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134 lines
6.6 KiB
HTML
134 lines
6.6 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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Permission is granted to copy, distribute and/or modify this document
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any later version published by the Free Software Foundation; with the
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<head>
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<title>Condition Code (GNU Compiler Collection (GCC) Internals)</title>
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<meta name="description" content="Condition Code (GNU Compiler Collection (GCC) Internals)">
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<meta name="keywords" content="Condition Code (GNU Compiler Collection (GCC) Internals)">
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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="Target-Macros.html#Target-Macros" rel="up" title="Target Macros">
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<link href="CC0-Condition-Codes.html#CC0-Condition-Codes" rel="next" title="CC0 Condition Codes">
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<link href="Anchored-Addresses.html#Anchored-Addresses" rel="prev" title="Anchored Addresses">
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</head>
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<body lang="en">
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<a name="Condition-Code"></a>
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<div class="header">
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<p>
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Next: <a href="Costs.html#Costs" accesskey="n" rel="next">Costs</a>, Previous: <a href="Anchored-Addresses.html#Anchored-Addresses" accesskey="p" rel="prev">Anchored Addresses</a>, Up: <a href="Target-Macros.html#Target-Macros" accesskey="u" rel="up">Target Macros</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="Condition-Code-Status"></a>
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<h3 class="section">18.15 Condition Code Status</h3>
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<a name="index-condition-code-status"></a>
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<p>The macros in this section can be split in two families, according to the
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two ways of representing condition codes in GCC.
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</p>
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<p>The first representation is the so called <code>(cc0)</code> representation
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(see <a href="Jump-Patterns.html#Jump-Patterns">Jump Patterns</a>), where all instructions can have an implicit
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clobber of the condition codes. The second is the condition code
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register representation, which provides better schedulability for
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architectures that do have a condition code register, but on which
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most instructions do not affect it. The latter category includes
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most RISC machines.
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</p>
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<p>The implicit clobbering poses a strong restriction on the placement of
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the definition and use of the condition code. In the past the definition
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and use were always adjacent. However, recent changes to support trapping
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arithmatic may result in the definition and user being in different blocks.
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Thus, there may be a <code>NOTE_INSN_BASIC_BLOCK</code> between them. Additionally,
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the definition may be the source of exception handling edges.
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</p>
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<p>These restrictions can prevent important
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optimizations on some machines. For example, on the IBM RS/6000, there
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is a delay for taken branches unless the condition code register is set
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three instructions earlier than the conditional branch. The instruction
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scheduler cannot perform this optimization if it is not permitted to
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separate the definition and use of the condition code register.
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</p>
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<p>For this reason, it is possible and suggested to use a register to
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represent the condition code for new ports. If there is a specific
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condition code register in the machine, use a hard register. If the
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condition code or comparison result can be placed in any general register,
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or if there are multiple condition registers, use a pseudo register.
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Registers used to store the condition code value will usually have a mode
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that is in class <code>MODE_CC</code>.
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</p>
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<p>Alternatively, you can use <code>BImode</code> if the comparison operator is
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specified already in the compare instruction. In this case, you are not
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interested in most macros in this section.
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</p>
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<table class="menu" border="0" cellspacing="0">
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<tr><td align="left" valign="top">• <a href="CC0-Condition-Codes.html#CC0-Condition-Codes" accesskey="1">CC0 Condition Codes</a>:</td><td> </td><td align="left" valign="top">Old style representation of condition codes.
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</td></tr>
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<tr><td align="left" valign="top">• <a href="MODE_005fCC-Condition-Codes.html#MODE_005fCC-Condition-Codes" accesskey="2">MODE_CC Condition Codes</a>:</td><td> </td><td align="left" valign="top">Modern representation of condition codes.
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</td></tr>
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</table>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Costs.html#Costs" accesskey="n" rel="next">Costs</a>, Previous: <a href="Anchored-Addresses.html#Anchored-Addresses" accesskey="p" rel="prev">Anchored Addresses</a>, Up: <a href="Target-Macros.html#Target-Macros" accesskey="u" rel="up">Target Macros</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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</body>
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</html>
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