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131 lines
6.8 KiB
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131 lines
6.8 KiB
HTML
4 years ago
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<html lang="en">
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<title>Incdec - GNU Compiler Collection (GCC) Internals</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="RTL.html#RTL" title="RTL">
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<link rel="prev" href="Side-Effects.html#Side-Effects" title="Side Effects">
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<link rel="next" href="Assembler.html#Assembler" title="Assembler">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being ``Funding Free Software'', the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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``GNU Free Documentation License''.
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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funds for GNU development.-->
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</head>
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<body>
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<div class="node">
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<a name="Incdec"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Assembler.html#Assembler">Assembler</a>,
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Previous: <a rel="previous" accesskey="p" href="Side-Effects.html#Side-Effects">Side Effects</a>,
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Up: <a rel="up" accesskey="u" href="RTL.html#RTL">RTL</a>
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<hr>
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</div>
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<h3 class="section">13.16 Embedded Side-Effects on Addresses</h3>
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<p><a name="index-RTL-preincrement-3052"></a><a name="index-RTL-postincrement-3053"></a><a name="index-RTL-predecrement-3054"></a><a name="index-RTL-postdecrement-3055"></a>
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Six special side-effect expression codes appear as memory addresses.
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<a name="index-pre_005fdec-3056"></a>
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<dl><dt><code>(pre_dec:</code><var>m</var> <var>x</var><code>)</code><dd>Represents the side effect of decrementing <var>x</var> by a standard
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amount and represents also the value that <var>x</var> has after being
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decremented. <var>x</var> must be a <code>reg</code> or <code>mem</code>, but most
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machines allow only a <code>reg</code>. <var>m</var> must be the machine mode
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for pointers on the machine in use. The amount <var>x</var> is decremented
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by is the length in bytes of the machine mode of the containing memory
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reference of which this expression serves as the address. Here is an
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example of its use:
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<pre class="smallexample"> (mem:DF (pre_dec:SI (reg:SI 39)))
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</pre>
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<p class="noindent">This says to decrement pseudo register 39 by the length of a <code>DFmode</code>
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value and use the result to address a <code>DFmode</code> value.
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<p><a name="index-pre_005finc-3057"></a><br><dt><code>(pre_inc:</code><var>m</var> <var>x</var><code>)</code><dd>Similar, but specifies incrementing <var>x</var> instead of decrementing it.
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<p><a name="index-post_005fdec-3058"></a><br><dt><code>(post_dec:</code><var>m</var> <var>x</var><code>)</code><dd>Represents the same side effect as <code>pre_dec</code> but a different
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value. The value represented here is the value <var>x</var> has <i>before</i>
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being decremented.
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<p><a name="index-post_005finc-3059"></a><br><dt><code>(post_inc:</code><var>m</var> <var>x</var><code>)</code><dd>Similar, but specifies incrementing <var>x</var> instead of decrementing it.
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<p><a name="index-post_005fmodify-3060"></a><br><dt><code>(post_modify:</code><var>m</var> <var>x</var> <var>y</var><code>)</code><dd>
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Represents the side effect of setting <var>x</var> to <var>y</var> and
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represents <var>x</var> before <var>x</var> is modified. <var>x</var> must be a
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<code>reg</code> or <code>mem</code>, but most machines allow only a <code>reg</code>.
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<var>m</var> must be the machine mode for pointers on the machine in use.
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<p>The expression <var>y</var> must be one of three forms:
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<code>(plus:</code><var>m</var> <var>x</var> <var>z</var><code>)</code>,
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<code>(minus:</code><var>m</var> <var>x</var> <var>z</var><code>)</code>, or
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<code>(plus:</code><var>m</var> <var>x</var> <var>i</var><code>)</code>,
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where <var>z</var> is an index register and <var>i</var> is a constant.
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<p>Here is an example of its use:
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<pre class="smallexample"> (mem:SF (post_modify:SI (reg:SI 42) (plus (reg:SI 42)
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(reg:SI 48))))
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</pre>
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<p>This says to modify pseudo register 42 by adding the contents of pseudo
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register 48 to it, after the use of what ever 42 points to.
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<p><a name="index-pre_005fmodify-3061"></a><br><dt><code>(pre_modify:</code><var>m</var> <var>x</var> <var>expr</var><code>)</code><dd>Similar except side effects happen before the use.
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</dl>
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<p>These embedded side effect expressions must be used with care. Instruction
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patterns may not use them. Until the ‘<samp><span class="samp">flow</span></samp>’ pass of the compiler,
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they may occur only to represent pushes onto the stack. The ‘<samp><span class="samp">flow</span></samp>’
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pass finds cases where registers are incremented or decremented in one
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instruction and used as an address shortly before or after; these cases are
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then transformed to use pre- or post-increment or -decrement.
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<p>If a register used as the operand of these expressions is used in
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another address in an insn, the original value of the register is used.
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Uses of the register outside of an address are not permitted within the
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same insn as a use in an embedded side effect expression because such
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insns behave differently on different machines and hence must be treated
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as ambiguous and disallowed.
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<p>An instruction that can be represented with an embedded side effect
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could also be represented using <code>parallel</code> containing an additional
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<code>set</code> to describe how the address register is altered. This is not
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done because machines that allow these operations at all typically
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allow them wherever a memory address is called for. Describing them as
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additional parallel stores would require doubling the number of entries
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in the machine description.
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</body></html>
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