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| <a name="Accessors"></a> |
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| Next: <a href="Special-Accessors.html#Special-Accessors" accesskey="n" rel="next">Special Accessors</a>, Previous: <a href="RTL-Classes.html#RTL-Classes" accesskey="p" rel="prev">RTL Classes</a>, Up: <a href="RTL.html#RTL" accesskey="u" rel="up">RTL</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="Access-to-Operands"></a> |
| <h3 class="section">13.3 Access to Operands</h3> |
| <a name="index-accessors"></a> |
| <a name="index-access-to-operands"></a> |
| <a name="index-operand-access"></a> |
| |
| <a name="index-XEXP"></a> |
| <a name="index-XINT"></a> |
| <a name="index-XWINT"></a> |
| <a name="index-XSTR"></a> |
| <p>Operands of expressions are accessed using the macros <code>XEXP</code>, |
| <code>XINT</code>, <code>XWINT</code> and <code>XSTR</code>. Each of these macros takes |
| two arguments: an expression-pointer (RTX) and an operand number |
| (counting from zero). Thus, |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">XEXP (<var>x</var>, 2) |
| </pre></div> |
| |
| <p>accesses operand 2 of expression <var>x</var>, as an expression. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">XINT (<var>x</var>, 2) |
| </pre></div> |
| |
| <p>accesses the same operand as an integer. <code>XSTR</code>, used in the same |
| fashion, would access it as a string. |
| </p> |
| <p>Any operand can be accessed as an integer, as an expression or as a string. |
| You must choose the correct method of access for the kind of value actually |
| stored in the operand. You would do this based on the expression code of |
| the containing expression. That is also how you would know how many |
| operands there are. |
| </p> |
| <p>For example, if <var>x</var> is a <code>subreg</code> expression, you know that it has |
| two operands which can be correctly accessed as <code>XEXP (<var>x</var>, 0)</code> |
| and <code>XINT (<var>x</var>, 1)</code>. If you did <code>XINT (<var>x</var>, 0)</code>, you |
| would get the address of the expression operand but cast as an integer; |
| that might occasionally be useful, but it would be cleaner to write |
| <code>(int) XEXP (<var>x</var>, 0)</code>. <code>XEXP (<var>x</var>, 1)</code> would also |
| compile without error, and would return the second, integer operand cast as |
| an expression pointer, which would probably result in a crash when |
| accessed. Nothing stops you from writing <code>XEXP (<var>x</var>, 28)</code> either, |
| but this will access memory past the end of the expression with |
| unpredictable results. |
| </p> |
| <p>Access to operands which are vectors is more complicated. You can use the |
| macro <code>XVEC</code> to get the vector-pointer itself, or the macros |
| <code>XVECEXP</code> and <code>XVECLEN</code> to access the elements and length of a |
| vector. |
| </p> |
| <dl compact="compact"> |
| <dd><a name="index-XVEC"></a> |
| </dd> |
| <dt><code>XVEC (<var>exp</var>, <var>idx</var>)</code></dt> |
| <dd><p>Access the vector-pointer which is operand number <var>idx</var> in <var>exp</var>. |
| </p> |
| <a name="index-XVECLEN"></a> |
| </dd> |
| <dt><code>XVECLEN (<var>exp</var>, <var>idx</var>)</code></dt> |
| <dd><p>Access the length (number of elements) in the vector which is |
| in operand number <var>idx</var> in <var>exp</var>. This value is an <code>int</code>. |
| </p> |
| <a name="index-XVECEXP"></a> |
| </dd> |
| <dt><code>XVECEXP (<var>exp</var>, <var>idx</var>, <var>eltnum</var>)</code></dt> |
| <dd><p>Access element number <var>eltnum</var> in the vector which is |
| in operand number <var>idx</var> in <var>exp</var>. This value is an RTX. |
| </p> |
| <p>It is up to you to make sure that <var>eltnum</var> is not negative |
| and is less than <code>XVECLEN (<var>exp</var>, <var>idx</var>)</code>. |
| </p></dd> |
| </dl> |
| |
| <p>All the macros defined in this section expand into lvalues and therefore |
| can be used to assign the operands, lengths and vector elements as well as |
| to access them. |
| </p> |
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