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| <a name="Bit_002dFields-1"></a> |
| <h3 class="section">13.11 Bit-Fields</h3> |
| <a name="index-bit_002dfields"></a> |
| |
| <p>Special expression codes exist to represent bit-field instructions. |
| </p> |
| <dl compact="compact"> |
| <dd><a name="index-sign_005fextract"></a> |
| <a name="index-BITS_005fBIG_005fENDIAN_002c-effect-on-sign_005fextract"></a> |
| </dd> |
| <dt><code>(sign_extract:<var>m</var> <var>loc</var> <var>size</var> <var>pos</var>)</code></dt> |
| <dd><p>This represents a reference to a sign-extended bit-field contained or |
| starting in <var>loc</var> (a memory or register reference). The bit-field |
| is <var>size</var> bits wide and starts at bit <var>pos</var>. The compilation |
| option <code>BITS_BIG_ENDIAN</code> says which end of the memory unit |
| <var>pos</var> counts from. |
| </p> |
| <p>If <var>loc</var> is in memory, its mode must be a single-byte integer mode. |
| If <var>loc</var> is in a register, the mode to use is specified by the |
| operand of the <code>insv</code> or <code>extv</code> pattern |
| (see <a href="Standard-Names.html#Standard-Names">Standard Names</a>) and is usually a full-word integer mode, |
| which is the default if none is specified. |
| </p> |
| <p>The mode of <var>pos</var> is machine-specific and is also specified |
| in the <code>insv</code> or <code>extv</code> pattern. |
| </p> |
| <p>The mode <var>m</var> is the same as the mode that would be used for |
| <var>loc</var> if it were a register. |
| </p> |
| <p>A <code>sign_extract</code> can not appear as an lvalue, or part thereof, |
| in RTL. |
| </p> |
| <a name="index-zero_005fextract"></a> |
| </dd> |
| <dt><code>(zero_extract:<var>m</var> <var>loc</var> <var>size</var> <var>pos</var>)</code></dt> |
| <dd><p>Like <code>sign_extract</code> but refers to an unsigned or zero-extended |
| bit-field. The same sequence of bits are extracted, but they |
| are filled to an entire word with zeros instead of by sign-extension. |
| </p> |
| <p>Unlike <code>sign_extract</code>, this type of expressions can be lvalues |
| in RTL; they may appear on the left side of an assignment, indicating |
| insertion of a value into the specified bit-field. |
| </p></dd> |
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