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176 lines
8.4 KiB
HTML
176 lines
8.4 KiB
HTML
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any later version published by the Free Software Foundation; with the
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Patterns (GNU Compiler Collection (GCC) Internals)</title>
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<meta name="description" content="Patterns (GNU Compiler Collection (GCC) Internals)">
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<meta name="keywords" content="Patterns (GNU Compiler Collection (GCC) Internals)">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Machine-Desc.html#Machine-Desc" rel="up" title="Machine Desc">
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<link href="Example.html#Example" rel="next" title="Example">
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<link href="Overview.html#Overview" rel="prev" title="Overview">
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</head>
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<body lang="en">
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<a name="Patterns"></a>
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<div class="header">
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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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</div>
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<hr>
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<a name="Everything-about-Instruction-Patterns"></a>
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<h3 class="section">17.2 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 indicates 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>An instruction condition cannot become more restrictive as compilation
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progresses. If the condition accepts a particular RTL instruction at
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one stage of compilation, it must continue to accept that instruction
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until the final pass. For example, ‘<samp>!reload_completed</samp>’ and
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‘<samp>can_create_pseudo_p ()</samp>’ are both invalid instruction conditions,
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because they are true during the earlier RTL passes and false during
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the later ones. For the same reason, if a condition accepts an
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instruction before register allocation, it cannot later try to control
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register allocation by excluding certain register or value combinations.
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</p>
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<p>Although a condition cannot become more restrictive as compilation
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progresses, the condition for a nameless pattern <em>can</em> become
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more permissive. For example, a nameless instruction can require
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‘<samp>reload_completed</samp>’ to be true, in which case it only matches
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after register allocation.
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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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<div class="header">
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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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</div>
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</body>
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</html>
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