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196 lines
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<title>Constant Definitions - GNU Compiler Collection (GCC) Internals</title>
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<a name="Constant-Definitions"></a>
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Next: <a rel="next" accesskey="n" href="Iterators.html#Iterators">Iterators</a>,
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Previous: <a rel="previous" accesskey="p" href="Define-Subst.html#Define-Subst">Define Subst</a>,
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Up: <a rel="up" accesskey="u" href="Machine-Desc.html#Machine-Desc">Machine Desc</a>
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<h3 class="section">16.22 Constant Definitions</h3>
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<p><a name="index-constant-definitions-3828"></a><a name="index-define_005fconstants-3829"></a>
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Using literal constants inside instruction patterns reduces legibility and
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can be a maintenance problem.
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<p>To overcome this problem, you may use the <code>define_constants</code>
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expression. It contains a vector of name-value pairs. From that
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point on, wherever any of the names appears in the MD file, it is as
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if the corresponding value had been written instead. You may use
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<code>define_constants</code> multiple times; each appearance adds more
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constants to the table. It is an error to redefine a constant with
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a different value.
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<p>To come back to the a29k load multiple example, instead of
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<pre class="smallexample"> (define_insn ""
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[(match_parallel 0 "load_multiple_operation"
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[(set (match_operand:SI 1 "gpc_reg_operand" "=r")
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(match_operand:SI 2 "memory_operand" "m"))
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(use (reg:SI 179))
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(clobber (reg:SI 179))])]
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""
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"loadm 0,0,%1,%2")
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</pre>
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<p>You could write:
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<pre class="smallexample"> (define_constants [
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(R_BP 177)
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(R_FC 178)
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(R_CR 179)
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(R_Q 180)
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])
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(define_insn ""
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[(match_parallel 0 "load_multiple_operation"
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[(set (match_operand:SI 1 "gpc_reg_operand" "=r")
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(match_operand:SI 2 "memory_operand" "m"))
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(use (reg:SI R_CR))
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(clobber (reg:SI R_CR))])]
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""
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"loadm 0,0,%1,%2")
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</pre>
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<p>The constants that are defined with a define_constant are also output
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in the insn-codes.h header file as #defines.
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<p><a name="index-enumerations-3830"></a><a name="index-define_005fc_005fenum-3831"></a>You can also use the machine description file to define enumerations.
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Like the constants defined by <code>define_constant</code>, these enumerations
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are visible to both the machine description file and the main C code.
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<p>The syntax is as follows:
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<pre class="smallexample"> (define_c_enum "<var>name</var>" [
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<var>value0</var>
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<var>value1</var>
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...
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<var>valuen</var>
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])
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</pre>
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<p>This definition causes the equivalent of the following C code to appear
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in <samp><span class="file">insn-constants.h</span></samp>:
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<pre class="smallexample"> enum <var>name</var> {
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<var>value0</var> = 0,
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<var>value1</var> = 1,
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...
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<var>valuen</var> = <var>n</var>
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};
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#define NUM_<var>cname</var>_VALUES (<var>n</var> + 1)
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</pre>
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<p>where <var>cname</var> is the capitalized form of <var>name</var>.
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It also makes each <var>valuei</var> available in the machine description
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file, just as if it had been declared with:
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<pre class="smallexample"> (define_constants [(<var>valuei</var> <var>i</var>)])
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</pre>
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<p>Each <var>valuei</var> is usually an upper-case identifier and usually
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begins with <var>cname</var>.
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<p>You can split the enumeration definition into as many statements as
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you like. The above example is directly equivalent to:
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<pre class="smallexample"> (define_c_enum "<var>name</var>" [<var>value0</var>])
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(define_c_enum "<var>name</var>" [<var>value1</var>])
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...
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(define_c_enum "<var>name</var>" [<var>valuen</var>])
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</pre>
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<p>Splitting the enumeration helps to improve the modularity of each
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individual <code>.md</code> file. For example, if a port defines its
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synchronization instructions in a separate <samp><span class="file">sync.md</span></samp> file,
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it is convenient to define all synchronization-specific enumeration
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values in <samp><span class="file">sync.md</span></samp> rather than in the main <samp><span class="file">.md</span></samp> file.
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<p>Some enumeration names have special significance to GCC:
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<dl>
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<dt><code>unspecv</code><dd><a name="index-unspec_005fvolatile-3832"></a>If an enumeration called <code>unspecv</code> is defined, GCC will use it
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when printing out <code>unspec_volatile</code> expressions. For example:
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<pre class="smallexample"> (define_c_enum "unspecv" [
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UNSPECV_BLOCKAGE
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])
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</pre>
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<p>causes GCC to print ‘<samp><span class="samp">(unspec_volatile ... 0)</span></samp>’ as:
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<pre class="smallexample"> (unspec_volatile ... UNSPECV_BLOCKAGE)
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</pre>
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<br><dt><code>unspec</code><dd><a name="index-unspec-3833"></a>If an enumeration called <code>unspec</code> is defined, GCC will use
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it when printing out <code>unspec</code> expressions. GCC will also use
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it when printing out <code>unspec_volatile</code> expressions unless an
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<code>unspecv</code> enumeration is also defined. You can therefore
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decide whether to keep separate enumerations for volatile and
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non-volatile expressions or whether to use the same enumeration
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for both.
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</dl>
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<p><a name="index-define_005fenum-3834"></a><a name="define_005fenum"></a>Another way of defining an enumeration is to use <code>define_enum</code>:
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<pre class="smallexample"> (define_enum "<var>name</var>" [
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<var>value0</var>
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<var>value1</var>
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...
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<var>valuen</var>
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])
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</pre>
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<p>This directive implies:
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<pre class="smallexample"> (define_c_enum "<var>name</var>" [
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<var>cname</var>_<var>cvalue0</var>
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<var>cname</var>_<var>cvalue1</var>
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...
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<var>cname</var>_<var>cvaluen</var>
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])
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</pre>
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<p><a name="index-define_005fenum_005fattr-3835"></a>where <var>cvaluei</var> is the capitalized form of <var>valuei</var>.
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However, unlike <code>define_c_enum</code>, the enumerations defined
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by <code>define_enum</code> can be used in attribute specifications
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(see <a href="define_005fenum_005fattr.html#define_005fenum_005fattr">define_enum_attr</a>).
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</body></html>
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