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