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<title>GNU Compiler Collection (GCC) Internals: Patterns</title>
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<a name="Patterns"></a>
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<p>
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Next: <a href="Example.html#Example" accesskey="n" rel="next">Example</a>, Previous: <a href="Overview.html#Overview" accesskey="p" rel="prev">Overview</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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<hr>
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<a name="Everything-about-Instruction-Patterns"></a>
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<h3 class="section">16.2 Everything about Instruction Patterns</h3>
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<a name="index-patterns"></a>
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<a name="index-instruction-patterns"></a>
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<a name="index-define_005finsn"></a>
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<p>A <code>define_insn</code> expression is used to define instruction patterns
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to which insns may be matched. A <code>define_insn</code> expression contains
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an incomplete RTL expression, with pieces to be filled in later, operand
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constraints that restrict how the pieces can be filled in, and an output
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template or C code to generate the assembler output.
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</p>
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<p>A <code>define_insn</code> is an RTL expression containing four or five operands:
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</p>
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<ol>
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<li> An optional name. The presence of a name indicate that this instruction
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pattern can perform a certain standard job for the RTL-generation
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pass of the compiler. This pass knows certain names and will use
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the instruction patterns with those names, if the names are defined
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in the machine description.
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<p>The absence of a name is indicated by writing an empty string
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where the name should go. Nameless instruction patterns are never
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used for generating RTL code, but they may permit several simpler insns
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to be combined later on.
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</p>
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<p>Names that are not thus known and used in RTL-generation have no
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effect; they are equivalent to no name at all.
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</p>
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<p>For the purpose of debugging the compiler, you may also specify a
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name beginning with the ‘<samp>*</samp>’ character. Such a name is used only
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for identifying the instruction in RTL dumps; it is equivalent to having
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a nameless pattern for all other purposes. Names beginning with the
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‘<samp>*</samp>’ character are not required to be unique.
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</p>
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</li><li> The <em>RTL template</em>: This is a vector of incomplete RTL expressions
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which describe the semantics of the instruction (see <a href="RTL-Template.html#RTL-Template">RTL Template</a>).
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It is incomplete because it may contain <code>match_operand</code>,
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<code>match_operator</code>, and <code>match_dup</code> expressions that stand for
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operands of the instruction.
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<p>If the vector has multiple elements, the RTL template is treated as a
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<code>parallel</code> expression.
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</p>
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</li><li> <a name="index-pattern-conditions"></a>
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<a name="index-conditions_002c-in-patterns"></a>
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The condition: This is a string which contains a C expression. When the
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compiler attempts to match RTL against a pattern, the condition is
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evaluated. If the condition evaluates to <code>true</code>, the match is
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permitted. The condition may be an empty string, which is treated
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as always <code>true</code>.
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<a name="index-named-patterns-and-conditions"></a>
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<p>For a named pattern, the condition may not depend on the data in the
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insn being matched, but only the target-machine-type flags. The compiler
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needs to test these conditions during initialization in order to learn
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exactly which named instructions are available in a particular run.
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</p>
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<a name="index-operands-1"></a>
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<p>For nameless patterns, the condition is applied only when matching an
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individual insn, and only after the insn has matched the pattern’s
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recognition template. The insn’s operands may be found in the vector
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<code>operands</code>.
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</p>
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<p>For an insn where the condition has once matched, it
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cannot later be used to control register allocation by excluding
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certain register or value combinations.
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</p>
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</li><li> The <em>output template</em> or <em>output statement</em>: This is either
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a string, or a fragment of C code which returns a string.
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<p>When simple substitution isn’t general enough, you can specify a piece
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of C code to compute the output. See <a href="Output-Statement.html#Output-Statement">Output Statement</a>.
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</p>
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</li><li> The <em>insn attributes</em>: This is an optional vector containing the values of
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attributes for insns matching this pattern (see <a href="Insn-Attributes.html#Insn-Attributes">Insn Attributes</a>).
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</li></ol>
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<hr>
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<p>
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Next: <a href="Example.html#Example" accesskey="n" rel="next">Example</a>, Previous: <a href="Overview.html#Overview" accesskey="p" rel="prev">Overview</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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