Add LLVM libcxxabi (#1063)

* third_party: Add libcxxabi

Added libcxxabi from LLVM 17.0.6
The library implements the Itanium C++ exception handling ABI.

* third_party/libcxxabi: Enable __cxa_thread_atexit

Enable `__cxa_thread_atexit` from libcxxabi.
`__cxa_thread_atexit_impl` is still implemented by the cosmo libc.
The original `__cxa_thread_atexit` has been removed.

* third_party/libcxx: Build with exceptions

Build libcxx with exceptions enabled.

- Removed `_LIBCPP_NO_EXCEPTIONS` from `__config`.
- Switched the exception implementation to `libcxxabi`. These two files
are taken from the same `libcxx` version as mentioned in `README.cosmo`.
- Removed `new_handler_fallback` in favor of `libcxxabi` implementation.
- Enable `-fexceptions` and `-frtti` for `libcxx`.
- Removed `THIRD_PARTY_LIBCXX` dependency from `libcxxabi` and
`libunwind`. These libraries do not use any runtime `libcxx` functions,
just headers.

* libc: Remove remaining redundant cxa functions

- `__cxa_pure_virtual` in `libcxxabi` is also a stub similar to the
existing one.
- `__cxa_guard_*` from `libcxxabi` is used instead of the ones from
Android.

Now there should be no more duplicate implementations.
`__cxa_thread_atexit_impl`, `__cxa_atexit`, and related supporting
functions, are still left to other libraries as in `libcxxabi`.

`libcxxabi` is also now added to `cosmopolitan.a` to make up for the
removed functions.

Affected in-tree libraries (`third_party/double-conversion`) have been
updated.

third_party/libcxxabi/cxa_thread_atexit.cc

@@ -0,0 +1,145 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "third_party/libcxxabi/abort_message.h"
+#include "third_party/libcxxabi/include/cxxabi.h"
+#include "third_party/libcxx/__threading_support"
+#ifndef _LIBCXXABI_HAS_NO_THREADS
+#if defined(__ELF__) && defined(_LIBCXXABI_LINK_PTHREAD_LIB)
+#pragma comment(lib, "pthread")
+#endif
+#endif
+
+#include "libc/isystem/stdlib.h"
+
+namespace __cxxabiv1 {
+
+  using Dtor = void(*)(void*);
+
+  extern "C"
+#ifndef HAVE___CXA_THREAD_ATEXIT_IMPL
+  // A weak symbol is used to detect this function's presence in the C library
+  // at runtime, even if libc++ is built against an older libc
+  _LIBCXXABI_WEAK
+#endif
+  int __cxa_thread_atexit_impl(Dtor, void*, void*);
+
+#ifndef HAVE___CXA_THREAD_ATEXIT_IMPL
+
+namespace {
+  // This implementation is used if the C library does not provide
+  // __cxa_thread_atexit_impl() for us.  It has a number of limitations that are
+  // difficult to impossible to address without ..._impl():
+  //
+  // - dso_symbol is ignored.  This means that a shared library may be unloaded
+  //   (via dlclose()) before its thread_local destructors have run.
+  //
+  // - thread_local destructors for the main thread are run by the destructor of
+  //   a static object.  This is later than expected; they should run before the
+  //   destructors of any objects with static storage duration.
+  //
+  // - thread_local destructors on non-main threads run on the first iteration
+  //   through the __libccpp_tls_key destructors.
+  //   std::notify_all_at_thread_exit() and similar functions must be careful to
+  //   wait until the second iteration to provide their intended ordering
+  //   guarantees.
+  //
+  // Another limitation, though one shared with ..._impl(), is that any
+  // thread_locals that are first initialized after non-thread_local global
+  // destructors begin to run will not be destroyed.  [basic.start.term] states
+  // that all thread_local destructors are sequenced before the destruction of
+  // objects with static storage duration, resulting in a contradiction if a
+  // thread_local is constructed after that point.  Thus we consider such
+  // programs ill-formed, and don't bother to run those destructors.  (If the
+  // program terminates abnormally after such a thread_local is constructed,
+  // the destructor is not expected to run and thus there is no contradiction.
+  // So construction still has to work.)
+
+  struct DtorList {
+    Dtor dtor;
+    void* obj;
+    DtorList* next;
+  };
+
+  // The linked list of thread-local destructors to run
+  __thread DtorList* dtors = nullptr;
+  // True if the destructors are currently scheduled to run on this thread
+  __thread bool dtors_alive = false;
+  // Used to trigger destructors on thread exit; value is ignored
+  std::__libcpp_tls_key dtors_key;
+
+  void run_dtors(void*) {
+    while (auto head = dtors) {
+      dtors = head->next;
+      head->dtor(head->obj);
+      ::free(head);
+    }
+
+    dtors_alive = false;
+  }
+
+  struct DtorsManager {
+    DtorsManager() {
+      // There is intentionally no matching std::__libcpp_tls_delete call, as
+      // __cxa_thread_atexit() may be called arbitrarily late (for example, from
+      // global destructors or atexit() handlers).
+      if (std::__libcpp_tls_create(&dtors_key, run_dtors) != 0) {
+        abort_message("std::__libcpp_tls_create() failed in __cxa_thread_atexit()");
+      }
+    }
+
+    ~DtorsManager() {
+      // std::__libcpp_tls_key destructors do not run on threads that call exit()
+      // (including when the main thread returns from main()), so we explicitly
+      // call the destructor here.  This runs at exit time (potentially earlier
+      // if libc++abi is dlclose()'d).  Any thread_locals initialized after this
+      // point will not be destroyed.
+      run_dtors(nullptr);
+    }
+  };
+} // namespace
+
+#endif // HAVE___CXA_THREAD_ATEXIT_IMPL
+
+extern "C" {
+
+  _LIBCXXABI_FUNC_VIS int __cxa_thread_atexit(Dtor dtor, void* obj, void* dso_symbol) throw() {
+#ifdef HAVE___CXA_THREAD_ATEXIT_IMPL
+    return __cxa_thread_atexit_impl(dtor, obj, dso_symbol);
+#else
+    if (__cxa_thread_atexit_impl) {
+      return __cxa_thread_atexit_impl(dtor, obj, dso_symbol);
+    } else {
+      // Initialize the dtors std::__libcpp_tls_key (uses __cxa_guard_*() for
+      // one-time initialization and __cxa_atexit() for destruction)
+      static DtorsManager manager;
+
+      if (!dtors_alive) {
+        if (std::__libcpp_tls_set(dtors_key, &dtors_key) != 0) {
+          return -1;
+        }
+        dtors_alive = true;
+      }
+
+      auto head = static_cast<DtorList*>(::malloc(sizeof(DtorList)));
+      if (!head) {
+        return -1;
+      }
+
+      head->dtor = dtor;
+      head->obj = obj;
+      head->next = dtors;
+      dtors = head;
+
+      return 0;
+    }
+#endif // HAVE___CXA_THREAD_ATEXIT_IMPL
+  }
+
+} // extern "C"
+} // namespace __cxxabiv1