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<h4 class="subsection">16.19.1 Defining Attributes and their Values</h4>
<p><a name="index-defining-attributes-and-their-values-3721"></a><a name="index-attributes_002c-defining-3722"></a>
<a name="index-define_005fattr-3723"></a>The <code>define_attr</code> expression is used to define each attribute required
by the target machine. It looks like:
<pre class="smallexample"> (define_attr <var>name</var> <var>list-of-values</var> <var>default</var>)
</pre>
<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><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><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-3724"></a>For each defined attribute, a number of definitions are written to the
<samp><span class="file">insn-attr.h</span></samp> file. For cases where an explicit set of values is
specified for an attribute, the following are defined:
<ul>
<li>A ‘<samp><span class="samp">#define</span></samp>’ is written for the symbol ‘<samp><span class="samp">HAVE_ATTR_</span><var>name</var></samp>’.
<li>An enumerated class is defined for ‘<samp><span class="samp">attr_</span><var>name</var></samp>’ with
elements of the form ‘<samp><var>upper-name</var><span class="samp">_</span><var>upper-value</var></samp>’ where
the attribute name and value are first converted to uppercase.
<li>A function ‘<samp><span class="samp">get_attr_</span><var>name</var></samp>’ is defined that is passed an insn and
returns the attribute value for that insn.
</ul>
<p>For example, if the following is present in the <samp><span class="file">md</span></samp> file:
<pre class="smallexample"> (define_attr "type" "branch,fp,load,store,arith" ...)
</pre>
<p class="noindent">the following lines will be written to the file <samp><span class="file">insn-attr.h</span></samp>.
<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>
<p>If the attribute takes numeric values, no <code>enum</code> type will be
defined and the function to obtain the attribute's value will return
<code>int</code>.
<p>There are attributes which are tied to a specific meaning. These
attributes are not free to use for other purposes:
<dl>
<dt><code>length</code><dd>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>.
<br><dt><code>enabled</code><dd>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>.
<br><dt><code>mnemonic</code><dd>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>.
</dl>
<p>For each of these special attributes, the corresponding
‘<samp><span class="samp">HAVE_ATTR_</span><var>name</var></samp>’ ‘<samp><span class="samp">#define</span></samp>’ is also written when the
attribute is not defined; in that case, it is defined as ‘<samp><span class="samp">0</span></samp>’.
<p><a name="index-define_005fenum_005fattr-3725"></a><a name="define_005fenum_005fattr"></a>Another way of defining an attribute is to use:
<pre class="smallexample"> (define_enum_attr "<var>attr</var>" "<var>enum</var>" <var>default</var>)
</pre>
<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="define_005fenum.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:
<pre class="smallexample"> (define_enum "processor" [
model_a
model_b
...
])
(define_enum_attr "arch" "processor"
(const (symbol_ref "target_arch")))
(define_enum_attr "tune" "processor"
(const (symbol_ref "target_tune")))
</pre>
<p>defines the same attributes as:
<pre class="smallexample"> (define_attr "arch" "model_a,model_b,..."
(const (symbol_ref "target_arch")))
(define_attr "tune" "model_a,model_b,..."
(const (symbol_ref "target_tune")))
</pre>
<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>).
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