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| <a name="Tagging-Insns"></a> |
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| Next: <a href="Attr-Example.html#Attr-Example" accesskey="n" rel="next">Attr Example</a>, Previous: <a href="Expressions.html#Expressions" accesskey="p" rel="prev">Expressions</a>, Up: <a href="Insn-Attributes.html#Insn-Attributes" accesskey="u" rel="up">Insn Attributes</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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| <a name="Assigning-Attribute-Values-to-Insns"></a> |
| <h4 class="subsection">16.19.3 Assigning Attribute Values to Insns</h4> |
| <a name="index-tagging-insns"></a> |
| <a name="index-assigning-attribute-values-to-insns"></a> |
| |
| <p>The value assigned to an attribute of an insn is primarily determined by |
| which pattern is matched by that insn (or which <code>define_peephole</code> |
| generated it). Every <code>define_insn</code> and <code>define_peephole</code> can |
| have an optional last argument to specify the values of attributes for |
| matching insns. The value of any attribute not specified in a particular |
| insn is set to the default value for that attribute, as specified in its |
| <code>define_attr</code>. Extensive use of default values for attributes |
| permits the specification of the values for only one or two attributes |
| in the definition of most insn patterns, as seen in the example in the |
| next section. |
| </p> |
| <p>The optional last argument of <code>define_insn</code> and |
| <code>define_peephole</code> is a vector of expressions, each of which defines |
| the value for a single attribute. The most general way of assigning an |
| attribute’s value is to use a <code>set</code> expression whose first operand is an |
| <code>attr</code> expression giving the name of the attribute being set. The |
| second operand of the <code>set</code> is an attribute expression |
| (see <a href="Expressions.html#Expressions">Expressions</a>) giving the value of the attribute. |
| </p> |
| <p>When the attribute value depends on the ‘<samp>alternative</samp>’ attribute |
| (i.e., which is the applicable alternative in the constraint of the |
| insn), the <code>set_attr_alternative</code> expression can be used. It |
| allows the specification of a vector of attribute expressions, one for |
| each alternative. |
| </p> |
| <a name="index-set_005fattr"></a> |
| <p>When the generality of arbitrary attribute expressions is not required, |
| the simpler <code>set_attr</code> expression can be used, which allows |
| specifying a string giving either a single attribute value or a list |
| of attribute values, one for each alternative. |
| </p> |
| <p>The form of each of the above specifications is shown below. In each case, |
| <var>name</var> is a string specifying the attribute to be set. |
| </p> |
| <dl compact="compact"> |
| <dt><code>(set_attr <var>name</var> <var>value-string</var>)</code></dt> |
| <dd><p><var>value-string</var> is either a string giving the desired attribute value, |
| or a string containing a comma-separated list giving the values for |
| succeeding alternatives. The number of elements must match the number |
| of alternatives in the constraint of the insn pattern. |
| </p> |
| <p>Note that it may be useful to specify ‘<samp>*</samp>’ for some alternative, in |
| which case the attribute will assume its default value for insns matching |
| that alternative. |
| </p> |
| <a name="index-set_005fattr_005falternative"></a> |
| </dd> |
| <dt><code>(set_attr_alternative <var>name</var> [<var>value1</var> <var>value2</var> …])</code></dt> |
| <dd><p>Depending on the alternative of the insn, the value will be one of the |
| specified values. This is a shorthand for using a <code>cond</code> with |
| tests on the ‘<samp>alternative</samp>’ attribute. |
| </p> |
| <a name="index-attr-1"></a> |
| </dd> |
| <dt><code>(set (attr <var>name</var>) <var>value</var>)</code></dt> |
| <dd><p>The first operand of this <code>set</code> must be the special RTL expression |
| <code>attr</code>, whose sole operand is a string giving the name of the |
| attribute being set. <var>value</var> is the value of the attribute. |
| </p></dd> |
| </dl> |
| |
| <p>The following shows three different ways of representing the same |
| attribute value specification: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(set_attr "type" "load,store,arith") |
| |
| (set_attr_alternative "type" |
| [(const_string "load") (const_string "store") |
| (const_string "arith")]) |
| |
| (set (attr "type") |
| (cond [(eq_attr "alternative" "1") (const_string "load") |
| (eq_attr "alternative" "2") (const_string "store")] |
| (const_string "arith"))) |
| </pre></div> |
| |
| <a name="index-define_005fasm_005fattributes"></a> |
| <p>The <code>define_asm_attributes</code> expression provides a mechanism to |
| specify the attributes assigned to insns produced from an <code>asm</code> |
| statement. It has the form: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(define_asm_attributes [<var>attr-sets</var>]) |
| </pre></div> |
| |
| <p>where <var>attr-sets</var> is specified the same as for both the |
| <code>define_insn</code> and the <code>define_peephole</code> expressions. |
| </p> |
| <p>These values will typically be the “worst case” attribute values. For |
| example, they might indicate that the condition code will be clobbered. |
| </p> |
| <p>A specification for a <code>length</code> attribute is handled specially. The |
| way to compute the length of an <code>asm</code> insn is to multiply the |
| length specified in the expression <code>define_asm_attributes</code> by the |
| number of machine instructions specified in the <code>asm</code> statement, |
| determined by counting the number of semicolons and newlines in the |
| string. Therefore, the value of the <code>length</code> attribute specified |
| in a <code>define_asm_attributes</code> should be the maximum possible length |
| of a single machine instruction. |
| </p> |
| <hr> |
| <div class="header"> |
| <p> |
| Next: <a href="Attr-Example.html#Attr-Example" accesskey="n" rel="next">Attr Example</a>, Previous: <a href="Expressions.html#Expressions" accesskey="p" rel="prev">Expressions</a>, Up: <a href="Insn-Attributes.html#Insn-Attributes" accesskey="u" rel="up">Insn Attributes</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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