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| <a name="Patterns"></a> |
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| <a name="Everything-about-Instruction-Patterns"></a> |
| <h3 class="section">16.2 Everything about Instruction Patterns</h3> |
| <a name="index-patterns"></a> |
| <a name="index-instruction-patterns"></a> |
| |
| <a name="index-define_005finsn"></a> |
| <p>Each instruction pattern contains an incomplete RTL expression, with pieces |
| to be filled in later, operand constraints that restrict how the pieces can |
| be filled in, and an output pattern or C code to generate the assembler |
| output, all wrapped up in a <code>define_insn</code> expression. |
| </p> |
| <p>A <code>define_insn</code> is an RTL expression containing four or five operands: |
| </p> |
| <ol> |
| <li> An optional name. The presence of a name indicate that this instruction |
| pattern can perform a certain standard job for the RTL-generation |
| pass of the compiler. This pass knows certain names and will use |
| the instruction patterns with those names, if the names are defined |
| in the machine description. |
| |
| <p>The absence of a name is indicated by writing an empty string |
| where the name should go. Nameless instruction patterns are never |
| used for generating RTL code, but they may permit several simpler insns |
| to be combined later on. |
| </p> |
| <p>Names that are not thus known and used in RTL-generation have no |
| effect; they are equivalent to no name at all. |
| </p> |
| <p>For the purpose of debugging the compiler, you may also specify a |
| name beginning with the ‘<samp>*</samp>’ character. Such a name is used only |
| for identifying the instruction in RTL dumps; it is entirely equivalent |
| to having a nameless pattern for all other purposes. |
| </p> |
| </li><li> The <em>RTL template</em> (see <a href="RTL-Template.html#RTL-Template">RTL Template</a>) is a vector of incomplete |
| RTL expressions which show what the instruction should look like. It is |
| incomplete because it may contain <code>match_operand</code>, |
| <code>match_operator</code>, and <code>match_dup</code> expressions that stand for |
| operands of the instruction. |
| |
| <p>If the vector has only one element, that element is the template for the |
| instruction pattern. If the vector has multiple elements, then the |
| instruction pattern is a <code>parallel</code> expression containing the |
| elements described. |
| </p> |
| </li><li> <a name="index-pattern-conditions"></a> |
| <a name="index-conditions_002c-in-patterns"></a> |
| A condition. This is a string which contains a C expression that is |
| the final test to decide whether an insn body matches this pattern. |
| |
| <a name="index-named-patterns-and-conditions"></a> |
| <p>For a named pattern, the condition (if present) may not depend on |
| the data in the insn being matched, but only the target-machine-type |
| flags. The compiler needs to test these conditions during |
| initialization in order to learn exactly which named instructions are |
| available in a particular run. |
| </p> |
| <a name="index-operands-1"></a> |
| <p>For nameless patterns, the condition is applied only when matching an |
| individual insn, and only after the insn has matched the pattern’s |
| recognition template. The insn’s operands may be found in the vector |
| <code>operands</code>. For an insn where the condition has once matched, it |
| can’t be used to control register allocation, for example by excluding |
| certain hard registers or hard register combinations. |
| </p> |
| </li><li> The <em>output template</em>: a string that says how to output matching |
| insns as assembler code. ‘<samp>%</samp>’ in this string specifies where |
| to substitute the value of an operand. See <a href="Output-Template.html#Output-Template">Output Template</a>. |
| |
| <p>When simple substitution isn’t general enough, you can specify a piece |
| of C code to compute the output. See <a href="Output-Statement.html#Output-Statement">Output Statement</a>. |
| </p> |
| </li><li> Optionally, a vector containing the values of attributes for insns matching |
| this pattern. See <a href="Insn-Attributes.html#Insn-Attributes">Insn Attributes</a>. |
| </li></ol> |
| |
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