cosmopolitan/libc/runtime/cxaguard.c

/*
 * Copyright (C) 2006 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
#include "libc/atomic.h"
#include "libc/intrin/atomic.h"
#include "libc/limits.h"
#include "third_party/nsync/futex.internal.h"

// This file contains C++ ABI support functions for one time
// constructors as defined in the "Run-time ABI for the ARM Architecture"
// section 4.4.2
//
// ARM C++ ABI and Itanium/x86 C++ ABI has different definition for
// one time construction:
//
//    ARM C++ ABI defines the LSB of guard variable should be tested
//    by compiler-generated code before calling __cxa_guard_acquire et al.
//
//    The Itanium/x86 C++ ABI defines the low-order _byte_ should be
//    tested instead.
//
//    Meanwhile, guard variable are 32bit aligned for ARM, and 64bit
//    aligned for x86.
//
// Reference documentation:
//
//    section 3.2.3 of ARM IHI 0041C (for ARM)
//    section 3.3.2 of the Itanium C++ ABI specification v1.83 (for x86).
//
// There is no C++ ABI available for other ARCH. But the gcc source
// shows all other ARCH follow the definition of Itanium/x86 C++ ABI.

// The Itanium/x86 C++ ABI (used by all architectures that aren't ARM32)
// mandates guard variables are 64-bit aligned, 64-bit values. The least
// significant byte is tested by the compiler-generated code before it's
// calling __cxa_guard_acquire.
union CxaGuardValue {
  atomic_int state;
  int64_t aligner;
};

// Set construction state values according to reference documentation. 0
// is the initialization value. Arm requires ((*gv & 1) == 1) after
// __cxa_guard_release, ((*gv & 3) == 0) after __cxa_guard_abort. X86
// requires first byte not modified by __cxa_guard_acquire, first byte
// is non-zero after __cxa_guard_release.
#define CONSTRUCTION_NOT_YET_STARTED         0
#define CONSTRUCTION_COMPLETE                1
#define CONSTRUCTION_UNDERWAY_WITHOUT_WAITER 0x100
#define CONSTRUCTION_UNDERWAY_WITH_WAITER    0x200

int __cxa_guard_acquire(union CxaGuardValue *gv) {
  int old_value = atomic_load_explicit(&gv->state, memory_order_relaxed);
  while (true) {
    if (old_value == CONSTRUCTION_COMPLETE) {
      // A load_acquire operation is needed before exiting with COMPLETE
      // state, as we must ensure that all the stores performed by the
      // construction function are observable on this CPU after we exit.
      atomic_thread_fence(memory_order_acquire);
      return 0;
    } else if (old_value == CONSTRUCTION_NOT_YET_STARTED) {
      if (!atomic_compare_exchange_weak_explicit(
              &gv->state, &old_value, CONSTRUCTION_UNDERWAY_WITHOUT_WAITER,
              memory_order_relaxed, memory_order_relaxed)) {
        continue;
      }
      // The acquire fence may not be needed. But as described in section 3.3.2
      // of the Itanium C++ ABI specification, it probably has to behave like
      // the acquisition of a mutex, which needs an acquire fence.
      atomic_thread_fence(memory_order_acquire);
      return 1;
    } else if (old_value == CONSTRUCTION_UNDERWAY_WITHOUT_WAITER) {
      if (!atomic_compare_exchange_weak_explicit(
              &gv->state, &old_value, CONSTRUCTION_UNDERWAY_WITH_WAITER,
              memory_order_relaxed, memory_order_relaxed)) {
        continue;
      }
    }
    nsync_futex_wait_(&gv->state, CONSTRUCTION_UNDERWAY_WITH_WAITER, 0, 0);
    old_value = atomic_load_explicit(&gv->state, memory_order_relaxed);
  }
}

void __cxa_guard_release(union CxaGuardValue *gv) {
  // Release fence is used to make all stores performed by the construction
  // function visible in other threads.
  int old_value = atomic_exchange_explicit(&gv->state, CONSTRUCTION_COMPLETE,
                                           memory_order_release);
  if (old_value == CONSTRUCTION_UNDERWAY_WITH_WAITER) {
    nsync_futex_wake_(&gv->state, INT_MAX, 0);
  }
}

void __cxa_guard_abort(union CxaGuardValue *gv) {
  // Release fence is used to make all stores performed by the construction
  // function visible in other threads.
  int old_value = atomic_exchange_explicit(
      &gv->state, CONSTRUCTION_NOT_YET_STARTED, memory_order_release);
  if (old_value == CONSTRUCTION_UNDERWAY_WITH_WAITER) {
    nsync_futex_wake_(&gv->state, INT_MAX, 0);
  }
}