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Previous: <a rel="previous" accesskey="p" href="Defining-Attributes.html#Defining-Attributes">Defining Attributes</a>,
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<h4 class="subsection">16.19.2 Attribute Expressions</h4>
<p><a name="index-attribute-expressions-3726"></a>
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:
<a name="index-g_t_0040code_007bconst_005fint_007d-and-attributes-3727"></a>
<dl><dt><code>(const_int </code><var>i</var><code>)</code><dd>The integer <var>i</var> specifies the value of a numeric attribute. <var>i</var>
must be non-negative.
<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-g_t_0040code_007bconst_005fstring_007d-and-attributes-3728"></a><br><dt><code>(const_string </code><var>value</var><code>)</code><dd>The string <var>value</var> specifies a constant attribute value.
If <var>value</var> is specified as ‘<samp><span class="samp">"*"</span></samp>’, it means that the default value of
the attribute is to be used for the insn containing this expression.
‘<samp><span class="samp">"*"</span></samp>’ obviously cannot be used in the <var>default</var> expression
of a <code>define_attr</code>.
<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-g_t_0040code_007bif_005fthen_005felse_007d-and-attributes-3729"></a><br><dt><code>(if_then_else </code><var>test</var> <var>true-value</var> <var>false-value</var><code>)</code><dd><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-g_t_0040code_007bcond_007d-and-attributes-3730"></a><br><dt><code>(cond [</code><var>test1</var> <var>value1</var><code> ...] </code><var>default</var><code>)</code><dd>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.
</dl>
<p><var>test</var> expressions can have one of the following forms:
<a name="index-g_t_0040code_007bconst_005fint_007d-and-attribute-tests-3731"></a>
<dl><dt><code>(const_int </code><var>i</var><code>)</code><dd>This test is true if <var>i</var> is nonzero and false otherwise.
<p><a name="index-g_t_0040code_007bnot_007d-and-attributes-3732"></a><a name="index-g_t_0040code_007bior_007d-and-attributes-3733"></a><a name="index-g_t_0040code_007band_007d-and-attributes-3734"></a><br><dt><code>(not </code><var>test</var><code>)</code><dt><code>(ior </code><var>test1</var> <var>test2</var><code>)</code><dt><code>(and </code><var>test1</var> <var>test2</var><code>)</code><dd>These tests are true if the indicated logical function is true.
<p><a name="index-g_t_0040code_007bmatch_005foperand_007d-and-attributes-3735"></a><br><dt><code>(match_operand:</code><var>m</var> <var>n</var> <var>pred</var> <var>constraints</var><code>)</code><dd>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>The <var>constraints</var> operand is ignored and should be the null string.
<p><a name="index-g_t_0040code_007bmatch_005ftest_007d-and-attributes-3736"></a><br><dt><code>(match_test </code><var>c-expr</var><code>)</code><dd>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:
<dl>
<dt><var>insn</var><dd>The rtl instruction under test.
<br><dt><var>which_alternative</var><dd>The <code>define_insn</code> alternative that <var>insn</var> matches.
See <a href="Output-Statement.html#Output-Statement">Output Statement</a>.
<br><dt><var>operands</var><dd>An array of <var>insn</var>'s rtl operands.
</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:
<pre class="smallexample"> (match_test "x & 2")
(match_test "(x & 2) != 0")
</pre>
<p><a name="index-g_t_0040code_007ble_007d-and-attributes-3737"></a><a name="index-g_t_0040code_007bleu_007d-and-attributes-3738"></a><a name="index-g_t_0040code_007blt_007d-and-attributes-3739"></a><a name="index-g_t_0040code_007bgt_007d-and-attributes-3740"></a><a name="index-g_t_0040code_007bgtu_007d-and-attributes-3741"></a><a name="index-g_t_0040code_007bge_007d-and-attributes-3742"></a><a name="index-g_t_0040code_007bgeu_007d-and-attributes-3743"></a><a name="index-g_t_0040code_007bne_007d-and-attributes-3744"></a><a name="index-g_t_0040code_007beq_007d-and-attributes-3745"></a><a name="index-g_t_0040code_007bplus_007d-and-attributes-3746"></a><a name="index-g_t_0040code_007bminus_007d-and-attributes-3747"></a><a name="index-g_t_0040code_007bmult_007d-and-attributes-3748"></a><a name="index-g_t_0040code_007bdiv_007d-and-attributes-3749"></a><a name="index-g_t_0040code_007bmod_007d-and-attributes-3750"></a><a name="index-g_t_0040code_007babs_007d-and-attributes-3751"></a><a name="index-g_t_0040code_007bneg_007d-and-attributes-3752"></a><a name="index-g_t_0040code_007bashift_007d-and-attributes-3753"></a><a name="index-g_t_0040code_007blshiftrt_007d-and-attributes-3754"></a><a name="index-g_t_0040code_007bashiftrt_007d-and-attributes-3755"></a><br><dt><code>(le </code><var>arith1</var> <var>arith2</var><code>)</code><dt><code>(leu </code><var>arith1</var> <var>arith2</var><code>)</code><dt><code>(lt </code><var>arith1</var> <var>arith2</var><code>)</code><dt><code>(ltu </code><var>arith1</var> <var>arith2</var><code>)</code><dt><code>(gt </code><var>arith1</var> <var>arith2</var><code>)</code><dt><code>(gtu </code><var>arith1</var> <var>arith2</var><code>)</code><dt><code>(ge </code><var>arith1</var> <var>arith2</var><code>)</code><dt><code>(geu </code><var>arith1</var> <var>arith2</var><code>)</code><dt><code>(ne </code><var>arith1</var> <var>arith2</var><code>)</code><dt><code>(eq </code><var>arith1</var> <var>arith2</var><code>)</code><dd>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-3756"></a><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
‘<samp><span class="samp">get_attr_...</span></samp>’ routine. It should normally be a global
variable.
<p><a name="index-eq_005fattr-3757"></a><br><dt><code>(eq_attr </code><var>name</var> <var>value</var><code>)</code><dd><var>name</var> is a string specifying the name of an attribute.
<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 ‘<samp><span class="samp">!</span></samp>’ followed by a
value or list. If <var>value</var> does not begin with a ‘<samp><span class="samp">!</span></samp>’, 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 ‘<samp><span class="samp">!</span></samp>’, this test is true if the attribute's value is
<em>not</em> in the specified list.
<p>For example,
<pre class="smallexample"> (eq_attr "type" "load,store")
</pre>
<p class="noindent">is equivalent to
<pre class="smallexample"> (ior (eq_attr "type" "load") (eq_attr "type" "store"))
</pre>
<p>If <var>name</var> specifies an attribute of ‘<samp><span class="samp">alternative</span></samp>’, 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,
<pre class="smallexample"> (eq_attr "alternative" "2,3")
</pre>
<p class="noindent">is equivalent to
<pre class="smallexample"> (ior (eq (symbol_ref "which_alternative") (const_int 2))
(eq (symbol_ref "which_alternative") (const_int 3)))
</pre>
<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-3758"></a><br><dt><code>(attr_flag </code><var>name</var><code>)</code><dd>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><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>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).
<pre class="smallexample"> (define_delay (eq_attr "type" "cbranch")
[(eq_attr "in_branch_delay" "true")
(and (eq_attr "in_branch_delay" "true")
(attr_flag "forward"))
(and (eq_attr "in_branch_delay" "true")
(attr_flag "backward"))])
</pre>
<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><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-3759"></a><br><dt><code>(attr </code><var>name</var><code>)</code><dd>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.
</dl>
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