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<title>GNU Compiler Collection (GCC) Internals: RTL Template</title>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="RTL-Template"></a>
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<div class="header">
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<p>
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Next: <a href="Output-Template.html#Output-Template" accesskey="n" rel="next">Output Template</a>, Previous: <a href="Example.html#Example" accesskey="p" rel="prev">Example</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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</div>
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<hr>
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<a name="RTL-Template-1"></a>
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<h3 class="section">16.4 RTL Template</h3>
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<a name="index-RTL-insn-template"></a>
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<a name="index-generating-insns"></a>
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<a name="index-insns_002c-generating"></a>
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<a name="index-recognizing-insns"></a>
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<a name="index-insns_002c-recognizing"></a>
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<p>The RTL template is used to define which insns match the particular pattern
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and how to find their operands. For named patterns, the RTL template also
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says how to construct an insn from specified operands.
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</p>
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<p>Construction involves substituting specified operands into a copy of the
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template. Matching involves determining the values that serve as the
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operands in the insn being matched. Both of these activities are
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controlled by special expression types that direct matching and
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substitution of the operands.
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</p>
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<dl compact="compact">
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<dd><a name="index-match_005foperand"></a>
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</dd>
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<dt><code>(match_operand:<var>m</var> <var>n</var> <var>predicate</var> <var>constraint</var>)</code></dt>
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<dd><p>This expression is a placeholder for operand number <var>n</var> of
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the insn. When constructing an insn, operand number <var>n</var>
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will be substituted at this point. When matching an insn, whatever
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appears at this position in the insn will be taken as operand
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number <var>n</var>; but it must satisfy <var>predicate</var> or this instruction
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pattern will not match at all.
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</p>
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<p>Operand numbers must be chosen consecutively counting from zero in
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each instruction pattern. There may be only one <code>match_operand</code>
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expression in the pattern for each operand number. Usually operands
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are numbered in the order of appearance in <code>match_operand</code>
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expressions. In the case of a <code>define_expand</code>, any operand numbers
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used only in <code>match_dup</code> expressions have higher values than all
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other operand numbers.
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</p>
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<p><var>predicate</var> is a string that is the name of a function that
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accepts two arguments, an expression and a machine mode.
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See <a href="Predicates.html#Predicates">Predicates</a>. During matching, the function will be called with
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the putative operand as the expression and <var>m</var> as the mode
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argument (if <var>m</var> is not specified, <code>VOIDmode</code> will be used,
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which normally causes <var>predicate</var> to accept any mode). If it
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returns zero, this instruction pattern fails to match.
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<var>predicate</var> may be an empty string; then it means no test is to be
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done on the operand, so anything which occurs in this position is
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valid.
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</p>
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<p>Most of the time, <var>predicate</var> will reject modes other than <var>m</var>—but
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not always. For example, the predicate <code>address_operand</code> uses
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<var>m</var> as the mode of memory ref that the address should be valid for.
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Many predicates accept <code>const_int</code> nodes even though their mode is
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<code>VOIDmode</code>.
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</p>
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<p><var>constraint</var> controls reloading and the choice of the best register
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class to use for a value, as explained later (see <a href="Constraints.html#Constraints">Constraints</a>).
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If the constraint would be an empty string, it can be omitted.
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</p>
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<p>People are often unclear on the difference between the constraint and the
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predicate. The predicate helps decide whether a given insn matches the
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pattern. The constraint plays no role in this decision; instead, it
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controls various decisions in the case of an insn which does match.
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</p>
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<a name="index-match_005fscratch"></a>
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</dd>
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<dt><code>(match_scratch:<var>m</var> <var>n</var> <var>constraint</var>)</code></dt>
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<dd><p>This expression is also a placeholder for operand number <var>n</var>
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and indicates that operand must be a <code>scratch</code> or <code>reg</code>
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expression.
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</p>
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<p>When matching patterns, this is equivalent to
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</p>
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<div class="smallexample">
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<pre class="smallexample">(match_operand:<var>m</var> <var>n</var> "scratch_operand" <var>constraint</var>)
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</pre></div>
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<p>but, when generating RTL, it produces a (<code>scratch</code>:<var>m</var>)
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expression.
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</p>
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<p>If the last few expressions in a <code>parallel</code> are <code>clobber</code>
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expressions whose operands are either a hard register or
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<code>match_scratch</code>, the combiner can add or delete them when
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necessary. See <a href="Side-Effects.html#Side-Effects">Side Effects</a>.
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</p>
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<a name="index-match_005fdup"></a>
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</dd>
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<dt><code>(match_dup <var>n</var>)</code></dt>
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<dd><p>This expression is also a placeholder for operand number <var>n</var>.
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It is used when the operand needs to appear more than once in the
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insn.
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</p>
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<p>In construction, <code>match_dup</code> acts just like <code>match_operand</code>:
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the operand is substituted into the insn being constructed. But in
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matching, <code>match_dup</code> behaves differently. It assumes that operand
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number <var>n</var> has already been determined by a <code>match_operand</code>
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appearing earlier in the recognition template, and it matches only an
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identical-looking expression.
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</p>
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<p>Note that <code>match_dup</code> should not be used to tell the compiler that
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a particular register is being used for two operands (example:
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<code>add</code> that adds one register to another; the second register is
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both an input operand and the output operand). Use a matching
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constraint (see <a href="Simple-Constraints.html#Simple-Constraints">Simple Constraints</a>) for those. <code>match_dup</code> is for the cases where one
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operand is used in two places in the template, such as an instruction
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that computes both a quotient and a remainder, where the opcode takes
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two input operands but the RTL template has to refer to each of those
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twice; once for the quotient pattern and once for the remainder pattern.
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</p>
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<a name="index-match_005foperator"></a>
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</dd>
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<dt><code>(match_operator:<var>m</var> <var>n</var> <var>predicate</var> [<var>operands</var>…])</code></dt>
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<dd><p>This pattern is a kind of placeholder for a variable RTL expression
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code.
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</p>
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<p>When constructing an insn, it stands for an RTL expression whose
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expression code is taken from that of operand <var>n</var>, and whose
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operands are constructed from the patterns <var>operands</var>.
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</p>
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<p>When matching an expression, it matches an expression if the function
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<var>predicate</var> returns nonzero on that expression <em>and</em> the
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patterns <var>operands</var> match the operands of the expression.
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</p>
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<p>Suppose that the function <code>commutative_operator</code> is defined as
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follows, to match any expression whose operator is one of the
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commutative arithmetic operators of RTL and whose mode is <var>mode</var>:
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</p>
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<div class="smallexample">
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<pre class="smallexample">int
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commutative_integer_operator (x, mode)
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rtx x;
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machine_mode mode;
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{
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enum rtx_code code = GET_CODE (x);
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if (GET_MODE (x) != mode)
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return 0;
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return (GET_RTX_CLASS (code) == RTX_COMM_ARITH
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|| code == EQ || code == NE);
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}
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</pre></div>
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<p>Then the following pattern will match any RTL expression consisting
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of a commutative operator applied to two general operands:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(match_operator:SI 3 "commutative_operator"
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[(match_operand:SI 1 "general_operand" "g")
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(match_operand:SI 2 "general_operand" "g")])
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</pre></div>
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<p>Here the vector <code>[<var>operands</var>…]</code> contains two patterns
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because the expressions to be matched all contain two operands.
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</p>
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<p>When this pattern does match, the two operands of the commutative
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operator are recorded as operands 1 and 2 of the insn. (This is done
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by the two instances of <code>match_operand</code>.) Operand 3 of the insn
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will be the entire commutative expression: use <code>GET_CODE
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(operands[3])</code> to see which commutative operator was used.
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</p>
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<p>The machine mode <var>m</var> of <code>match_operator</code> works like that of
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<code>match_operand</code>: it is passed as the second argument to the
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predicate function, and that function is solely responsible for
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deciding whether the expression to be matched “has” that mode.
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</p>
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<p>When constructing an insn, argument 3 of the gen-function will specify
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the operation (i.e. the expression code) for the expression to be
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made. It should be an RTL expression, whose expression code is copied
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into a new expression whose operands are arguments 1 and 2 of the
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gen-function. The subexpressions of argument 3 are not used;
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only its expression code matters.
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</p>
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<p>When <code>match_operator</code> is used in a pattern for matching an insn,
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it usually best if the operand number of the <code>match_operator</code>
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is higher than that of the actual operands of the insn. This improves
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register allocation because the register allocator often looks at
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operands 1 and 2 of insns to see if it can do register tying.
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</p>
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<p>There is no way to specify constraints in <code>match_operator</code>. The
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operand of the insn which corresponds to the <code>match_operator</code>
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never has any constraints because it is never reloaded as a whole.
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However, if parts of its <var>operands</var> are matched by
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<code>match_operand</code> patterns, those parts may have constraints of
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their own.
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</p>
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<a name="index-match_005fop_005fdup"></a>
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</dd>
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<dt><code>(match_op_dup:<var>m</var> <var>n</var>[<var>operands</var>…])</code></dt>
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<dd><p>Like <code>match_dup</code>, except that it applies to operators instead of
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operands. When constructing an insn, operand number <var>n</var> will be
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substituted at this point. But in matching, <code>match_op_dup</code> behaves
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differently. It assumes that operand number <var>n</var> has already been
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determined by a <code>match_operator</code> appearing earlier in the
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recognition template, and it matches only an identical-looking
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expression.
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</p>
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<a name="index-match_005fparallel"></a>
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</dd>
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<dt><code>(match_parallel <var>n</var> <var>predicate</var> [<var>subpat</var>…])</code></dt>
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<dd><p>This pattern is a placeholder for an insn that consists of a
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<code>parallel</code> expression with a variable number of elements. This
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expression should only appear at the top level of an insn pattern.
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</p>
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<p>When constructing an insn, operand number <var>n</var> will be substituted at
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this point. When matching an insn, it matches if the body of the insn
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is a <code>parallel</code> expression with at least as many elements as the
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vector of <var>subpat</var> expressions in the <code>match_parallel</code>, if each
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<var>subpat</var> matches the corresponding element of the <code>parallel</code>,
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<em>and</em> the function <var>predicate</var> returns nonzero on the
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<code>parallel</code> that is the body of the insn. It is the responsibility
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of the predicate to validate elements of the <code>parallel</code> beyond
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those listed in the <code>match_parallel</code>.
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</p>
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<p>A typical use of <code>match_parallel</code> is to match load and store
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multiple expressions, which can contain a variable number of elements
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in a <code>parallel</code>. For example,
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</p>
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<div class="smallexample">
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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></div>
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<p>This example comes from <samp>a29k.md</samp>. The function
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<code>load_multiple_operation</code> is defined in <samp>a29k.c</samp> and checks
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that subsequent elements in the <code>parallel</code> are the same as the
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<code>set</code> in the pattern, except that they are referencing subsequent
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registers and memory locations.
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</p>
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<p>An insn that matches this pattern might look like:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(parallel
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[(set (reg:SI 20) (mem:SI (reg:SI 100)))
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(use (reg:SI 179))
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(clobber (reg:SI 179))
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(set (reg:SI 21)
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(mem:SI (plus:SI (reg:SI 100)
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(const_int 4))))
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(set (reg:SI 22)
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(mem:SI (plus:SI (reg:SI 100)
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(const_int 8))))])
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</pre></div>
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<a name="index-match_005fpar_005fdup"></a>
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</dd>
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<dt><code>(match_par_dup <var>n</var> [<var>subpat</var>…])</code></dt>
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<dd><p>Like <code>match_op_dup</code>, but for <code>match_parallel</code> instead of
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<code>match_operator</code>.
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</p>
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</dd>
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</dl>
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Next: <a href="Output-Template.html#Output-Template" accesskey="n" rel="next">Output Template</a>, Previous: <a href="Example.html#Example" accesskey="p" rel="prev">Example</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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