GCC内建原子操作

参见：http://gcc.gnu.org/onlinedocs/gcc/_005f_005fsync-Builtins.html

The following built-in functionsare intended to be compatible with those describedin the Intel Itanium Processor-specific Application Binary Interface,section 7.4. As such, they depart from the normal GCC practice of usingthe ‘__builtin_’ prefix, and further that they are overloaded such thatthey work on multiple types.

The definition given in the Intel documentation allows only for the use ofthe types int, long, long long as well as their unsignedcounterparts. GCC allows any integral scalar or pointer type that is1, 2, 4 or 8 bytes in length.

Not all operations are supported by all target processors. If a particularoperation cannot be implemented on the target processor, a warning isgenerated and a call an external function is generated. The externalfunction 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 orbackward. Further, instructions are issued as necessary to prevent theprocessor from speculating loads across the operation and from queuing storesafter 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'snot clear what is meant by that; it could mean that only thefollowing variables are protected, or it could mean that these variablesshould in addition be protected. At present GCC ignores this list andprotects all variables that are globally accessible. If in the futurewe make some use of this list, an empty list will continue to mean allglobally accessible variables.

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, andreturns the value that had previously been in memory. That is,

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

Note: GCC 4.4 and later implement __sync_fetch_and_nandas *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, andreturn the new value. That is,

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

Note: GCC 4.4 and later implement __sync_nand_and_fetchas *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 currentvalue of *ptr is oldval, then write newval into*ptr.

The “bool” version returns true if the comparison is successful andnewval is written. The “val” version returns the contentsof *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-setoperation, but rather an atomic exchange operation. It writes valueinto *ptr, and returns the previous contents of*ptr.

Many targets have only minimal support for such locks, and do not supporta full exchange operation. In this case, a target may support reducedfunctionality here by which the only valid value to store is theimmediate constant 1. The exact value actually stored in *ptris 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 bespeculated to) before the operation, but previous memory stores may notbe globally visible yet, and previous memory loads may not yet besatisfied.

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 allprevious memory loads have been satisfied, but following memory readsare not prevented from being speculated to before the barrier.