xref: /bionic/libstdc++/src/one_time_construction.cpp

/*
 * Copyright 2006 The Android Open Source Project
 */

#include <stddef.h>
#include <sys/atomics.h>
#include <endian.h>
#include <private/bionic_futex.h>
#include <private/bionic_atomic_inline.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.


#if defined(__arm__)
// The ARM C++ ABI mandates that guard variable are
// 32-bit aligned, 32-bit values. And only its LSB is tested by
// the compiler-generated code before calling
// __cxa_guard_acquire.
//
typedef union {
    int volatile state;
    int32_t aligner;
} _guard_t;

const static int ready = 0x1;
const static int pending = 0x2;
const static int waiting = 0x6;

#else   // GCC sources indicates all none-arm follow the same ABI
// The Itanium/x86 C++ ABI mandates that guard variables
// are 64-bit aligned, 64-bit values. Also, the least-significant
// byte is tested by the compiler-generated code before, we calling
// __cxa_guard_acquire. We can access it through the first
// 32-bit word in the union below.
//
typedef union {
    int volatile state;
    int64_t aligner;
} _guard_t;

const static int ready     = letoh32(0x1);
const static int pending   = letoh32(0x100);
const static int waiting   = letoh32(0x10000);
#endif

extern "C" int __cxa_guard_acquire(_guard_t* gv)
{
    // 0 -> pending, return 1
    // pending -> waiting, wait and return 0
    // waiting: untouched, wait and return 0
    // ready: untouched, return 0

retry:
    if (__bionic_cmpxchg(0, pending, &gv->state) == 0) {
        ANDROID_MEMBAR_FULL();
        return 1;
    }
    __bionic_cmpxchg(pending, waiting, &gv->state); // Indicate there is a waiter
    __futex_wait(&gv->state, waiting, NULL);

    if (gv->state != ready) // __cxa_guard_abort was called, let every thread try since there is no return code for this condition
        goto retry;

    ANDROID_MEMBAR_FULL();
    return 0;
}

extern "C" void __cxa_guard_release(_guard_t* gv)
{
    // pending -> ready
    // waiting -> ready, and wake

    ANDROID_MEMBAR_FULL();
    if (__bionic_cmpxchg(pending, ready, &gv->state) == 0) {
        return;
    }

    gv->state = ready;
    __futex_wake(&gv->state, 0x7fffffff);
}

extern "C" void __cxa_guard_abort(_guard_t* gv)
{
    ANDROID_MEMBAR_FULL();
    gv->state= 0;
    __futex_wake(&gv->state, 0x7fffffff);
}