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<title>GNU Compiler Collection (GCC) Internals: Expressions</title>

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<a name="Expressions"></a>
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<p>
Next: <a href="Tagging-Insns.html#Tagging-Insns" accesskey="n" rel="next">Tagging Insns</a>, Previous: <a href="Defining-Attributes.html#Defining-Attributes" accesskey="p" rel="prev">Defining Attributes</a>, Up: <a href="Insn-Attributes.html#Insn-Attributes" accesskey="u" rel="up">Insn Attributes</a> &nbsp; [<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>
</div>
<hr>
<a name="Attribute-Expressions"></a>
<h4 class="subsection">16.19.2 Attribute Expressions</h4>
<a name="index-attribute-expressions"></a>

<p>RTL expressions used to define attributes use the codes described above
plus a few specific to attribute definitions, to be discussed below.
Attribute value expressions must have one of the following forms:
</p>
<dl compact="compact">
<dd><a name="index-const_005fint-and-attributes"></a>
</dd>
<dt><code>(const_int <var>i</var>)</code></dt>
<dd><p>The integer <var>i</var> specifies the value of a numeric attribute.  <var>i</var>
must be non-negative.
</p>
<p>The value of a numeric attribute can be specified either with a
<code>const_int</code>, or as an integer represented as a string in
<code>const_string</code>, <code>eq_attr</code> (see below), <code>attr</code>,
<code>symbol_ref</code>, simple arithmetic expressions, and <code>set_attr</code>
overrides on specific instructions (see <a href="Tagging-Insns.html#Tagging-Insns">Tagging Insns</a>).
</p>
<a name="index-const_005fstring-and-attributes"></a>
</dd>
<dt><code>(const_string <var>value</var>)</code></dt>
<dd><p>The string <var>value</var> specifies a constant attribute value.
If <var>value</var> is specified as &lsquo;<samp>&quot;*&quot;</samp>&rsquo;, it means that the default value of
the attribute is to be used for the insn containing this expression.
&lsquo;<samp>&quot;*&quot;</samp>&rsquo; obviously cannot be used in the <var>default</var> expression
of a <code>define_attr</code>.
</p>
<p>If the attribute whose value is being specified is numeric, <var>value</var>
must be a string containing a non-negative integer (normally
<code>const_int</code> would be used in this case).  Otherwise, it must
contain one of the valid values for the attribute.
</p>
<a name="index-if_005fthen_005felse-and-attributes"></a>
</dd>
<dt><code>(if_then_else <var>test</var> <var>true-value</var> <var>false-value</var>)</code></dt>
<dd><p><var>test</var> specifies an attribute test, whose format is defined below.
The value of this expression is <var>true-value</var> if <var>test</var> is true,
otherwise it is <var>false-value</var>.
</p>
<a name="index-cond-and-attributes"></a>
</dd>
<dt><code>(cond [<var>test1</var> <var>value1</var> &hellip;] <var>default</var>)</code></dt>
<dd><p>The first operand of this expression is a vector containing an even
number of expressions and consisting of pairs of <var>test</var> and <var>value</var>
expressions.  The value of the <code>cond</code> expression is that of the
<var>value</var> corresponding to the first true <var>test</var> expression.  If
none of the <var>test</var> expressions are true, the value of the <code>cond</code>
expression is that of the <var>default</var> expression.
</p></dd>
</dl>

<p><var>test</var> expressions can have one of the following forms:
</p>
<dl compact="compact">
<dd><a name="index-const_005fint-and-attribute-tests"></a>
</dd>
<dt><code>(const_int <var>i</var>)</code></dt>
<dd><p>This test is true if <var>i</var> is nonzero and false otherwise.
</p>
<a name="index-not-and-attributes"></a>
<a name="index-ior-and-attributes"></a>
<a name="index-and-and-attributes"></a>
</dd>
<dt><code>(not <var>test</var>)</code></dt>
<dt><code>(ior <var>test1</var> <var>test2</var>)</code></dt>
<dt><code>(and <var>test1</var> <var>test2</var>)</code></dt>
<dd><p>These tests are true if the indicated logical function is true.
</p>
<a name="index-match_005foperand-and-attributes"></a>
</dd>
<dt><code>(match_operand:<var>m</var> <var>n</var> <var>pred</var> <var>constraints</var>)</code></dt>
<dd><p>This test is true if operand <var>n</var> of the insn whose attribute value
is being determined has mode <var>m</var> (this part of the test is ignored
if <var>m</var> is <code>VOIDmode</code>) and the function specified by the string
<var>pred</var> returns a nonzero value when passed operand <var>n</var> and mode
<var>m</var> (this part of the test is ignored if <var>pred</var> is the null
string).
</p>
<p>The <var>constraints</var> operand is ignored and should be the null string.
</p>
<a name="index-match_005ftest-and-attributes"></a>
</dd>
<dt><code>(match_test <var>c-expr</var>)</code></dt>
<dd><p>The test is true if C expression <var>c-expr</var> is true.  In non-constant
attributes, <var>c-expr</var> has access to the following variables:
</p>
<dl compact="compact">
<dt><var>insn</var></dt>
<dd><p>The rtl instruction under test.
</p></dd>
<dt><var>which_alternative</var></dt>
<dd><p>The <code>define_insn</code> alternative that <var>insn</var> matches.
See <a href="Output-Statement.html#Output-Statement">Output Statement</a>.
</p></dd>
<dt><var>operands</var></dt>
<dd><p>An array of <var>insn</var>&rsquo;s rtl operands.
</p></dd>
</dl>

<p><var>c-expr</var> behaves like the condition in a C <code>if</code> statement,
so there is no need to explicitly convert the expression into a boolean
0 or 1 value.  For example, the following two tests are equivalent:
</p>
<div class="smallexample">
<pre class="smallexample">(match_test &quot;x &amp; 2&quot;)
(match_test &quot;(x &amp; 2) != 0&quot;)
</pre></div>

<a name="index-le-and-attributes"></a>
<a name="index-leu-and-attributes"></a>
<a name="index-lt-and-attributes"></a>
<a name="index-gt-and-attributes"></a>
<a name="index-gtu-and-attributes"></a>
<a name="index-ge-and-attributes"></a>
<a name="index-geu-and-attributes"></a>
<a name="index-ne-and-attributes"></a>
<a name="index-eq-and-attributes"></a>
<a name="index-plus-and-attributes"></a>
<a name="index-minus-and-attributes"></a>
<a name="index-mult-and-attributes"></a>
<a name="index-div-and-attributes"></a>
<a name="index-mod-and-attributes"></a>
<a name="index-abs-and-attributes"></a>
<a name="index-neg-and-attributes"></a>
<a name="index-ashift-and-attributes"></a>
<a name="index-lshiftrt-and-attributes"></a>
<a name="index-ashiftrt-and-attributes"></a>
</dd>
<dt><code>(le <var>arith1</var> <var>arith2</var>)</code></dt>
<dt><code>(leu <var>arith1</var> <var>arith2</var>)</code></dt>
<dt><code>(lt <var>arith1</var> <var>arith2</var>)</code></dt>
<dt><code>(ltu <var>arith1</var> <var>arith2</var>)</code></dt>
<dt><code>(gt <var>arith1</var> <var>arith2</var>)</code></dt>
<dt><code>(gtu <var>arith1</var> <var>arith2</var>)</code></dt>
<dt><code>(ge <var>arith1</var> <var>arith2</var>)</code></dt>
<dt><code>(geu <var>arith1</var> <var>arith2</var>)</code></dt>
<dt><code>(ne <var>arith1</var> <var>arith2</var>)</code></dt>
<dt><code>(eq <var>arith1</var> <var>arith2</var>)</code></dt>
<dd><p>These tests are true if the indicated comparison of the two arithmetic
expressions is true.  Arithmetic expressions are formed with
<code>plus</code>, <code>minus</code>, <code>mult</code>, <code>div</code>, <code>mod</code>,
<code>abs</code>, <code>neg</code>, <code>and</code>, <code>ior</code>, <code>xor</code>, <code>not</code>,
<code>ashift</code>, <code>lshiftrt</code>, and <code>ashiftrt</code> expressions.
</p>
<a name="index-get_005fattr"></a>
<p><code>const_int</code> and <code>symbol_ref</code> are always valid terms (see <a href="Insn-Lengths.html#Insn-Lengths">Insn Lengths</a>,for additional forms).  <code>symbol_ref</code> is a string
denoting a C expression that yields an <code>int</code> when evaluated by the
&lsquo;<samp>get_attr_&hellip;</samp>&rsquo; routine.  It should normally be a global
variable.
</p>
<a name="index-eq_005fattr"></a>
</dd>
<dt><code>(eq_attr <var>name</var> <var>value</var>)</code></dt>
<dd><p><var>name</var> is a string specifying the name of an attribute.
</p>
<p><var>value</var> is a string that is either a valid value for attribute
<var>name</var>, a comma-separated list of values, or &lsquo;<samp>!</samp>&rsquo; followed by a
value or list.  If <var>value</var> does not begin with a &lsquo;<samp>!</samp>&rsquo;, this
test is true if the value of the <var>name</var> attribute of the current
insn is in the list specified by <var>value</var>.  If <var>value</var> begins
with a &lsquo;<samp>!</samp>&rsquo;, this test is true if the attribute&rsquo;s value is
<em>not</em> in the specified list.
</p>
<p>For example,
</p>
<div class="smallexample">
<pre class="smallexample">(eq_attr &quot;type&quot; &quot;load,store&quot;)
</pre></div>

<p>is equivalent to
</p>
<div class="smallexample">
<pre class="smallexample">(ior (eq_attr &quot;type&quot; &quot;load&quot;) (eq_attr &quot;type&quot; &quot;store&quot;))
</pre></div>

<p>If <var>name</var> specifies an attribute of &lsquo;<samp>alternative</samp>&rsquo;, it refers to the
value of the compiler variable <code>which_alternative</code>
(see <a href="Output-Statement.html#Output-Statement">Output Statement</a>) and the values must be small integers.  For
example,
</p>
<div class="smallexample">
<pre class="smallexample">(eq_attr &quot;alternative&quot; &quot;2,3&quot;)
</pre></div>

<p>is equivalent to
</p>
<div class="smallexample">
<pre class="smallexample">(ior (eq (symbol_ref &quot;which_alternative&quot;) (const_int 2))
     (eq (symbol_ref &quot;which_alternative&quot;) (const_int 3)))
</pre></div>

<p>Note that, for most attributes, an <code>eq_attr</code> test is simplified in cases
where the value of the attribute being tested is known for all insns matching
a particular pattern.  This is by far the most common case.
</p>
<a name="index-attr_005fflag"></a>
</dd>
<dt><code>(attr_flag <var>name</var>)</code></dt>
<dd><p>The value of an <code>attr_flag</code> expression is true if the flag
specified by <var>name</var> is true for the <code>insn</code> currently being
scheduled.
</p>
<p><var>name</var> is a string specifying one of a fixed set of flags to test.
Test the flags <code>forward</code> and <code>backward</code> to determine the
direction of a conditional branch.
</p>
<p>This example describes a conditional branch delay slot which
can be nullified for forward branches that are taken (annul-true) or
for backward branches which are not taken (annul-false).
</p>
<div class="smallexample">
<pre class="smallexample">(define_delay (eq_attr &quot;type&quot; &quot;cbranch&quot;)
  [(eq_attr &quot;in_branch_delay&quot; &quot;true&quot;)
   (and (eq_attr &quot;in_branch_delay&quot; &quot;true&quot;)
        (attr_flag &quot;forward&quot;))
   (and (eq_attr &quot;in_branch_delay&quot; &quot;true&quot;)
        (attr_flag &quot;backward&quot;))])
</pre></div>

<p>The <code>forward</code> and <code>backward</code> flags are false if the current
<code>insn</code> being scheduled is not a conditional branch.
</p>
<p><code>attr_flag</code> is only used during delay slot scheduling and has no
meaning to other passes of the compiler.
</p>
<a name="index-attr"></a>
</dd>
<dt><code>(attr <var>name</var>)</code></dt>
<dd><p>The value of another attribute is returned.  This is most useful
for numeric attributes, as <code>eq_attr</code> and <code>attr_flag</code>
produce more efficient code for non-numeric attributes.
</p></dd>
</dl>

<hr>
<div class="header">
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Next: <a href="Tagging-Insns.html#Tagging-Insns" accesskey="n" rel="next">Tagging Insns</a>, Previous: <a href="Defining-Attributes.html#Defining-Attributes" accesskey="p" rel="prev">Defining Attributes</a>, Up: <a href="Insn-Attributes.html#Insn-Attributes" accesskey="u" rel="up">Insn Attributes</a> &nbsp; [<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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