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<title>Using the GNU Compiler Collection (GCC): Statement Attributes</title>
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<a name="Statement-Attributes"></a>
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<a name="Statement-Attributes-1"></a>
<h3 class="section">6.38 Statement Attributes</h3>
<a name="index-Statement-Attributes"></a>
<p>GCC allows attributes to be set on null statements. See <a href="Attribute-Syntax.html#Attribute-Syntax">Attribute Syntax</a>,
for details of the exact syntax for using attributes. Other attributes are
available for functions (see <a href="Function-Attributes.html#Function-Attributes">Function Attributes</a>), variables
(see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>), labels (see <a href="Label-Attributes.html#Label-Attributes">Label Attributes</a>), enumerators
(see <a href="Enumerator-Attributes.html#Enumerator-Attributes">Enumerator Attributes</a>), and for types (see <a href="Type-Attributes.html#Type-Attributes">Type Attributes</a>).
</p>
<dl compact="compact">
<dd><a name="index-fallthrough-statement-attribute"></a>
</dd>
<dt><code>fallthrough</code></dt>
<dd><p>The <code>fallthrough</code> attribute with a null statement serves as a
fallthrough statement. It hints to the compiler that a statement
that falls through to another case label, or user-defined label
in a switch statement is intentional and thus the
<samp>-Wimplicit-fallthrough</samp> warning must not trigger. The
fallthrough attribute may appear at most once in each attribute
list, and may not be mixed with other attributes. It can only
be used in a switch statement (the compiler will issue an error
otherwise), after a preceding statement and before a logically
succeeding case label, or user-defined label.
</p>
<p>This example uses the <code>fallthrough</code> statement attribute to indicate that
the <samp>-Wimplicit-fallthrough</samp> warning should not be emitted:
</p>
<div class="smallexample">
<pre class="smallexample">switch (cond)
{
case 1:
bar (1);
__attribute__((fallthrough));
case 2:
…
}
</pre></div>
<a name="index-assume-statement-attribute"></a>
</dd>
<dt><code>assume</code></dt>
<dd><p>The <code>assume</code> attribute with a null statement serves as portable
assumption. It should have a single argument, a conditional expression,
which is not evaluated. If the argument would evaluate to true
at the point where it appears, it has no effect, otherwise there
is undefined behavior. This is a GNU variant of the ISO C++23
standard <code>assume</code> attribute, but it can be used in any version of
both C and C++.
</p>
<div class="smallexample">
<pre class="smallexample">int
foo (int x, int y)
{
__attribute__((assume(x == 42)));
__attribute__((assume(++y == 43)));
return x + y;
}
</pre></div>
<p><code>y</code> is not actually incremented and the compiler can but does not
have to optimize it to just <code>return 42 + 42;</code>.
</p>
</dd>
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