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<h3 class="section">6.50 Legacy <code>__sync</code> Built-in Functions for Atomic Memory Access</h3>
<p>The following built-in functions
are intended to be compatible with those described
in the <cite>Intel Itanium Processor-specific Application Binary Interface</cite>,
section 7.4. As such, they depart from the normal GCC practice of using
the ‘<samp><span class="samp">__builtin_</span></samp>’ prefix, and further that they are overloaded such that
they work on multiple types.
<p>The definition given in the Intel documentation allows only for the use of
the types <code>int</code>, <code>long</code>, <code>long long</code> as well as their unsigned
counterparts. GCC allows any integral scalar or pointer type that is
1, 2, 4 or 8 bytes in length.
<p>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
‘<samp><span class="samp">_</span><var>n</var></samp>’ where <var>n</var> is the size of the data type.
<!-- ??? Should we have a mechanism to suppress this warning? This is almost -->
<!-- useful for implementing the operation under the control of an external -->
<!-- mutex. -->
<p>In most cases, these built-in functions are considered a <dfn>full barrier</dfn>.
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.
<p>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 <em>only</em> the
following variables are protected, or it could mean that these variables
should in addition be protected. At present GCC ignores this list and
protects all variables that are globally accessible. If in the future
we make some use of this list, an empty list will continue to mean all
globally accessible variables.
<dl>
<dt><var>type</var><code> __sync_fetch_and_add (</code><var>type</var><code> *ptr, </code><var>type</var><code> value, ...)</code><dt><var>type</var><code> __sync_fetch_and_sub (</code><var>type</var><code> *ptr, </code><var>type</var><code> value, ...)</code><dt><var>type</var><code> __sync_fetch_and_or (</code><var>type</var><code> *ptr, </code><var>type</var><code> value, ...)</code><dt><var>type</var><code> __sync_fetch_and_and (</code><var>type</var><code> *ptr, </code><var>type</var><code> value, ...)</code><dt><var>type</var><code> __sync_fetch_and_xor (</code><var>type</var><code> *ptr, </code><var>type</var><code> value, ...)</code><dt><var>type</var><code> __sync_fetch_and_nand (</code><var>type</var><code> *ptr, </code><var>type</var><code> value, ...)</code><dd><a name="index-g_t_005f_005fsync_005ffetch_005fand_005fadd-3519"></a><a name="index-g_t_005f_005fsync_005ffetch_005fand_005fsub-3520"></a><a name="index-g_t_005f_005fsync_005ffetch_005fand_005for-3521"></a><a name="index-g_t_005f_005fsync_005ffetch_005fand_005fand-3522"></a><a name="index-g_t_005f_005fsync_005ffetch_005fand_005fxor-3523"></a><a name="index-g_t_005f_005fsync_005ffetch_005fand_005fnand-3524"></a>These built-in functions perform the operation suggested by the name, and
returns the value that had previously been in memory. That is,
<pre class="smallexample"> { tmp = *ptr; *ptr <var>op</var>= value; return tmp; }
{ tmp = *ptr; *ptr = ~(tmp & value); return tmp; } // nand
</pre>
<p><em>Note:</em> GCC 4.4 and later implement <code>__sync_fetch_and_nand</code>
as <code>*ptr = ~(tmp & value)</code> instead of <code>*ptr = ~tmp & value</code>.
<br><dt><var>type</var><code> __sync_add_and_fetch (</code><var>type</var><code> *ptr, </code><var>type</var><code> value, ...)</code><dt><var>type</var><code> __sync_sub_and_fetch (</code><var>type</var><code> *ptr, </code><var>type</var><code> value, ...)</code><dt><var>type</var><code> __sync_or_and_fetch (</code><var>type</var><code> *ptr, </code><var>type</var><code> value, ...)</code><dt><var>type</var><code> __sync_and_and_fetch (</code><var>type</var><code> *ptr, </code><var>type</var><code> value, ...)</code><dt><var>type</var><code> __sync_xor_and_fetch (</code><var>type</var><code> *ptr, </code><var>type</var><code> value, ...)</code><dt><var>type</var><code> __sync_nand_and_fetch (</code><var>type</var><code> *ptr, </code><var>type</var><code> value, ...)</code><dd><a name="index-g_t_005f_005fsync_005fadd_005fand_005ffetch-3525"></a><a name="index-g_t_005f_005fsync_005fsub_005fand_005ffetch-3526"></a><a name="index-g_t_005f_005fsync_005for_005fand_005ffetch-3527"></a><a name="index-g_t_005f_005fsync_005fand_005fand_005ffetch-3528"></a><a name="index-g_t_005f_005fsync_005fxor_005fand_005ffetch-3529"></a><a name="index-g_t_005f_005fsync_005fnand_005fand_005ffetch-3530"></a>These built-in functions perform the operation suggested by the name, and
return the new value. That is,
<pre class="smallexample"> { *ptr <var>op</var>= value; return *ptr; }
{ *ptr = ~(*ptr & value); return *ptr; } // nand
</pre>
<p><em>Note:</em> GCC 4.4 and later implement <code>__sync_nand_and_fetch</code>
as <code>*ptr = ~(*ptr & value)</code> instead of
<code>*ptr = ~*ptr & value</code>.
<br><dt><code>bool __sync_bool_compare_and_swap (</code><var>type</var><code> *ptr, </code><var>type</var><code> oldval, </code><var>type</var><code> newval, ...)</code><dt><var>type</var><code> __sync_val_compare_and_swap (</code><var>type</var><code> *ptr, </code><var>type</var><code> oldval, </code><var>type</var><code> newval, ...)</code><dd><a name="index-g_t_005f_005fsync_005fbool_005fcompare_005fand_005fswap-3531"></a><a name="index-g_t_005f_005fsync_005fval_005fcompare_005fand_005fswap-3532"></a>These built-in functions perform an atomic compare and swap.
That is, if the current
value of <code>*</code><var>ptr</var> is <var>oldval</var>, then write <var>newval</var> into
<code>*</code><var>ptr</var>.
<p>The “bool” version returns true if the comparison is successful and
<var>newval</var> is written. The “val” version returns the contents
of <code>*</code><var>ptr</var> before the operation.
<br><dt><code>__sync_synchronize (...)</code><dd><a name="index-g_t_005f_005fsync_005fsynchronize-3533"></a>This built-in function issues a full memory barrier.
<br><dt><var>type</var><code> __sync_lock_test_and_set (</code><var>type</var><code> *ptr, </code><var>type</var><code> value, ...)</code><dd><a name="index-g_t_005f_005fsync_005flock_005ftest_005fand_005fset-3534"></a>This built-in function, as described by Intel, is not a traditional test-and-set
operation, but rather an atomic exchange operation. It writes <var>value</var>
into <code>*</code><var>ptr</var>, and returns the previous contents of
<code>*</code><var>ptr</var>.
<p>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 <em>only</em> valid value to store is the
immediate constant 1. The exact value actually stored in <code>*</code><var>ptr</var>
is implementation defined.
<p>This built-in function is not a full barrier,
but rather an <dfn>acquire barrier</dfn>.
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.
<br><dt><code>void __sync_lock_release (</code><var>type</var><code> *ptr, ...)</code><dd><a name="index-g_t_005f_005fsync_005flock_005frelease-3535"></a>This built-in function releases the lock acquired by
<code>__sync_lock_test_and_set</code>.
Normally this means writing the constant 0 to <code>*</code><var>ptr</var>.
<p>This built-in function is not a full barrier,
but rather a <dfn>release barrier</dfn>.
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.
</dl>
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