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| <a name="Incdec"></a> |
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| <a name="Embedded-Side_002dEffects-on-Addresses"></a> |
| <h3 class="section">13.16 Embedded Side-Effects on Addresses</h3> |
| <a name="index-RTL-preincrement"></a> |
| <a name="index-RTL-postincrement"></a> |
| <a name="index-RTL-predecrement"></a> |
| <a name="index-RTL-postdecrement"></a> |
| |
| <p>Six special side-effect expression codes appear as memory addresses. |
| </p> |
| <dl compact="compact"> |
| <dd><a name="index-pre_005fdec"></a> |
| </dd> |
| <dt><code>(pre_dec:<var>m</var> <var>x</var>)</code></dt> |
| <dd><p>Represents the side effect of decrementing <var>x</var> by a standard |
| amount and represents also the value that <var>x</var> has after being |
| decremented. <var>x</var> must be a <code>reg</code> or <code>mem</code>, but most |
| machines allow only a <code>reg</code>. <var>m</var> must be the machine mode |
| for pointers on the machine in use. The amount <var>x</var> is decremented |
| by is the length in bytes of the machine mode of the containing memory |
| reference of which this expression serves as the address. Here is an |
| example of its use: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(mem:DF (pre_dec:SI (reg:SI 39))) |
| </pre></div> |
| |
| <p>This says to decrement pseudo register 39 by the length of a <code>DFmode</code> |
| value and use the result to address a <code>DFmode</code> value. |
| </p> |
| <a name="index-pre_005finc"></a> |
| </dd> |
| <dt><code>(pre_inc:<var>m</var> <var>x</var>)</code></dt> |
| <dd><p>Similar, but specifies incrementing <var>x</var> instead of decrementing it. |
| </p> |
| <a name="index-post_005fdec"></a> |
| </dd> |
| <dt><code>(post_dec:<var>m</var> <var>x</var>)</code></dt> |
| <dd><p>Represents the same side effect as <code>pre_dec</code> but a different |
| value. The value represented here is the value <var>x</var> has <i>before</i> |
| being decremented. |
| </p> |
| <a name="index-post_005finc"></a> |
| </dd> |
| <dt><code>(post_inc:<var>m</var> <var>x</var>)</code></dt> |
| <dd><p>Similar, but specifies incrementing <var>x</var> instead of decrementing it. |
| </p> |
| <a name="index-post_005fmodify"></a> |
| </dd> |
| <dt><code>(post_modify:<var>m</var> <var>x</var> <var>y</var>)</code></dt> |
| <dd> |
| <p>Represents the side effect of setting <var>x</var> to <var>y</var> and |
| represents <var>x</var> before <var>x</var> is modified. <var>x</var> must be a |
| <code>reg</code> or <code>mem</code>, but most machines allow only a <code>reg</code>. |
| <var>m</var> must be the machine mode for pointers on the machine in use. |
| </p> |
| <p>The expression <var>y</var> must be one of three forms: |
| <code>(plus:<var>m</var> <var>x</var> <var>z</var>)</code>, |
| <code>(minus:<var>m</var> <var>x</var> <var>z</var>)</code>, or |
| <code>(plus:<var>m</var> <var>x</var> <var>i</var>)</code>, |
| where <var>z</var> is an index register and <var>i</var> is a constant. |
| </p> |
| <p>Here is an example of its use: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">(mem:SF (post_modify:SI (reg:SI 42) (plus (reg:SI 42) |
| (reg:SI 48)))) |
| </pre></div> |
| |
| <p>This says to modify pseudo register 42 by adding the contents of pseudo |
| register 48 to it, after the use of what ever 42 points to. |
| </p> |
| <a name="index-pre_005fmodify"></a> |
| </dd> |
| <dt><code>(pre_modify:<var>m</var> <var>x</var> <var>expr</var>)</code></dt> |
| <dd><p>Similar except side effects happen before the use. |
| </p></dd> |
| </dl> |
| |
| <p>These embedded side effect expressions must be used with care. Instruction |
| patterns may not use them. Until the ‘<samp>flow</samp>’ pass of the compiler, |
| they may occur only to represent pushes onto the stack. The ‘<samp>flow</samp>’ |
| pass finds cases where registers are incremented or decremented in one |
| instruction and used as an address shortly before or after; these cases are |
| then transformed to use pre- or post-increment or -decrement. |
| </p> |
| <p>If a register used as the operand of these expressions is used in |
| another address in an insn, the original value of the register is used. |
| Uses of the register outside of an address are not permitted within the |
| same insn as a use in an embedded side effect expression because such |
| insns behave differently on different machines and hence must be treated |
| as ambiguous and disallowed. |
| </p> |
| <p>An instruction that can be represented with an embedded side effect |
| could also be represented using <code>parallel</code> containing an additional |
| <code>set</code> to describe how the address register is altered. This is not |
| done because machines that allow these operations at all typically |
| allow them wherever a memory address is called for. Describing them as |
| additional parallel stores would require doubling the number of entries |
| in the machine description. |
| </p> |
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