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 <a name="Insn-Lengths"></a>
 <div class="header">
 <p>
 Next: <a href="Constant-Attributes.html#Constant-Attributes" accesskey="n" rel="next">Constant Attributes</a>, Previous: <a href="Attr-Example.html#Attr-Example" accesskey="p" rel="prev">Attr Example</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="Computing-the-Length-of-an-Insn"></a>
 <h4 class="subsection">16.19.5 Computing the Length of an Insn</h4>
 <a name="index-insn-lengths_002c-computing"></a>
 <a name="index-computing-the-length-of-an-insn"></a>

 <p>For many machines, multiple types of branch instructions are provided, each
 for different length branch displacements.  In most cases, the assembler
 will choose the correct instruction to use.  However, when the assembler
 cannot do so, GCC can when a special attribute, the <code>length</code>
 attribute, is defined.  This attribute must be defined to have numeric
 values by specifying a null string in its <code>define_attr</code>.
 </p>
 <p>In the case of the <code>length</code> attribute, two additional forms of
 arithmetic terms are allowed in test expressions:
 </p>
 <dl compact="compact">
 <dd><a name="index-match_005fdup-and-attributes"></a>
 </dd>
 <dt><code>(match_dup <var>n</var>)</code></dt>
 <dd><p>This refers to the address of operand <var>n</var> of the current insn, which
 must be a <code>label_ref</code>.
 </p>
 <a name="index-pc-and-attributes"></a>
 </dd>
 <dt><code>(pc)</code></dt>
 <dd><p>For non-branch instructions and backward branch instructions, this refers
 to the address of the current insn.  But for forward branch instructions,
 this refers to the address of the next insn, because the length of the
 current insn is to be computed.
 </p></dd>
 </dl>

 <a name="index-addr_005fvec_002c-length-of"></a>
 <a name="index-addr_005fdiff_005fvec_002c-length-of"></a>
 <p>For normal insns, the length will be determined by value of the
 <code>length</code> attribute.  In the case of <code>addr_vec</code> and
 <code>addr_diff_vec</code> insn patterns, the length is computed as
 the number of vectors multiplied by the size of each vector.
 </p>
 <p>Lengths are measured in addressable storage units (bytes).
 </p>
 <p>The following macros can be used to refine the length computation:
 </p>
 <dl compact="compact">
 <dd><a name="index-ADJUST_005fINSN_005fLENGTH"></a>
 </dd>
 <dt><code>ADJUST_INSN_LENGTH (<var>insn</var>, <var>length</var>)</code></dt>
 <dd><p>If defined, modifies the length assigned to instruction <var>insn</var> as a
 function of the context in which it is used.  <var>length</var> is an lvalue
 that contains the initially computed length of the insn and should be
 updated with the correct length of the insn.
 </p>
 <p>This macro will normally not be required.  A case in which it is
 required is the ROMP.  On this machine, the size of an <code>addr_vec</code>
 insn must be increased by two to compensate for the fact that alignment
 may be required.
 </p></dd>
 </dl>

 <a name="index-get_005fattr_005flength"></a>
 <p>The routine that returns <code>get_attr_length</code> (the value of the
 <code>length</code> attribute) can be used by the output routine to
 determine the form of the branch instruction to be written, as the
 example below illustrates.
 </p>
 <p>As an example of the specification of variable-length branches, consider
 the IBM 360.  If we adopt the convention that a register will be set to
 the starting address of a function, we can jump to labels within 4k of
 the start using a four-byte instruction.  Otherwise, we need a six-byte
 sequence to load the address from memory and then branch to it.
 </p>
 <p>On such a machine, a pattern for a branch instruction might be specified
 as follows:
 </p>
 <div class="smallexample">
 <pre class="smallexample">(define_insn &quot;jump&quot;
   [(set (pc)
         (label_ref (match_operand 0 &quot;&quot; &quot;&quot;)))]
   &quot;&quot;
 {
    return (get_attr_length (insn) == 4
            ? &quot;b %l0&quot; : &quot;l r15,=a(%l0); br r15&quot;);
 }
   [(set (attr &quot;length&quot;)
         (if_then_else (lt (match_dup 0) (const_int 4096))
                       (const_int 4)
                       (const_int 6)))])
 </pre></div>

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	<a name="Insn-Lengths"></a>
	<div class="header">
	<p>
	Next: <a href="Constant-Attributes.html#Constant-Attributes" accesskey="n" rel="next">Constant Attributes</a>, Previous: <a href="Attr-Example.html#Attr-Example" accesskey="p" rel="prev">Attr Example</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>
	</div>
	<hr>
	<a name="Computing-the-Length-of-an-Insn"></a>
	<h4 class="subsection">16.19.5 Computing the Length of an Insn</h4>
	<a name="index-insn-lengths_002c-computing"></a>
	<a name="index-computing-the-length-of-an-insn"></a>

	<p>For many machines, multiple types of branch instructions are provided, each
	for different length branch displacements. In most cases, the assembler
	will choose the correct instruction to use. However, when the assembler
	cannot do so, GCC can when a special attribute, the <code>length</code>
	attribute, is defined. This attribute must be defined to have numeric
	values by specifying a null string in its <code>define_attr</code>.
	</p>
	<p>In the case of the <code>length</code> attribute, two additional forms of
	arithmetic terms are allowed in test expressions:
	</p>
	<dl compact="compact">
	<dd><a name="index-match_005fdup-and-attributes"></a>
	</dd>
	<dt><code>(match_dup <var>n</var>)</code></dt>
	<dd><p>This refers to the address of operand <var>n</var> of the current insn, which
	must be a <code>label_ref</code>.
	</p>
	<a name="index-pc-and-attributes"></a>
	</dd>
	<dt><code>(pc)</code></dt>
	<dd><p>For non-branch instructions and backward branch instructions, this refers
	to the address of the current insn. But for forward branch instructions,
	this refers to the address of the next insn, because the length of the
	current insn is to be computed.
	</p></dd>
	</dl>

	<a name="index-addr_005fvec_002c-length-of"></a>
	<a name="index-addr_005fdiff_005fvec_002c-length-of"></a>
	<p>For normal insns, the length will be determined by value of the
	<code>length</code> attribute. In the case of <code>addr_vec</code> and
	<code>addr_diff_vec</code> insn patterns, the length is computed as
	the number of vectors multiplied by the size of each vector.
	</p>
	<p>Lengths are measured in addressable storage units (bytes).
	</p>
	<p>The following macros can be used to refine the length computation:
	</p>
	<dl compact="compact">
	<dd><a name="index-ADJUST_005fINSN_005fLENGTH"></a>
	</dd>
	<dt><code>ADJUST_INSN_LENGTH (<var>insn</var>, <var>length</var>)</code></dt>
	<dd><p>If defined, modifies the length assigned to instruction <var>insn</var> as a
	function of the context in which it is used. <var>length</var> is an lvalue
	that contains the initially computed length of the insn and should be
	updated with the correct length of the insn.
	</p>
	<p>This macro will normally not be required. A case in which it is
	required is the ROMP. On this machine, the size of an <code>addr_vec</code>
	insn must be increased by two to compensate for the fact that alignment
	may be required.
	</p></dd>
	</dl>

	<a name="index-get_005fattr_005flength"></a>
	<p>The routine that returns <code>get_attr_length</code> (the value of the
	<code>length</code> attribute) can be used by the output routine to
	determine the form of the branch instruction to be written, as the
	example below illustrates.
	</p>
	<p>As an example of the specification of variable-length branches, consider
	the IBM 360. If we adopt the convention that a register will be set to
	the starting address of a function, we can jump to labels within 4k of
	the start using a four-byte instruction. Otherwise, we need a six-byte
	sequence to load the address from memory and then branch to it.
	</p>
	<p>On such a machine, a pattern for a branch instruction might be specified
	as follows:
	</p>
	<div class="smallexample">
	<pre class="smallexample">(define_insn "jump"
	[(set (pc)
	(label_ref (match_operand 0 "" "")))]
	""
	{
	return (get_attr_length (insn) == 4
	? "b %l0" : "l r15,=a(%l0); br r15");
	}
	[(set (attr "length")
	(if_then_else (lt (match_dup 0) (const_int 4096))
	(const_int 4)
	(const_int 6)))])
	</pre></div>

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