back End] There is a problem with the generated C code.

Additional Details

Expected: exit status 0 (g++ 15.3.1: 0).
Actual (linux-gcc-release and linux-gcc-debug): 3 of 3 runs:

C++ runtime abort: terminate() called by the exception handling mechanism

and Aborted, exit status 134. A throw std::runtime_error("x") written in
the program itself is caught. std::string("abc").at(10) aborts the same way.

The problem is in how EDG's runtime library works with libstdc++.so rather
than in the generated C as such; none of the issue types fits exactly.
libedgrt has no bridge for foreign exceptions: the only __cxa_* throw
routine it defines is __cxa_throw_bad_array_new_length
(lib_src/vec_newdel.c:1143). bases/docker/dev-env/gcc/edg_eccp_config:33
links -lstdc++ -lgcc_s -lpthread.

Possible directions (untested):

• libedgrt could define the std::__throw_* helpers (__throw_out_of_range_fmt, __throw_length_error, __throw_invalid_argument, ...) to throw with EDG's mechanism. That would cover throws that start in header code compiled by EDG, such as vector::at, and possibly some inside the .so, depending on symbol interposition.
• The runtime could catch foreign (Itanium) exceptions at the boundary and rethrow them through EDG's mechanism.

Backtraces of the Boost failures (gdb at the abort):

• Boost.JSON value: __cxa_throw (libstdc++.so.6) ←
  std::__throw_ios_failure (libstdc++.so.6) ← std::basic_ios<char>::clear
  (libstdc++.so.6) ← boost::json::operator>>(std::istream &, value &)
• Boost.Regex regex_regress: __cxa_throw ← std::__throw_out_of_range_fmt
  (libstdc++.so.6) ← std::vector<std::pair<size_t, size_t>>::_M_range_check
  ← at ← boost::basic_regex<char>::subexpression
• Boost.Charconv github_issue_122: __cxa_throw ←
  std::__throw_runtime_error ← std::locale::facet::_S_create_c_locale ←
  std::locale::_Impl::_Impl (all libstdc++.so.6)

Environment: edgcpp/dev-env Docker image (Fedora 43, GCC 15.3.1) on
x86-64, boost branch at c36e03a67c. EDG_USE_SYSTEM_HEADERS=1 makes eccp
use GCC 15's headers (libstdc++) and pass --gnu_version=150301
(bases/docker/dev-env/gcc/edg_eccp_config).

Reproducing (Standard) Configuration(s)

linux-gcc-release

Reproducing (Non-standard) Configuration

No response

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• The reproducing source code that I have provided does not depend on headers that I have not included with my submission.
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