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<h4 class="subsection">16.7.2 Defining Machine-Specific Predicates</h4>
<p><a name="index-defining-predicates-3301"></a><a name="index-define_005fpredicate-3302"></a><a name="index-define_005fspecial_005fpredicate-3303"></a>
Many machines have requirements for their operands that cannot be
expressed precisely using the generic predicates. You can define
additional predicates using <code>define_predicate</code> and
<code>define_special_predicate</code> expressions. These expressions have
three operands:
<ul>
<li>The name of the predicate, as it will be referred to in
<code>match_operand</code> or <code>match_operator</code> expressions.
<li>An RTL expression which evaluates to true if the predicate allows the
operand <var>op</var>, false if it does not. This expression can only use
the following RTL codes:
<dl>
<dt><code>MATCH_OPERAND</code><dd>When written inside a predicate expression, a <code>MATCH_OPERAND</code>
expression evaluates to true if the predicate it names would allow
<var>op</var>. The operand number and constraint are ignored. Due to
limitations in <samp><span class="command">genrecog</span></samp>, you can only refer to generic
predicates and predicates that have already been defined.
<br><dt><code>MATCH_CODE</code><dd>This expression evaluates to true if <var>op</var> or a specified
subexpression of <var>op</var> has one of a given list of RTX codes.
<p>The first operand of this expression is a string constant containing a
comma-separated list of RTX code names (in lower case). These are the
codes for which the <code>MATCH_CODE</code> will be true.
<p>The second operand is a string constant which indicates what
subexpression of <var>op</var> to examine. If it is absent or the empty
string, <var>op</var> itself is examined. Otherwise, the string constant
must be a sequence of digits and/or lowercase letters. Each character
indicates a subexpression to extract from the current expression; for
the first character this is <var>op</var>, for the second and subsequent
characters it is the result of the previous character. A digit
<var>n</var> extracts ‘<samp><span class="samp">XEXP (</span><var>e</var><span class="samp">, </span><var>n</var><span class="samp">)<!-- /@w --></span></samp>’; a letter <var>l</var>
extracts ‘<samp><span class="samp">XVECEXP (</span><var>e</var><span class="samp">, 0, </span><var>n</var><span class="samp">)<!-- /@w --></span></samp>’ where <var>n</var> is the
alphabetic ordinal of <var>l</var> (0 for `a', 1 for 'b', and so on). The
<code>MATCH_CODE</code> then examines the RTX code of the subexpression
extracted by the complete string. It is not possible to extract
components of an <code>rtvec</code> that is not at position 0 within its RTX
object.
<br><dt><code>MATCH_TEST</code><dd>This expression has one operand, a string constant containing a C
expression. The predicate's arguments, <var>op</var> and <var>mode</var>, are
available with those names in the C expression. The <code>MATCH_TEST</code>
evaluates to true if the C expression evaluates to a nonzero value.
<code>MATCH_TEST</code> expressions must not have side effects.
<br><dt><code>AND</code><dt><code>IOR</code><dt><code>NOT</code><dt><code>IF_THEN_ELSE</code><dd>The basic ‘<samp><span class="samp">MATCH_</span></samp>’ expressions can be combined using these
logical operators, which have the semantics of the C operators
‘<samp><span class="samp">&&</span></samp>’, ‘<samp><span class="samp">||</span></samp>’, ‘<samp><span class="samp">!</span></samp>’, and ‘<samp><span class="samp">? :<!-- /@w --></span></samp>’ respectively. As
in Common Lisp, you may give an <code>AND</code> or <code>IOR</code> expression an
arbitrary number of arguments; this has exactly the same effect as
writing a chain of two-argument <code>AND</code> or <code>IOR</code> expressions.
</dl>
<li>An optional block of C code, which should execute
‘<samp><span class="samp">return true<!-- /@w --></span></samp>’ if the predicate is found to match and
‘<samp><span class="samp">return false<!-- /@w --></span></samp>’ if it does not. It must not have any side
effects. The predicate arguments, <var>op</var> and <var>mode</var>, are
available with those names.
<p>If a code block is present in a predicate definition, then the RTL
expression must evaluate to true <em>and</em> the code block must
execute ‘<samp><span class="samp">return true<!-- /@w --></span></samp>’ for the predicate to allow the operand.
The RTL expression is evaluated first; do not re-check anything in the
code block that was checked in the RTL expression.
</ul>
<p>The program <samp><span class="command">genrecog</span></samp> scans <code>define_predicate</code> and
<code>define_special_predicate</code> expressions to determine which RTX
codes are possibly allowed. You should always make this explicit in
the RTL predicate expression, using <code>MATCH_OPERAND</code> and
<code>MATCH_CODE</code>.
<p>Here is an example of a simple predicate definition, from the IA64
machine description:
<pre class="smallexample"> ;; <span class="roman">True if </span><var>op</var><span class="roman"> is a </span><code>SYMBOL_REF</code><span class="roman"> which refers to the sdata section.</span>
(define_predicate "small_addr_symbolic_operand"
(and (match_code "symbol_ref")
(match_test "SYMBOL_REF_SMALL_ADDR_P (op)")))
</pre>
<p class="noindent">And here is another, showing the use of the C block.
<pre class="smallexample"> ;; <span class="roman">True if </span><var>op</var><span class="roman"> is a register operand that is (or could be) a GR reg.</span>
(define_predicate "gr_register_operand"
(match_operand 0 "register_operand")
{
unsigned int regno;
if (GET_CODE (op) == SUBREG)
op = SUBREG_REG (op);
regno = REGNO (op);
return (regno >= FIRST_PSEUDO_REGISTER || GENERAL_REGNO_P (regno));
})
</pre>
<p>Predicates written with <code>define_predicate</code> automatically include
a test that <var>mode</var> is <code>VOIDmode</code>, or <var>op</var> has the same
mode as <var>mode</var>, or <var>op</var> is a <code>CONST_INT</code> or
<code>CONST_DOUBLE</code>. They do <em>not</em> check specifically for
integer <code>CONST_DOUBLE</code>, nor do they test that the value of either
kind of constant fits in the requested mode. This is because
target-specific predicates that take constants usually have to do more
stringent value checks anyway. If you need the exact same treatment
of <code>CONST_INT</code> or <code>CONST_DOUBLE</code> that the generic predicates
provide, use a <code>MATCH_OPERAND</code> subexpression to call
<code>const_int_operand</code>, <code>const_double_operand</code>, or
<code>immediate_operand</code>.
<p>Predicates written with <code>define_special_predicate</code> do not get any
automatic mode checks, and are treated as having special mode handling
by <samp><span class="command">genrecog</span></samp>.
<p>The program <samp><span class="command">genpreds</span></samp> is responsible for generating code to
test predicates. It also writes a header file containing function
declarations for all machine-specific predicates. It is not necessary
to declare these predicates in <samp><var>cpu</var><span class="file">-protos.h</span></samp>.
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