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