[libcxx] linking error with -fno-exceptions

[futog]

#include <new>
#include <cassert>
#include <cstdlib>

int cnt = 0;
alignas(32) char DummyData[32 * 3];
void* operator new(std::size_t size) {
    return DummyData;
}

void operator delete(void*) noexcept {
    cnt++;
}

void test_it ( void );

int main() {
    test_it();
}

void test_it(void) {
    std::nothrow_t cval_nothrow;
    void* p = ::operator new(sizeof(int), cval_nothrow);

    assert(p == DummyData);

    ::operator delete(p, cval_nothrow);

    assert(cnt == 1);
}

built with fno-exceptions and linked with a libcxx built with fno-exceptions, gives the following error:

ld.lld: error: undefined symbol: __start___lcxx_override
>>> referenced by stdlib_new_delete.cpp.o:(operator new(unsigned int, std::nothrow_t const&)) in archive ../libc++abi.a
>>> referenced by stdlib_new_delete.cpp.o:(operator new(unsigned int, std::nothrow_t const&)) in archive ../libc++abi.a
>>> the encapsulation symbol needs to be retained under --gc-sections properly; consider -z nostart-stop-gc (see https://lld.llvm.org/ELF/start-stop-gc)

ld.lld: error: undefined symbol: __stop___lcxx_override
>>> referenced by stdlib_new_delete.cpp.o:(operator new(unsigned int, std::nothrow_t const&)) in archive ../libc++abi.a
>>> referenced by stdlib_new_delete.cpp.o:(operator new(unsigned int, std::nothrow_t const&)) in archive ../libc++abi.a

I think the reason is that the __lcxx_override section is not referenced anywhere, given the current fno-exception implementation of operator new(unsigned int, std::nothrow_t const&)

Based 3145265, this should not work runtime anyway, but I guess the intention here is not a linking error that is not that easy to decode.

[philnik777]

Could you give some more details? On godbolt this works just fine: https://godbolt.org/z/PvTT3vnrc

[futog]

Do we have libc++ compiled with fno-exceptions on godbolt? I can copy the relevant parts from the library and the map file.

libcxxabi/src/stdlib_new_delete.cpp:

_LIBCPP_WEAK void* operator new(size_t size, const std::nothrow_t&) noexcept {
#ifdef _LIBCPP_HAS_NO_EXCEPTIONS
#  if _LIBCPP_CAN_DETECT_OVERRIDDEN_FUNCTION
  _LIBCPP_ASSERT_SHIM(
      !std::__is_function_overridden(static_cast<void* (*)(std::size_t)>(&operator new)),
      "libc++ was configured with exceptions disabled and `operator new(size_t)` has been overridden, "
      "but `operator new(size_t, nothrow_t)` has not been overridden. This is problematic because "
      "`operator new(size_t, nothrow_t)` must call `operator new(size_t)`, which will terminate in case "
      "it fails to allocate, making it impossible for `operator new(size_t, nothrow_t)` to fulfill its "
      "contract (since it should return nullptr upon failure). Please make sure you override "
      "`operator new(size_t, nothrow_t)` as well.");
#  endif

  return operator_new_impl(size);
#else
  void* p = nullptr;
  try {
    p = ::operator new(size);
  } catch (...) {
  }
  return p;
#endif
}

and this is the relevant part of the map file:

80012f04 80012f04       74     4         ./libc++abi.a(stdlib_new_delete.cpp.o):(.text._ZnwjRKSt9nothrow_t)
80012f04 80012f04        0     1                 $x
80012f04 80012f04       74     1                 operator new(unsigned int, std::nothrow_t const&)
80012f44 80012f44        0     1                 .L0 
80012f5c 80012f5c        0     1                 .L0 
80012f6c 80012f6c        0     1                 .L0 
80012f78 80012f78        4     4         ./libc++abi.a(stdlib_new_delete.cpp.o):(.text._ZdlPvRKSt9nothrow_t)
80012f78 80012f78        0     1                 $x
80012f78 80012f78        4     1                 operator delete(void*, std::nothrow_t const&)

[frederick-vs-ja]

Have you tried replacing nothrow_t versions together?

[futog]

Have you tried replacing nothrow_t versions together?

yes, same thing.

[ldionne]

I tried this locally on macOS and I get the following error at runtime, as expected:

# .---command stderr------------
# | libc++abi: libc++ was configured with exceptions disabled and `operator new(size_t)` has been overridden, but `operator new(size_t, nothrow_t)` has not been overridden. This is problematic because `operator new(size_t, nothrow_t)` must call `operator new(size_t)`, which will terminate in case it fails to allocate, making it impossible for `operator new(size_t, nothrow_t)` to fulfill its contract (since it should return nullptr upon failure). Please make sure you override `operator new(size_t, nothrow_t)` as well.
# `-----------------------------

What platform are you running this on? Can you share your CMake configuration and how you run the test? The very best would even be to share some kind of Docker image that reproduces, that way we'd be able to reproduce with certainty.

[futog]

This is not a libcxx problem. It is possible to create a small reproducer.

The problem comes only if -Wl,--gc-sections is used. The error message says so, I just missed it:
>>> the encapsulation symbol needs to be retained under --gc-sections properly; consider -z nostart-stop-gc (see https://lld.llvm.org/ELF/start-stop-gc)

Thank you for your time looking into this.