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<title>Bit-Fields - GNU Compiler Collection (GCC) Internals</title>
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<a name="Bit-Fields"></a>
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Next: <a rel="next" accesskey="n" href="Vector-Operations.html#Vector-Operations">Vector Operations</a>,
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Previous: <a rel="previous" accesskey="p" href="Comparisons.html#Comparisons">Comparisons</a>,
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<h3 class="section">13.11 Bit-Fields</h3>
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<p><a name="index-bit_002dfields-3001"></a>
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Special expression codes exist to represent bit-field instructions.
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<a name="index-sign_005fextract-3002"></a>
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<a name="index-g_t_0040code_007bBITS_005fBIG_005fENDIAN_007d_002c-effect-on-_0040code_007bsign_005fextract_007d-3003"></a>
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<dl><dt><code>(sign_extract:</code><var>m</var> <var>loc</var> <var>size</var> <var>pos</var><code>)</code><dd>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>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>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>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>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><a name="index-zero_005fextract-3004"></a><br><dt><code>(zero_extract:</code><var>m</var> <var>loc</var> <var>size</var> <var>pos</var><code>)</code><dd>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>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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</dl>
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