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<a name="RTL-Template"></a>
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<hr>
<a name="RTL-Template-1"></a>
<h3 class="section">17.4 RTL Template</h3>
<a name="index-RTL-insn-template"></a>
<a name="index-generating-insns"></a>
<a name="index-insns_002c-generating"></a>
<a name="index-recognizing-insns"></a>
<a name="index-insns_002c-recognizing"></a>

<p>The RTL template is used to define which insns match the particular pattern
and how to find their operands.  For named patterns, the RTL template also
says how to construct an insn from specified operands.
</p>
<p>Construction involves substituting specified operands into a copy of the
template.  Matching involves determining the values that serve as the
operands in the insn being matched.  Both of these activities are
controlled by special expression types that direct matching and
substitution of the operands.
</p>
<dl compact="compact">
<dd><a name="index-match_005foperand"></a>
</dd>
<dt><code>(match_operand:<var>m</var> <var>n</var> <var>predicate</var> <var>constraint</var>)</code></dt>
<dd><p>This expression is a placeholder for operand number <var>n</var> of
the insn.  When constructing an insn, operand number <var>n</var>
will be substituted at this point.  When matching an insn, whatever
appears at this position in the insn will be taken as operand
number <var>n</var>; but it must satisfy <var>predicate</var> or this instruction
pattern will not match at all.
</p>
<p>Operand numbers must be chosen consecutively counting from zero in
each instruction pattern.  There may be only one <code>match_operand</code>
expression in the pattern for each operand number.  Usually operands
are numbered in the order of appearance in <code>match_operand</code>
expressions.  In the case of a <code>define_expand</code>, any operand numbers
used only in <code>match_dup</code> expressions have higher values than all
other operand numbers.
</p>
<p><var>predicate</var> is a string that is the name of a function that
accepts two arguments, an expression and a machine mode.
See <a href="Predicates.html#Predicates">Predicates</a>.  During matching, the function will be called with
the putative operand as the expression and <var>m</var> as the mode
argument (if <var>m</var> is not specified, <code>VOIDmode</code> will be used,
which normally causes <var>predicate</var> to accept any mode).  If it
returns zero, this instruction pattern fails to match.
<var>predicate</var> may be an empty string; then it means no test is to be
done on the operand, so anything which occurs in this position is
valid.
</p>
<p>Most of the time, <var>predicate</var> will reject modes other than <var>m</var>&mdash;but
not always.  For example, the predicate <code>address_operand</code> uses
<var>m</var> as the mode of memory ref that the address should be valid for.
Many predicates accept <code>const_int</code> nodes even though their mode is
<code>VOIDmode</code>.
</p>
<p><var>constraint</var> controls reloading and the choice of the best register
class to use for a value, as explained later (see <a href="Constraints.html#Constraints">Constraints</a>).
If the constraint would be an empty string, it can be omitted.
</p>
<p>People are often unclear on the difference between the constraint and the
predicate.  The predicate helps decide whether a given insn matches the
pattern.  The constraint plays no role in this decision; instead, it
controls various decisions in the case of an insn which does match.
</p>
<a name="index-match_005fscratch"></a>
</dd>
<dt><code>(match_scratch:<var>m</var> <var>n</var> <var>constraint</var>)</code></dt>
<dd><p>This expression is also a placeholder for operand number <var>n</var>
and indicates that operand must be a <code>scratch</code> or <code>reg</code>
expression.
</p>
<p>When matching patterns, this is equivalent to
</p>
<div class="smallexample">
<pre class="smallexample">(match_operand:<var>m</var> <var>n</var> &quot;scratch_operand&quot; <var>constraint</var>)
</pre></div>

<p>but, when generating RTL, it produces a (<code>scratch</code>:<var>m</var>)
expression.
</p>
<p>If the last few expressions in a <code>parallel</code> are <code>clobber</code>
expressions whose operands are either a hard register or
<code>match_scratch</code>, the combiner can add or delete them when
necessary.  See <a href="Side-Effects.html#Side-Effects">Side Effects</a>.
</p>
<a name="index-match_005fdup"></a>
</dd>
<dt><code>(match_dup <var>n</var>)</code></dt>
<dd><p>This expression is also a placeholder for operand number <var>n</var>.
It is used when the operand needs to appear more than once in the
insn.
</p>
<p>In construction, <code>match_dup</code> acts just like <code>match_operand</code>:
the operand is substituted into the insn being constructed.  But in
matching, <code>match_dup</code> behaves differently.  It assumes that operand
number <var>n</var> has already been determined by a <code>match_operand</code>
appearing earlier in the recognition template, and it matches only an
identical-looking expression.
</p>
<p>Note that <code>match_dup</code> should not be used to tell the compiler that
a particular register is being used for two operands (example:
<code>add</code> that adds one register to another; the second register is
both an input operand and the output operand).  Use a matching
constraint (see <a href="Simple-Constraints.html#Simple-Constraints">Simple Constraints</a>) for those.  <code>match_dup</code> is for the cases where one
operand is used in two places in the template, such as an instruction
that computes both a quotient and a remainder, where the opcode takes
two input operands but the RTL template has to refer to each of those
twice; once for the quotient pattern and once for the remainder pattern.
</p>
<a name="index-match_005foperator"></a>
</dd>
<dt><code>(match_operator:<var>m</var> <var>n</var> <var>predicate</var> [<var>operands</var>&hellip;])</code></dt>
<dd><p>This pattern is a kind of placeholder for a variable RTL expression
code.
</p>
<p>When constructing an insn, it stands for an RTL expression whose
expression code is taken from that of operand <var>n</var>, and whose
operands are constructed from the patterns <var>operands</var>.
</p>
<p>When matching an expression, it matches an expression if the function
<var>predicate</var> returns nonzero on that expression <em>and</em> the
patterns <var>operands</var> match the operands of the expression.
</p>
<p>Suppose that the function <code>commutative_operator</code> is defined as
follows, to match any expression whose operator is one of the
commutative arithmetic operators of RTL and whose mode is <var>mode</var>:
</p>
<div class="smallexample">
<pre class="smallexample">int
commutative_integer_operator (x, mode)
     rtx x;
     machine_mode mode;
{
  enum rtx_code code = GET_CODE (x);
  if (GET_MODE (x) != mode)
    return 0;
  return (GET_RTX_CLASS (code) == RTX_COMM_ARITH
          || code == EQ || code == NE);
}
</pre></div>

<p>Then the following pattern will match any RTL expression consisting
of a commutative operator applied to two general operands:
</p>
<div class="smallexample">
<pre class="smallexample">(match_operator:SI 3 &quot;commutative_operator&quot;
  [(match_operand:SI 1 &quot;general_operand&quot; &quot;g&quot;)
   (match_operand:SI 2 &quot;general_operand&quot; &quot;g&quot;)])
</pre></div>

<p>Here the vector <code>[<var>operands</var>&hellip;]</code> contains two patterns
because the expressions to be matched all contain two operands.
</p>
<p>When this pattern does match, the two operands of the commutative
operator are recorded as operands 1 and 2 of the insn.  (This is done
by the two instances of <code>match_operand</code>.)  Operand 3 of the insn
will be the entire commutative expression: use <code>GET_CODE
(operands[3])</code> to see which commutative operator was used.
</p>
<p>The machine mode <var>m</var> of <code>match_operator</code> works like that of
<code>match_operand</code>: it is passed as the second argument to the
predicate function, and that function is solely responsible for
deciding whether the expression to be matched &ldquo;has&rdquo; that mode.
</p>
<p>When constructing an insn, argument 3 of the gen-function will specify
the operation (i.e. the expression code) for the expression to be
made.  It should be an RTL expression, whose expression code is copied
into a new expression whose operands are arguments 1 and 2 of the
gen-function.  The subexpressions of argument 3 are not used;
only its expression code matters.
</p>
<p>When <code>match_operator</code> is used in a pattern for matching an insn,
it usually best if the operand number of the <code>match_operator</code>
is higher than that of the actual operands of the insn.  This improves
register allocation because the register allocator often looks at
operands 1 and 2 of insns to see if it can do register tying.
</p>
<p>There is no way to specify constraints in <code>match_operator</code>.  The
operand of the insn which corresponds to the <code>match_operator</code>
never has any constraints because it is never reloaded as a whole.
However, if parts of its <var>operands</var> are matched by
<code>match_operand</code> patterns, those parts may have constraints of
their own.
</p>
<a name="index-match_005fop_005fdup"></a>
</dd>
<dt><code>(match_op_dup:<var>m</var> <var>n</var>[<var>operands</var>&hellip;])</code></dt>
<dd><p>Like <code>match_dup</code>, except that it applies to operators instead of
operands.  When constructing an insn, operand number <var>n</var> will be
substituted at this point.  But in matching, <code>match_op_dup</code> behaves
differently.  It assumes that operand number <var>n</var> has already been
determined by a <code>match_operator</code> appearing earlier in the
recognition template, and it matches only an identical-looking
expression.
</p>
<a name="index-match_005fparallel"></a>
</dd>
<dt><code>(match_parallel <var>n</var> <var>predicate</var> [<var>subpat</var>&hellip;])</code></dt>
<dd><p>This pattern is a placeholder for an insn that consists of a
<code>parallel</code> expression with a variable number of elements.  This
expression should only appear at the top level of an insn pattern.
</p>
<p>When constructing an insn, operand number <var>n</var> will be substituted at
this point.  When matching an insn, it matches if the body of the insn
is a <code>parallel</code> expression with at least as many elements as the
vector of <var>subpat</var> expressions in the <code>match_parallel</code>, if each
<var>subpat</var> matches the corresponding element of the <code>parallel</code>,
<em>and</em> the function <var>predicate</var> returns nonzero on the
<code>parallel</code> that is the body of the insn.  It is the responsibility
of the predicate to validate elements of the <code>parallel</code> beyond
those listed in the <code>match_parallel</code>.
</p>
<p>A typical use of <code>match_parallel</code> is to match load and store
multiple expressions, which can contain a variable number of elements
in a <code>parallel</code>.  For example,
</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>This example comes from <samp>a29k.md</samp>.  The function
<code>load_multiple_operation</code> is defined in <samp>a29k.c</samp> and checks
that subsequent elements in the <code>parallel</code> are the same as the
<code>set</code> in the pattern, except that they are referencing subsequent
registers and memory locations.
</p>
<p>An insn that matches this pattern might look like:
</p>
<div class="smallexample">
<pre class="smallexample">(parallel
 [(set (reg:SI 20) (mem:SI (reg:SI 100)))
  (use (reg:SI 179))
  (clobber (reg:SI 179))
  (set (reg:SI 21)
       (mem:SI (plus:SI (reg:SI 100)
                        (const_int 4))))
  (set (reg:SI 22)
       (mem:SI (plus:SI (reg:SI 100)
                        (const_int 8))))])
</pre></div>

<a name="index-match_005fpar_005fdup"></a>
</dd>
<dt><code>(match_par_dup <var>n</var> [<var>subpat</var>&hellip;])</code></dt>
<dd><p>Like <code>match_op_dup</code>, but for <code>match_parallel</code> instead of
<code>match_operator</code>.
</p>
</dd>
</dl>

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