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<title>GNU Compiler Collection (GCC) Internals: Defining Attributes</title>
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<a name="Defining-Attributes"></a>
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<div class="header">
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<p>
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Next: <a href="Expressions.html#Expressions" accesskey="n" rel="next">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="Defining-Attributes-and-their-Values"></a>
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<h4 class="subsection">16.19.1 Defining Attributes and their Values</h4>
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<a name="index-defining-attributes-and-their-values"></a>
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<a name="index-attributes_002c-defining"></a>
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<a name="index-define_005fattr"></a>
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<p>The <code>define_attr</code> expression is used to define each attribute required
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by the target machine. It looks like:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(define_attr <var>name</var> <var>list-of-values</var> <var>default</var>)
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</pre></div>
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<p><var>name</var> is a string specifying the name of the attribute being
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defined. Some attributes are used in a special way by the rest of the
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compiler. The <code>enabled</code> attribute can be used to conditionally
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enable or disable insn alternatives (see <a href="Disable-Insn-Alternatives.html#Disable-Insn-Alternatives">Disable Insn Alternatives</a>). The <code>predicable</code> attribute, together with a
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suitable <code>define_cond_exec</code> (see <a href="Conditional-Execution.html#Conditional-Execution">Conditional Execution</a>), can
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be used to automatically generate conditional variants of instruction
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patterns. The <code>mnemonic</code> attribute can be used to check for the
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instruction mnemonic (see <a href="Mnemonic-Attribute.html#Mnemonic-Attribute">Mnemonic Attribute</a>). The compiler
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internally uses the names <code>ce_enabled</code> and <code>nonce_enabled</code>,
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so they should not be used elsewhere as alternative names.
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</p>
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<p><var>list-of-values</var> is either a string that specifies a comma-separated
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list of values that can be assigned to the attribute, or a null string to
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indicate that the attribute takes numeric values.
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</p>
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<p><var>default</var> is an attribute expression that gives the value of this
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attribute for insns that match patterns whose definition does not include
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an explicit value for this attribute. See <a href="Attr-Example.html#Attr-Example">Attr Example</a>, for more
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information on the handling of defaults. See <a href="Constant-Attributes.html#Constant-Attributes">Constant Attributes</a>,
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for information on attributes that do not depend on any particular insn.
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</p>
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<a name="index-insn_002dattr_002eh"></a>
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<p>For each defined attribute, a number of definitions are written to the
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<samp>insn-attr.h</samp> file. For cases where an explicit set of values is
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specified for an attribute, the following are defined:
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</p>
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<ul>
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<li> A ‘<samp>#define</samp>’ is written for the symbol ‘<samp>HAVE_ATTR_<var>name</var></samp>’.
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</li><li> An enumerated class is defined for ‘<samp>attr_<var>name</var></samp>’ with
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elements of the form ‘<samp><var>upper-name</var>_<var>upper-value</var></samp>’ where
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the attribute name and value are first converted to uppercase.
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</li><li> A function ‘<samp>get_attr_<var>name</var></samp>’ is defined that is passed an insn and
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returns the attribute value for that insn.
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</li></ul>
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<p>For example, if the following is present in the <samp>md</samp> file:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(define_attr "type" "branch,fp,load,store,arith" …)
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</pre></div>
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<p>the following lines will be written to the file <samp>insn-attr.h</samp>.
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define HAVE_ATTR_type 1
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enum attr_type {TYPE_BRANCH, TYPE_FP, TYPE_LOAD,
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TYPE_STORE, TYPE_ARITH};
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extern enum attr_type get_attr_type ();
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</pre></div>
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<p>If the attribute takes numeric values, no <code>enum</code> type will be
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defined and the function to obtain the attribute’s value will return
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<code>int</code>.
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</p>
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<p>There are attributes which are tied to a specific meaning. These
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attributes are not free to use for other purposes:
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</p>
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<dl compact="compact">
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<dt><code>length</code></dt>
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<dd><p>The <code>length</code> attribute is used to calculate the length of emitted
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code chunks. This is especially important when verifying branch
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distances. See <a href="Insn-Lengths.html#Insn-Lengths">Insn Lengths</a>.
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</p>
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</dd>
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<dt><code>enabled</code></dt>
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<dd><p>The <code>enabled</code> attribute can be defined to prevent certain
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alternatives of an insn definition from being used during code
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generation. See <a href="Disable-Insn-Alternatives.html#Disable-Insn-Alternatives">Disable Insn Alternatives</a>.
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</p>
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</dd>
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<dt><code>mnemonic</code></dt>
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<dd><p>The <code>mnemonic</code> attribute can be defined to implement instruction
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specific checks in e.g. the pipeline description.
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See <a href="Mnemonic-Attribute.html#Mnemonic-Attribute">Mnemonic Attribute</a>.
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</p></dd>
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</dl>
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<p>For each of these special attributes, the corresponding
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‘<samp>HAVE_ATTR_<var>name</var></samp>’ ‘<samp>#define</samp>’ is also written when the
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attribute is not defined; in that case, it is defined as ‘<samp>0</samp>’.
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</p>
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<a name="index-define_005fenum_005fattr"></a>
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<a name="define_005fenum_005fattr"></a><p>Another way of defining an attribute is to use:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(define_enum_attr "<var>attr</var>" "<var>enum</var>" <var>default</var>)
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</pre></div>
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<p>This works in just the same way as <code>define_attr</code>, except that
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the list of values is taken from a separate enumeration called
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<var>enum</var> (see <a href="Constant-Definitions.html#define_005fenum">define_enum</a>). This form allows you to use
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the same list of values for several attributes without having to
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repeat the list each time. For example:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(define_enum "processor" [
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model_a
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model_b
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…
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])
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(define_enum_attr "arch" "processor"
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(const (symbol_ref "target_arch")))
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(define_enum_attr "tune" "processor"
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(const (symbol_ref "target_tune")))
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</pre></div>
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<p>defines the same attributes as:
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</p>
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<div class="smallexample">
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<pre class="smallexample">(define_attr "arch" "model_a,model_b,…"
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(const (symbol_ref "target_arch")))
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(define_attr "tune" "model_a,model_b,…"
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(const (symbol_ref "target_tune")))
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</pre></div>
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<p>but without duplicating the processor list. The second example defines two
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separate C enums (<code>attr_arch</code> and <code>attr_tune</code>) whereas the first
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defines a single C enum (<code>processor</code>).
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</p><hr>
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<div class="header">
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<p>
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Next: <a href="Expressions.html#Expressions" accesskey="n" rel="next">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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