lib/atomic.c

/*===-- atomic.c - Implement support functions for atomic operations.------===
 *
 *                     The LLVM Compiler Infrastructure
 *
 * This file is dual licensed under the MIT and the University of Illinois Open
 * Source Licenses. See LICENSE.TXT for details.
 *
 *===----------------------------------------------------------------------===
 *
 *  atomic.c defines a set of functions for performing atomic accesses on
 *  arbitrary-sized memory locations.  This design uses locks that should
 *  be fast in the uncontended case, for two reasons:
 * 
 *  1) This code must work with C programs that do not link to anything
 *     (including pthreads) and so it should not depend on any pthread
 *     functions.
 *  2) Atomic operations, rather than explicit mutexes, are most commonly used
 *     on code where contended operations are rate.
 * 
 *  To avoid needing a per-object lock, this code allocates an array of
 *  locks and hashes the object pointers to find the one that it should use.
 *  For operations that must be atomic on two locations, the lower lock is
 *  always acquired first, to avoid deadlock.
 *
 *===----------------------------------------------------------------------===
 */

#if defined(__clang__)

#include <stdint.h>
#include <string.h>

// Clang objects if you redefine a builtin.  This little hack allows us to
// define a function with the same name as an intrinsic.
#pragma redefine_extname __atomic_load_c __atomic_load
#pragma redefine_extname __atomic_store_c __atomic_store
#pragma redefine_extname __atomic_exchange_c __atomic_exchange
#pragma redefine_extname __atomic_compare_exchange_c __atomic_compare_exchange

/// Number of locks.  This allocates one page on 32-bit platforms, two on
/// 64-bit.  This can be specified externally if a different trade between
/// memory usage and contention probability is required for a given platform.
#ifndef SPINLOCK_COUNT
#define SPINLOCK_COUNT (1<<10)
#endif
static const long SPINLOCK_MASK = SPINLOCK_COUNT - 1;

////////////////////////////////////////////////////////////////////////////////
// Platform-specific lock implementation.  Falls back to spinlocks if none is
// defined.  Each platform should define the Lock type, and corresponding
// lock() and unlock() functions.
////////////////////////////////////////////////////////////////////////////////
#ifdef __FreeBSD__
#include <errno.h>
#include <sys/types.h>
#include <machine/atomic.h>
#include <sys/umtx.h>
typedef struct _usem Lock;
inline static void unlock(Lock *l) {
  __c11_atomic_store((_Atomic(uint32_t)*)&l->_count, 1, __ATOMIC_RELEASE);
  __c11_atomic_thread_fence(__ATOMIC_SEQ_CST);
  if (l->_has_waiters)
      _umtx_op(l, UMTX_OP_SEM_WAKE, 1, 0, 0);
}
inline static void lock(Lock *l) {
  uint32_t old = 1;
  while (!__c11_atomic_compare_exchange_weak((_Atomic(uint32_t)*)&l->_count, &old,
        0, __ATOMIC_ACQUIRE, __ATOMIC_RELAXED)) {
    _umtx_op(l, UMTX_OP_SEM_WAIT, 0, 0, 0);
    old = 1;
  }
}
/// locks for atomic operations
static Lock locks[SPINLOCK_COUNT] = { [0 ...  SPINLOCK_COUNT-1] = {0,1,0} };
#else
typedef _Atomic(uintptr_t) Lock;
/// Unlock a lock.  This is a release operation.
inline static void unlock(Lock *l) {
  __c11_atomic_store(l, 0, __ATOMIC_RELEASE);
}
/// Locks a lock.  In the current implementation, this is potentially
/// unbounded in the contended case.
inline static void lock(Lock *l) {
  uintptr_t old = 0;
  while (!__c11_atomic_compare_exchange_weak(l, &old, 1, __ATOMIC_ACQUIRE,
        __ATOMIC_RELAXED))
    old = 0;
}
/// locks for atomic operations
static Lock locks[SPINLOCK_COUNT];
#endif


/// Returns a lock to use for a given pointer.  
static inline Lock *lock_for_pointer(void *ptr) {
  intptr_t hash = (intptr_t)ptr;
  // Disregard the lowest 4 bits.  We want all values that may be part of the
  // same memory operation to hash to the same value and therefore use the same
  // lock.  
  hash >>= 4;
  // Use the next bits as the basis for the hash
  intptr_t low = hash & SPINLOCK_MASK;
  // Now use the high(er) set of bits to perturb the hash, so that we don't
  // get collisions from atomic fields in a single object
  hash >>= 16;
  hash ^= low;
  // Return a pointer to the word to use
  return locks + (hash & SPINLOCK_MASK);
}

/// Macros for determining whether a size is lock free.  Clang can not yet
/// codegen __atomic_is_lock_free(16), so for now we assume 16-byte values are
/// not lock free.
#define IS_LOCK_FREE_1 __c11_atomic_is_lock_free(1)
#define IS_LOCK_FREE_2 __c11_atomic_is_lock_free(2)
#define IS_LOCK_FREE_4 __c11_atomic_is_lock_free(4)
#define IS_LOCK_FREE_8 __c11_atomic_is_lock_free(8)
#define IS_LOCK_FREE_16 0

/// Macro that calls the compiler-generated lock-free versions of functions
/// when they exist.
#define LOCK_FREE_CASES() \
  do {\
  switch (size) {\
    case 2:\
      if (IS_LOCK_FREE_2) {\
        LOCK_FREE_ACTION(uint16_t);\
      }\
    case 4:\
      if (IS_LOCK_FREE_4) {\
        LOCK_FREE_ACTION(uint32_t);\
      }\
    case 8:\
      if (IS_LOCK_FREE_8) {\
        LOCK_FREE_ACTION(uint64_t);\
      }\
    case 16:\
      if (IS_LOCK_FREE_16) {\
        /* FIXME: __uint128_t isn't available on 32 bit platforms.
        LOCK_FREE_ACTION(__uint128_t);*/\
      }\
  }\
  } while (0)


/// An atomic load operation.  This is atomic with respect to the source
/// pointer only.
void __atomic_load_c(int size, void *src, void *dest, int model) {
#define LOCK_FREE_ACTION(type) \
    *((type*)dest) = __c11_atomic_load((_Atomic(type)*)src, model);\
    return;
  LOCK_FREE_CASES();
#undef LOCK_FREE_ACTION
  Lock *l = lock_for_pointer(src);
  lock(l);
  memcpy(dest, src, size);
  unlock(l);
}

/// An atomic store operation.  This is atomic with respect to the destination
/// pointer only.
void __atomic_store_c(int size, void *dest, void *src, int model) {
#define LOCK_FREE_ACTION(type) \
    __c11_atomic_store((_Atomic(type)*)dest, *(type*)dest, model);\
    return;
  LOCK_FREE_CASES();
#undef LOCK_FREE_ACTION
  Lock *l = lock_for_pointer(dest);
  lock(l);
  memcpy(dest, src, size);
  unlock(l);
}

/// Atomic compare and exchange operation.  If the value at *ptr is identical
/// to the value at *expected, then this copies value at *desired to *ptr.  If
/// they  are not, then this stores the current value from *ptr in *expected.
///
/// This function returns 1 if the exchange takes place or 0 if it fails. 
int __atomic_compare_exchange_c(int size, void *ptr, void *expected,
    void *desired, int success, int failure) {
#define LOCK_FREE_ACTION(type) \
  return __c11_atomic_compare_exchange_strong((_Atomic(type)*)ptr, (type*)expected,\
      *(type*)desired, success, failure)
  LOCK_FREE_CASES();
#undef LOCK_FREE_ACTION
  Lock *l = lock_for_pointer(ptr);
  lock(l);
  if (memcmp(ptr, expected, size) == 0) {
    memcpy(ptr, desired, size);
    unlock(l);
    return 1;
  }
  memcpy(expected, ptr, size);
  unlock(l);
  return 0;
}

/// Performs an atomic exchange operation between two pointers.  This is atomic
/// with respect to the target address.
void __atomic_exchange_c(int size, void *ptr, void *val, void *old, int model) {
#define LOCK_FREE_ACTION(type) \
    *(type*)old = __c11_atomic_exchange((_Atomic(type)*)ptr, *(type*)val,\
        model);\
    return;
  LOCK_FREE_CASES();
#undef LOCK_FREE_ACTION
  Lock *l = lock_for_pointer(ptr);
  lock(l);
  memcpy(old, ptr, size);
  memcpy(ptr, val, size);
  unlock(l);
}

////////////////////////////////////////////////////////////////////////////////
// Where the size is known at compile time, the compiler may emit calls to
// specialised versions of the above functions.
////////////////////////////////////////////////////////////////////////////////
#define OPTIMISED_CASES\
  OPTIMISED_CASE(1, IS_LOCK_FREE_1, uint8_t)\
  OPTIMISED_CASE(2, IS_LOCK_FREE_2, uint16_t)\
  OPTIMISED_CASE(4, IS_LOCK_FREE_4, uint32_t)\
  OPTIMISED_CASE(8, IS_LOCK_FREE_8, uint64_t)\
  /* FIXME: __uint128_t isn't available on 32 bit platforms.
  OPTIMISED_CASE(16, IS_LOCK_FREE_16, __uint128_t)*/\

#define OPTIMISED_CASE(n, lockfree, type)\
type __atomic_load_##n(type *src, int model) {\
  if (lockfree)\
    return __c11_atomic_load((_Atomic(type)*)src, model);\
  Lock *l = lock_for_pointer(src);\
  lock(l);\
  type val = *src;\
  unlock(l);\
  return val;\
}
OPTIMISED_CASES
#undef OPTIMISED_CASE

#define OPTIMISED_CASE(n, lockfree, type)\
void  __atomic_store_##n(type *dest, type val, int model) {\
  if (lockfree) {\
    __c11_atomic_store((_Atomic(type)*)dest, val, model);\
    return;\
  }\
  Lock *l = lock_for_pointer(dest);\
  lock(l);\
  *dest = val;\
  unlock(l);\
  return;\
}
OPTIMISED_CASES
#undef OPTIMISED_CASE

#define OPTIMISED_CASE(n, lockfree, type)\
type __atomic_exchange_##n(type *dest, type val, int model) {\
  if (lockfree)\
    return __c11_atomic_exchange((_Atomic(type)*)dest, val, model);\
  Lock *l = lock_for_pointer(dest);\
  lock(l);\
  type tmp = *dest;\
  *dest = val;\
  unlock(l);\
  return tmp;\
}
OPTIMISED_CASES
#undef OPTIMISED_CASE

#define OPTIMISED_CASE(n, lockfree, type)\
int __atomic_compare_exchange_##n(type *ptr, type *expected, type desired,\
    int success, int failure) {\
  if (lockfree)\
    return __c11_atomic_compare_exchange_strong((_Atomic(type)*)ptr, expected, desired,\
        success, failure);\
  Lock *l = lock_for_pointer(ptr);\
  lock(l);\
  if (*ptr == *expected) {\
    *ptr = desired;\
    unlock(l);\
    return 1;\
  }\
  *expected = *ptr;\
  unlock(l);\
  return 0;\
}
OPTIMISED_CASES
#undef OPTIMISED_CASE

////////////////////////////////////////////////////////////////////////////////
// Atomic read-modify-write operations for integers of various sizes.
////////////////////////////////////////////////////////////////////////////////
#define ATOMIC_RMW(n, lockfree, type, opname, op) \
type __atomic_fetch_##opname##_##n(type *ptr, type val, int model) {\
  if (lockfree) \
    return __c11_atomic_fetch_##opname((_Atomic(type)*)ptr, val, model);\
  Lock *l = lock_for_pointer(ptr);\
  lock(l);\
  type tmp = *ptr;\
  *ptr = tmp op val;\
  unlock(l);\
  return tmp;\
}

#define OPTIMISED_CASE(n, lockfree, type) ATOMIC_RMW(n, lockfree, type, add, +)
OPTIMISED_CASES
#undef OPTIMISED_CASE
#define OPTIMISED_CASE(n, lockfree, type) ATOMIC_RMW(n, lockfree, type, sub, -)
OPTIMISED_CASES
#undef OPTIMISED_CASE
#define OPTIMISED_CASE(n, lockfree, type) ATOMIC_RMW(n, lockfree, type, and, &)
OPTIMISED_CASES
#undef OPTIMISED_CASE
#define OPTIMISED_CASE(n, lockfree, type) ATOMIC_RMW(n, lockfree, type, or, |)
OPTIMISED_CASES
#undef OPTIMISED_CASE
#define OPTIMISED_CASE(n, lockfree, type) ATOMIC_RMW(n, lockfree, type, xor, ^)
OPTIMISED_CASES
#undef OPTIMISED_CASE

#endif // __clang__