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<title>Using the GNU Compiler Collection (GCC): AArch64 Function Attributes</title>

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<a name="AArch64-Function-Attributes"></a>
<div class="header">
<p>
Next: <a href="ARC-Function-Attributes.html#ARC-Function-Attributes" accesskey="n" rel="next">ARC Function Attributes</a>, Previous: <a href="Common-Function-Attributes.html#Common-Function-Attributes" accesskey="p" rel="prev">Common Function Attributes</a>, Up: <a href="Function-Attributes.html#Function-Attributes" accesskey="u" rel="up">Function Attributes</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="AArch64-Function-Attributes-1"></a>
<h4 class="subsection">6.31.2 AArch64 Function Attributes</h4>

<p>The following target-specific function attributes are available for the
AArch64 target.  For the most part, these options mirror the behavior of
similar command-line options (see <a href="AArch64-Options.html#AArch64-Options">AArch64 Options</a>), but on a
per-function basis.
</p>
<dl compact="compact">
<dt><code>general-regs-only</code></dt>
<dd><a name="index-general_002dregs_002donly-function-attribute_002c-AArch64"></a>
<p>Indicates that no floating-point or Advanced SIMD registers should be
used when generating code for this function.  If the function explicitly
uses floating-point code, then the compiler gives an error.  This is
the same behavior as that of the command-line option
<samp>-mgeneral-regs-only</samp>.
</p>
</dd>
<dt><code>fix-cortex-a53-835769</code></dt>
<dd><a name="index-fix_002dcortex_002da53_002d835769-function-attribute_002c-AArch64"></a>
<p>Indicates that the workaround for the Cortex-A53 erratum 835769 should be
applied to this function.  To explicitly disable the workaround for this
function specify the negated form: <code>no-fix-cortex-a53-835769</code>.
This corresponds to the behavior of the command line options
<samp>-mfix-cortex-a53-835769</samp> and <samp>-mno-fix-cortex-a53-835769</samp>.
</p>
</dd>
<dt><code>cmodel=</code></dt>
<dd><a name="index-cmodel_003d-function-attribute_002c-AArch64"></a>
<p>Indicates that code should be generated for a particular code model for
this function.  The behavior and permissible arguments are the same as
for the command line option <samp>-mcmodel=</samp>.
</p>
</dd>
<dt><code>strict-align</code></dt>
<dd><a name="index-strict_002dalign-function-attribute_002c-AArch64"></a>
<p>Indicates that the compiler should not assume that unaligned memory references
are handled by the system.  The behavior is the same as for the command-line
option <samp>-mstrict-align</samp>.
</p>
</dd>
<dt><code>omit-leaf-frame-pointer</code></dt>
<dd><a name="index-omit_002dleaf_002dframe_002dpointer-function-attribute_002c-AArch64"></a>
<p>Indicates that the frame pointer should be omitted for a leaf function call.
To keep the frame pointer, the inverse attribute
<code>no-omit-leaf-frame-pointer</code> can be specified.  These attributes have
the same behavior as the command-line options <samp>-momit-leaf-frame-pointer</samp>
and <samp>-mno-omit-leaf-frame-pointer</samp>.
</p>
</dd>
<dt><code>tls-dialect=</code></dt>
<dd><a name="index-tls_002ddialect_003d-function-attribute_002c-AArch64"></a>
<p>Specifies the TLS dialect to use for this function.  The behavior and
permissible arguments are the same as for the command-line option
<samp>-mtls-dialect=</samp>.
</p>
</dd>
<dt><code>arch=</code></dt>
<dd><a name="index-arch_003d-function-attribute_002c-AArch64"></a>
<p>Specifies the architecture version and architectural extensions to use
for this function.  The behavior and permissible arguments are the same as
for the <samp>-march=</samp> command-line option.
</p>
</dd>
<dt><code>tune=</code></dt>
<dd><a name="index-tune_003d-function-attribute_002c-AArch64"></a>
<p>Specifies the core for which to tune the performance of this function.
The behavior and permissible arguments are the same as for the <samp>-mtune=</samp>
command-line option.
</p>
</dd>
<dt><code>cpu=</code></dt>
<dd><a name="index-cpu_003d-function-attribute_002c-AArch64"></a>
<p>Specifies the core for which to tune the performance of this function and also
whose architectural features to use.  The behavior and valid arguments are the
same as for the <samp>-mcpu=</samp> command-line option.
</p>
</dd>
<dt><code>sign-return-address</code></dt>
<dd><a name="index-sign_002dreturn_002daddress-function-attribute_002c-AArch64"></a>
<p>Select the function scope on which return address signing will be applied.  The
behavior and permissible arguments are the same as for the command-line option
<samp>-msign-return-address=</samp>.  The default value is <code>none</code>.
</p>
</dd>
</dl>

<p>The above target attributes can be specified as follows:
</p>
<div class="smallexample">
<pre class="smallexample">__attribute__((target(&quot;<var>attr-string</var>&quot;)))
int
f (int a)
{
  return a + 5;
}
</pre></div>

<p>where <code><var>attr-string</var></code> is one of the attribute strings specified above.
</p>
<p>Additionally, the architectural extension string may be specified on its
own.  This can be used to turn on and off particular architectural extensions
without having to specify a particular architecture version or core.  Example:
</p>
<div class="smallexample">
<pre class="smallexample">__attribute__((target(&quot;+crc+nocrypto&quot;)))
int
foo (int a)
{
  return a + 5;
}
</pre></div>

<p>In this example <code>target(&quot;+crc+nocrypto&quot;)</code> enables the <code>crc</code>
extension and disables the <code>crypto</code> extension for the function <code>foo</code>
without modifying an existing <samp>-march=</samp> or <samp>-mcpu</samp> option.
</p>
<p>Multiple target function attributes can be specified by separating them with
a comma.  For example:
</p><div class="smallexample">
<pre class="smallexample">__attribute__((target(&quot;arch=armv8-a+crc+crypto,tune=cortex-a53&quot;)))
int
foo (int a)
{
  return a + 5;
}
</pre></div>

<p>is valid and compiles function <code>foo</code> for ARMv8-A with <code>crc</code>
and <code>crypto</code> extensions and tunes it for <code>cortex-a53</code>.
</p>
<a name="Inlining-rules"></a>
<h4 class="subsubsection">6.31.2.1 Inlining rules</h4>
<p>Specifying target attributes on individual functions or performing link-time
optimization across translation units compiled with different target options
can affect function inlining rules:
</p>
<p>In particular, a caller function can inline a callee function only if the
architectural features available to the callee are a subset of the features
available to the caller.
For example: A function <code>foo</code> compiled with <samp>-march=armv8-a+crc</samp>,
or tagged with the equivalent <code>arch=armv8-a+crc</code> attribute,
can inline a function <code>bar</code> compiled with <samp>-march=armv8-a+nocrc</samp>
because the all the architectural features that function <code>bar</code> requires
are available to function <code>foo</code>.  Conversely, function <code>bar</code> cannot
inline function <code>foo</code>.
</p>
<p>Additionally inlining a function compiled with <samp>-mstrict-align</samp> into a
function compiled without <code>-mstrict-align</code> is not allowed.
However, inlining a function compiled without <samp>-mstrict-align</samp> into a
function compiled with <samp>-mstrict-align</samp> is allowed.
</p>
<p>Note that CPU tuning options and attributes such as the <samp>-mcpu=</samp>,
<samp>-mtune=</samp> do not inhibit inlining unless the CPU specified by the
<samp>-mcpu=</samp> option or the <code>cpu=</code> attribute conflicts with the
architectural feature rules specified above.
</p>
<hr>
<div class="header">
<p>
Next: <a href="ARC-Function-Attributes.html#ARC-Function-Attributes" accesskey="n" rel="next">ARC Function Attributes</a>, Previous: <a href="Common-Function-Attributes.html#Common-Function-Attributes" accesskey="p" rel="prev">Common Function Attributes</a>, Up: <a href="Function-Attributes.html#Function-Attributes" accesskey="u" rel="up">Function Attributes</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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