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<a name="Constant-Definitions"></a>
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<a name="Constant-Definitions-1"></a>
<h3 class="section">17.22 Constant Definitions</h3>
<a name="index-constant-definitions"></a>
<a name="index-define_005fconstants"></a>

<p>Using literal constants inside instruction patterns reduces legibility and
can be a maintenance problem.
</p>
<p>To overcome this problem, you may use the <code>define_constants</code>
expression.  It contains a vector of name-value pairs.  From that
point on, wherever any of the names appears in the MD file, it is as
if the corresponding value had been written instead.  You may use
<code>define_constants</code> multiple times; each appearance adds more
constants to the table.  It is an error to redefine a constant with
a different value.
</p>
<p>To come back to the a29k load multiple example, instead of
</p>
<div class="smallexample">
<pre class="smallexample">(define_insn &quot;&quot;
  [(match_parallel 0 &quot;load_multiple_operation&quot;
     [(set (match_operand:SI 1 &quot;gpc_reg_operand&quot; &quot;=r&quot;)
           (match_operand:SI 2 &quot;memory_operand&quot; &quot;m&quot;))
      (use (reg:SI 179))
      (clobber (reg:SI 179))])]
  &quot;&quot;
  &quot;loadm 0,0,%1,%2&quot;)
</pre></div>

<p>You could write:
</p>
<div class="smallexample">
<pre class="smallexample">(define_constants [
    (R_BP 177)
    (R_FC 178)
    (R_CR 179)
    (R_Q  180)
])

(define_insn &quot;&quot;
  [(match_parallel 0 &quot;load_multiple_operation&quot;
     [(set (match_operand:SI 1 &quot;gpc_reg_operand&quot; &quot;=r&quot;)
           (match_operand:SI 2 &quot;memory_operand&quot; &quot;m&quot;))
      (use (reg:SI R_CR))
      (clobber (reg:SI R_CR))])]
  &quot;&quot;
  &quot;loadm 0,0,%1,%2&quot;)
</pre></div>

<p>The constants that are defined with a define_constant are also output
in the insn-codes.h header file as #defines.
</p>
<a name="index-enumerations"></a>
<a name="index-define_005fc_005fenum"></a>
<p>You can also use the machine description file to define enumerations.
Like the constants defined by <code>define_constant</code>, these enumerations
are visible to both the machine description file and the main C code.
</p>
<p>The syntax is as follows:
</p>
<div class="smallexample">
<pre class="smallexample">(define_c_enum &quot;<var>name</var>&quot; [
  <var>value0</var>
  <var>value1</var>
  &hellip;
  <var>valuen</var>
])
</pre></div>

<p>This definition causes the equivalent of the following C code to appear
in <samp>insn-constants.h</samp>:
</p>
<div class="smallexample">
<pre class="smallexample">enum <var>name</var> {
  <var>value0</var> = 0,
  <var>value1</var> = 1,
  &hellip;
  <var>valuen</var> = <var>n</var>
};
#define NUM_<var>cname</var>_VALUES (<var>n</var> + 1)
</pre></div>

<p>where <var>cname</var> is the capitalized form of <var>name</var>.
It also makes each <var>valuei</var> available in the machine description
file, just as if it had been declared with:
</p>
<div class="smallexample">
<pre class="smallexample">(define_constants [(<var>valuei</var> <var>i</var>)])
</pre></div>

<p>Each <var>valuei</var> is usually an upper-case identifier and usually
begins with <var>cname</var>.
</p>
<p>You can split the enumeration definition into as many statements as
you like.  The above example is directly equivalent to:
</p>
<div class="smallexample">
<pre class="smallexample">(define_c_enum &quot;<var>name</var>&quot; [<var>value0</var>])
(define_c_enum &quot;<var>name</var>&quot; [<var>value1</var>])
&hellip;
(define_c_enum &quot;<var>name</var>&quot; [<var>valuen</var>])
</pre></div>

<p>Splitting the enumeration helps to improve the modularity of each
individual <code>.md</code> file.  For example, if a port defines its
synchronization instructions in a separate <samp>sync.md</samp> file,
it is convenient to define all synchronization-specific enumeration
values in <samp>sync.md</samp> rather than in the main <samp>.md</samp> file.
</p>
<p>Some enumeration names have special significance to GCC:
</p>
<dl compact="compact">
<dt><code>unspecv</code></dt>
<dd><a name="index-unspec_005fvolatile-1"></a>
<p>If an enumeration called <code>unspecv</code> is defined, GCC will use it
when printing out <code>unspec_volatile</code> expressions.  For example:
</p>
<div class="smallexample">
<pre class="smallexample">(define_c_enum &quot;unspecv&quot; [
  UNSPECV_BLOCKAGE
])
</pre></div>

<p>causes GCC to print &lsquo;<samp>(unspec_volatile &hellip; 0)</samp>&rsquo; as:
</p>
<div class="smallexample">
<pre class="smallexample">(unspec_volatile ... UNSPECV_BLOCKAGE)
</pre></div>

</dd>
<dt><code>unspec</code></dt>
<dd><a name="index-unspec-1"></a>
<p>If an enumeration called <code>unspec</code> is defined, GCC will use
it when printing out <code>unspec</code> expressions.  GCC will also use
it when printing out <code>unspec_volatile</code> expressions unless an
<code>unspecv</code> enumeration is also defined.  You can therefore
decide whether to keep separate enumerations for volatile and
non-volatile expressions or whether to use the same enumeration
for both.
</p></dd>
</dl>

<a name="index-define_005fenum"></a>
<a name="define_005fenum"></a><p>Another way of defining an enumeration is to use <code>define_enum</code>:
</p>
<div class="smallexample">
<pre class="smallexample">(define_enum &quot;<var>name</var>&quot; [
  <var>value0</var>
  <var>value1</var>
  &hellip;
  <var>valuen</var>
])
</pre></div>

<p>This directive implies:
</p>
<div class="smallexample">
<pre class="smallexample">(define_c_enum &quot;<var>name</var>&quot; [
  <var>cname</var>_<var>cvalue0</var>
  <var>cname</var>_<var>cvalue1</var>
  &hellip;
  <var>cname</var>_<var>cvaluen</var>
])
</pre></div>

<a name="index-define_005fenum_005fattr-1"></a>
<p>where <var>cvaluei</var> is the capitalized form of <var>valuei</var>.
However, unlike <code>define_c_enum</code>, the enumerations defined
by <code>define_enum</code> can be used in attribute specifications
(see <a href="Defining-Attributes.html#define_005fenum_005fattr">define_enum_attr</a>).
</p><hr>
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