127 lines
5.4 KiB
HTML
127 lines
5.4 KiB
HTML
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<head>
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<title>GNU Compiler Collection (GCC) Internals: Bit-Fields</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals: Bit-Fields">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="RTL.html#RTL" rel="up" title="RTL">
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Bit_002dFields"></a>
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<div class="header">
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<p>
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Next: <a href="Vector-Operations.html#Vector-Operations" accesskey="n" rel="next">Vector Operations</a>, Previous: <a href="Comparisons.html#Comparisons" accesskey="p" rel="prev">Comparisons</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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</div>
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<hr>
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<a name="Bit_002dFields-1"></a>
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<h3 class="section">13.11 Bit-Fields</h3>
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<a name="index-bit_002dfields"></a>
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<p>Special expression codes exist to represent bit-field instructions.
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</p>
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<dl compact="compact">
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<dd><a name="index-sign_005fextract"></a>
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<a name="index-BITS_005fBIG_005fENDIAN_002c-effect-on-sign_005fextract"></a>
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</dd>
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<dt><code>(sign_extract:<var>m</var> <var>loc</var> <var>size</var> <var>pos</var>)</code></dt>
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<dd><p>This represents a reference to a sign-extended bit-field contained or
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starting in <var>loc</var> (a memory or register reference). The bit-field
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is <var>size</var> bits wide and starts at bit <var>pos</var>. The compilation
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option <code>BITS_BIG_ENDIAN</code> says which end of the memory unit
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<var>pos</var> counts from.
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</p>
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<p>If <var>loc</var> is in memory, its mode must be a single-byte integer mode.
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If <var>loc</var> is in a register, the mode to use is specified by the
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operand of the <code>insv</code> or <code>extv</code> pattern
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(see <a href="Standard-Names.html#Standard-Names">Standard Names</a>) and is usually a full-word integer mode,
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which is the default if none is specified.
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</p>
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<p>The mode of <var>pos</var> is machine-specific and is also specified
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in the <code>insv</code> or <code>extv</code> pattern.
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</p>
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<p>The mode <var>m</var> is the same as the mode that would be used for
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<var>loc</var> if it were a register.
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</p>
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<p>A <code>sign_extract</code> can not appear as an lvalue, or part thereof,
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in RTL.
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</p>
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<a name="index-zero_005fextract"></a>
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</dd>
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<dt><code>(zero_extract:<var>m</var> <var>loc</var> <var>size</var> <var>pos</var>)</code></dt>
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<dd><p>Like <code>sign_extract</code> but refers to an unsigned or zero-extended
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bit-field. The same sequence of bits are extracted, but they
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are filled to an entire word with zeros instead of by sign-extension.
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</p>
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<p>Unlike <code>sign_extract</code>, this type of expressions can be lvalues
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in RTL; they may appear on the left side of an assignment, indicating
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insertion of a value into the specified bit-field.
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</p></dd>
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</dl>
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</body>
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</html>
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