Using the GNU Compiler Collection (GCC): __sync Builtins

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### 6.51 Legacy `__sync` Built-in Functions for Atomic Memory Access

The following built-in functions are intended to be compatible with those described in the Intel Itanium Processor-specific Application Binary Interface, section 7.4. As such, they depart from normal GCC practice by not using the ‘`__builtin_`’ prefix and also by being overloaded so that they work on multiple types.

The definition given in the Intel documentation allows only for the use of the types `int`, `long`, `long long` or their unsigned counterparts. GCC allows any scalar type that is 1, 2, 4 or 8 bytes in size other than the C type `_Bool` or the C++ type `bool`. Operations on pointer arguments are performed as if the operands were of the `uintptr_t` type. That is, they are not scaled by the size of the type to which the pointer points.

These functions are implemented in terms of the ‘`__atomic`’ builtins (see __atomic Builtins). They should not be used for new code which should use the ‘`__atomic`’ builtins instead.

Not all operations are supported by all target processors. If a particular operation cannot be implemented on the target processor, a warning is generated and a call to an external function is generated. The external function carries the same name as the built-in version, with an additional suffix ‘`_n`’ where n is the size of the data type.

In most cases, these built-in functions are considered a full barrier. That is, no memory operand is moved across the operation, either forward or backward. Further, instructions are issued as necessary to prevent the processor from speculating loads across the operation and from queuing stores after the operation.

All of the routines are described in the Intel documentation to take “an optional list of variables protected by the memory barrier”. It’s not clear what is meant by that; it could mean that only the listed variables are protected, or it could mean a list of additional variables to be protected. The list is ignored by GCC which treats it as empty. GCC interprets an empty list as meaning that all globally accessible variables should be protected.

`type __sync_fetch_and_add (type *ptr, type value, ...)`
`type __sync_fetch_and_sub (type *ptr, type value, ...)`
`type __sync_fetch_and_or (type *ptr, type value, ...)`
`type __sync_fetch_and_and (type *ptr, type value, ...)`
`type __sync_fetch_and_xor (type *ptr, type value, ...)`
`type __sync_fetch_and_nand (type *ptr, type value, ...)`

  These built-in functions perform the operation suggested by the name, and returns the value that had previously been in memory. That is, operations on integer operands have the following semantics. Operations on pointer arguments are performed as if the operands were of the `uintptr_t` type. That is, they are not scaled by the size of the type to which the pointer points.

      { tmp = *ptr; *ptr op= value; return tmp; }
      { tmp = *ptr; *ptr = ~(tmp & value); return tmp; }   // nand

  The object pointed to by the first argument must be of integer or pointer type. It must not be a Boolean type.

  Note: GCC 4.4 and later implement `__sync_fetch_and_nand` as `*ptr = ~(tmp & value)` instead of `*ptr = ~tmp & value`.

`type __sync_add_and_fetch (type *ptr, type value, ...)`
`type __sync_sub_and_fetch (type *ptr, type value, ...)`
`type __sync_or_and_fetch (type *ptr, type value, ...)`
`type __sync_and_and_fetch (type *ptr, type value, ...)`
`type __sync_xor_and_fetch (type *ptr, type value, ...)`
`type __sync_nand_and_fetch (type *ptr, type value, ...)`

  These built-in functions perform the operation suggested by the name, and return the new value. That is, operations on integer operands have the following semantics. Operations on pointer operands are performed as if the operand’s type were `uintptr_t`.

      { *ptr op= value; return *ptr; }
      { *ptr = ~(*ptr & value); return *ptr; }   // nand

  The same constraints on arguments apply as for the corresponding `__sync_op_and_fetch` built-in functions.

  Note: GCC 4.4 and later implement `__sync_nand_and_fetch` as `*ptr = ~(*ptr & value)` instead of `*ptr = ~*ptr & value`.

`bool __sync_bool_compare_and_swap (type *ptr, type oldval, type newval, ...)`
`type __sync_val_compare_and_swap (type *ptr, type oldval, type newval, ...)`

  These built-in functions perform an atomic compare and swap. That is, if the current value of `*ptr` is oldval, then write newval into `*ptr`.

  The “bool” version returns true if the comparison is successful and newval is written. The “val” version returns the contents of `*ptr` before the operation.

`__sync_synchronize (...)`

  This built-in function issues a full memory barrier.

`type __sync_lock_test_and_set (type *ptr, type value, ...)`

  This built-in function, as described by Intel, is not a traditional test-and-set operation, but rather an atomic exchange operation. It writes value into `*ptr`, and returns the previous contents of `*ptr`.

  Many targets have only minimal support for such locks, and do not support a full exchange operation. In this case, a target may support reduced functionality here by which the only valid value to store is the immediate constant 1. The exact value actually stored in `*ptr` is implementation defined.

  This built-in function is not a full barrier, but rather an acquire barrier. This means that references after the operation cannot move to (or be speculated to) before the operation, but previous memory stores may not be globally visible yet, and previous memory loads may not yet be satisfied.

`void __sync_lock_release (type *ptr, ...)`

  This built-in function releases the lock acquired by `__sync_lock_test_and_set`. Normally this means writing the constant 0 to `*ptr`.

  This built-in function is not a full barrier, but rather a release barrier. This means that all previous memory stores are globally visible, and all previous memory loads have been satisfied, but following memory reads are not prevented from being speculated to before the barrier.

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