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<title>Defining Attributes (GNU Compiler Collection (GCC) Internals)</title>

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<a name="Defining-Attributes"></a>
<div class="header">
<p>
Next: <a href="Expressions.html#Expressions" accesskey="n" rel="next">Expressions</a>, Up: <a href="Insn-Attributes.html#Insn-Attributes" accesskey="u" rel="up">Insn 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="Defining-Attributes-and-their-Values"></a>
<h4 class="subsection">17.19.1 Defining Attributes and their Values</h4>
<a name="index-defining-attributes-and-their-values"></a>
<a name="index-attributes_002c-defining"></a>

<a name="index-define_005fattr"></a>
<p>The <code>define_attr</code> expression is used to define each attribute required
by the target machine.  It looks like:
</p>
<div class="smallexample">
<pre class="smallexample">(define_attr <var>name</var> <var>list-of-values</var> <var>default</var>)
</pre></div>

<p><var>name</var> is a string specifying the name of the attribute being
defined.  Some attributes are used in a special way by the rest of the
compiler. The <code>enabled</code> attribute can be used to conditionally
enable or disable insn alternatives (see <a href="Disable-Insn-Alternatives.html#Disable-Insn-Alternatives">Disable Insn Alternatives</a>). The <code>predicable</code> attribute, together with a
suitable <code>define_cond_exec</code> (see <a href="Conditional-Execution.html#Conditional-Execution">Conditional Execution</a>), can
be used to automatically generate conditional variants of instruction
patterns. The <code>mnemonic</code> attribute can be used to check for the
instruction mnemonic (see <a href="Mnemonic-Attribute.html#Mnemonic-Attribute">Mnemonic Attribute</a>).  The compiler
internally uses the names <code>ce_enabled</code> and <code>nonce_enabled</code>,
so they should not be used elsewhere as alternative names.
</p>
<p><var>list-of-values</var> is either a string that specifies a comma-separated
list of values that can be assigned to the attribute, or a null string to
indicate that the attribute takes numeric values.
</p>
<p><var>default</var> is an attribute expression that gives the value of this
attribute for insns that match patterns whose definition does not include
an explicit value for this attribute.  See <a href="Attr-Example.html#Attr-Example">Attr Example</a>, for more
information on the handling of defaults.  See <a href="Constant-Attributes.html#Constant-Attributes">Constant Attributes</a>,
for information on attributes that do not depend on any particular insn.
</p>
<a name="index-insn_002dattr_002eh"></a>
<p>For each defined attribute, a number of definitions are written to the
<samp>insn-attr.h</samp> file.  For cases where an explicit set of values is
specified for an attribute, the following are defined:
</p>
<ul>
<li> A &lsquo;<samp>#define</samp>&rsquo; is written for the symbol &lsquo;<samp>HAVE_ATTR_<var>name</var></samp>&rsquo;.

</li><li> An enumerated class is defined for &lsquo;<samp>attr_<var>name</var></samp>&rsquo; with
elements of the form &lsquo;<samp><var>upper-name</var>_<var>upper-value</var></samp>&rsquo; where
the attribute name and value are first converted to uppercase.

</li><li> A function &lsquo;<samp>get_attr_<var>name</var></samp>&rsquo; is defined that is passed an insn and
returns the attribute value for that insn.
</li></ul>

<p>For example, if the following is present in the <samp>md</samp> file:
</p>
<div class="smallexample">
<pre class="smallexample">(define_attr &quot;type&quot; &quot;branch,fp,load,store,arith&quot; &hellip;)
</pre></div>

<p>the following lines will be written to the file <samp>insn-attr.h</samp>.
</p>
<div class="smallexample">
<pre class="smallexample">#define HAVE_ATTR_type 1
enum attr_type {TYPE_BRANCH, TYPE_FP, TYPE_LOAD,
                 TYPE_STORE, TYPE_ARITH};
extern enum attr_type get_attr_type ();
</pre></div>

<p>If the attribute takes numeric values, no <code>enum</code> type will be
defined and the function to obtain the attribute&rsquo;s value will return
<code>int</code>.
</p>
<p>There are attributes which are tied to a specific meaning.  These
attributes are not free to use for other purposes:
</p>
<dl compact="compact">
<dt><code>length</code></dt>
<dd><p>The <code>length</code> attribute is used to calculate the length of emitted
code chunks.  This is especially important when verifying branch
distances. See <a href="Insn-Lengths.html#Insn-Lengths">Insn Lengths</a>.
</p>
</dd>
<dt><code>enabled</code></dt>
<dd><p>The <code>enabled</code> attribute can be defined to prevent certain
alternatives of an insn definition from being used during code
generation. See <a href="Disable-Insn-Alternatives.html#Disable-Insn-Alternatives">Disable Insn Alternatives</a>.
</p>
</dd>
<dt><code>mnemonic</code></dt>
<dd><p>The <code>mnemonic</code> attribute can be defined to implement instruction
specific checks in e.g. the pipeline description.
See <a href="Mnemonic-Attribute.html#Mnemonic-Attribute">Mnemonic Attribute</a>.
</p></dd>
</dl>

<p>For each of these special attributes, the corresponding
&lsquo;<samp>HAVE_ATTR_<var>name</var></samp>&rsquo; &lsquo;<samp>#define</samp>&rsquo; is also written when the
attribute is not defined; in that case, it is defined as &lsquo;<samp>0</samp>&rsquo;.
</p>
<a name="index-define_005fenum_005fattr"></a>
<a name="define_005fenum_005fattr"></a><p>Another way of defining an attribute is to use:
</p>
<div class="smallexample">
<pre class="smallexample">(define_enum_attr &quot;<var>attr</var>&quot; &quot;<var>enum</var>&quot; <var>default</var>)
</pre></div>

<p>This works in just the same way as <code>define_attr</code>, except that
the list of values is taken from a separate enumeration called
<var>enum</var> (see <a href="Constant-Definitions.html#define_005fenum">define_enum</a>).  This form allows you to use
the same list of values for several attributes without having to
repeat the list each time.  For example:
</p>
<div class="smallexample">
<pre class="smallexample">(define_enum &quot;processor&quot; [
  model_a
  model_b
  &hellip;
])
(define_enum_attr &quot;arch&quot; &quot;processor&quot;
  (const (symbol_ref &quot;target_arch&quot;)))
(define_enum_attr &quot;tune&quot; &quot;processor&quot;
  (const (symbol_ref &quot;target_tune&quot;)))
</pre></div>

<p>defines the same attributes as:
</p>
<div class="smallexample">
<pre class="smallexample">(define_attr &quot;arch&quot; &quot;model_a,model_b,&hellip;&quot;
  (const (symbol_ref &quot;target_arch&quot;)))
(define_attr &quot;tune&quot; &quot;model_a,model_b,&hellip;&quot;
  (const (symbol_ref &quot;target_tune&quot;)))
</pre></div>

<p>but without duplicating the processor list.  The second example defines two
separate C enums (<code>attr_arch</code> and <code>attr_tune</code>) whereas the first
defines a single C enum (<code>processor</code>).
</p><hr>
<div class="header">
<p>
Next: <a href="Expressions.html#Expressions" accesskey="n" rel="next">Expressions</a>, Up: <a href="Insn-Attributes.html#Insn-Attributes" accesskey="u" rel="up">Insn 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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