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 <a name="Constant-Definitions"></a>
 <div class="header">
 <p>
 Next: <a href="Iterators.html#Iterators" accesskey="n" rel="next">Iterators</a>, Previous: <a href="Define-Subst.html#Define-Subst" accesskey="p" rel="prev">Define Subst</a>, Up: <a href="Machine-Desc.html#Machine-Desc" accesskey="u" rel="up">Machine Desc</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="Constant-Definitions-1"></a>
 <h3 class="section">16.22 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>
 <div class="header">
 <p>
 Next: <a href="Iterators.html#Iterators" accesskey="n" rel="next">Iterators</a>, Previous: <a href="Define-Subst.html#Define-Subst" accesskey="p" rel="prev">Define Subst</a>, Up: <a href="Machine-Desc.html#Machine-Desc" accesskey="u" rel="up">Machine Desc</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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	<a name="Constant-Definitions"></a>
	<div class="header">
	<p>
	Next: <a href="Iterators.html#Iterators" accesskey="n" rel="next">Iterators</a>, Previous: <a href="Define-Subst.html#Define-Subst" accesskey="p" rel="prev">Define Subst</a>, Up: <a href="Machine-Desc.html#Machine-Desc" accesskey="u" rel="up">Machine Desc</a>   [<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="Constant-Definitions-1"></a>
	<h3 class="section">16.22 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 ""
	[(match_parallel 0 "load_multiple_operation"
	[(set (match_operand:SI 1 "gpc_reg_operand" "=r")
	(match_operand:SI 2 "memory_operand" "m"))
	(use (reg:SI 179))
	(clobber (reg:SI 179))])]
	""
	"loadm 0,0,%1,%2")
	</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 ""
	[(match_parallel 0 "load_multiple_operation"
	[(set (match_operand:SI 1 "gpc_reg_operand" "=r")
	(match_operand:SI 2 "memory_operand" "m"))
	(use (reg:SI R_CR))
	(clobber (reg:SI R_CR))])]
	""
	"loadm 0,0,%1,%2")
	</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 "<var>name</var>" [
	<var>value0</var>
	<var>value1</var>
	…
	<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,
	…
	<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 "<var>name</var>" [<var>value0</var>])
	(define_c_enum "<var>name</var>" [<var>value1</var>])
	…
	(define_c_enum "<var>name</var>" [<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 "unspecv" [
	UNSPECV_BLOCKAGE
	])
	</pre></div>

	<p>causes GCC to print ‘<samp>(unspec_volatile … 0)</samp>’ 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 "<var>name</var>" [
	<var>value0</var>
	<var>value1</var>
	…
	<var>valuen</var>
	])
	</pre></div>

	<p>This directive implies:
	</p>
	<div class="smallexample">
	<pre class="smallexample">(define_c_enum "<var>name</var>" [
	<var>cname</var>_<var>cvalue0</var>
	<var>cname</var>_<var>cvalue1</var>
	…
	<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>
	<div class="header">
	<p>
	Next: <a href="Iterators.html#Iterators" accesskey="n" rel="next">Iterators</a>, Previous: <a href="Define-Subst.html#Define-Subst" accesskey="p" rel="prev">Define Subst</a>, Up: <a href="Machine-Desc.html#Machine-Desc" accesskey="u" rel="up">Machine Desc</a>   [<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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