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124 lines
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124 lines
6.2 KiB
HTML
4 years ago
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<html lang="en">
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<title>CC0 Condition Codes - 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="Condition-Code.html#Condition-Code" title="Condition Code">
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<link rel="next" href="MODE_005fCC-Condition-Codes.html#MODE_005fCC-Condition-Codes" title="MODE_CC Condition Codes">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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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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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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</head>
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<body>
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<div class="node">
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<a name="CC0-Condition-Codes"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="MODE_005fCC-Condition-Codes.html#MODE_005fCC-Condition-Codes">MODE_CC Condition Codes</a>,
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Up: <a rel="up" accesskey="u" href="Condition-Code.html#Condition-Code">Condition Code</a>
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<hr>
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</div>
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<h4 class="subsection">17.15.1 Representation of condition codes using <code>(cc0)</code></h4>
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<p><a name="index-cc0-4428"></a>
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<a name="index-cc_005fstatus-4429"></a>The file <samp><span class="file">conditions.h</span></samp> defines a variable <code>cc_status</code> to
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describe how the condition code was computed (in case the interpretation of
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the condition code depends on the instruction that it was set by). This
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variable contains the RTL expressions on which the condition code is
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currently based, and several standard flags.
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<p>Sometimes additional machine-specific flags must be defined in the machine
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description header file. It can also add additional machine-specific
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information by defining <code>CC_STATUS_MDEP</code>.
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<div class="defun">
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— Macro: <b>CC_STATUS_MDEP</b><var><a name="index-CC_005fSTATUS_005fMDEP-4430"></a></var><br>
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<blockquote><p>C code for a data type which is used for declaring the <code>mdep</code>
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component of <code>cc_status</code>. It defaults to <code>int</code>.
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<p>This macro is not used on machines that do not use <code>cc0</code>.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>CC_STATUS_MDEP_INIT</b><var><a name="index-CC_005fSTATUS_005fMDEP_005fINIT-4431"></a></var><br>
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<blockquote><p>A C expression to initialize the <code>mdep</code> field to “empty”.
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The default definition does nothing, since most machines don't use
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the field anyway. If you want to use the field, you should probably
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define this macro to initialize it.
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<p>This macro is not used on machines that do not use <code>cc0</code>.
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</p></blockquote></div>
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<div class="defun">
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— Macro: <b>NOTICE_UPDATE_CC</b> (<var>exp, insn</var>)<var><a name="index-NOTICE_005fUPDATE_005fCC-4432"></a></var><br>
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<blockquote><p>A C compound statement to set the components of <code>cc_status</code>
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appropriately for an insn <var>insn</var> whose body is <var>exp</var>. It is
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this macro's responsibility to recognize insns that set the condition
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code as a byproduct of other activity as well as those that explicitly
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set <code>(cc0)</code>.
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<p>This macro is not used on machines that do not use <code>cc0</code>.
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<p>If there are insns that do not set the condition code but do alter
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other machine registers, this macro must check to see whether they
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invalidate the expressions that the condition code is recorded as
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reflecting. For example, on the 68000, insns that store in address
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registers do not set the condition code, which means that usually
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<code>NOTICE_UPDATE_CC</code> can leave <code>cc_status</code> unaltered for such
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insns. But suppose that the previous insn set the condition code
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based on location ‘<samp><span class="samp">a4@(102)</span></samp>’ and the current insn stores a new
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value in ‘<samp><span class="samp">a4</span></samp>’. Although the condition code is not changed by
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this, it will no longer be true that it reflects the contents of
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‘<samp><span class="samp">a4@(102)</span></samp>’. Therefore, <code>NOTICE_UPDATE_CC</code> must alter
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<code>cc_status</code> in this case to say that nothing is known about the
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condition code value.
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<p>The definition of <code>NOTICE_UPDATE_CC</code> must be prepared to deal
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with the results of peephole optimization: insns whose patterns are
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<code>parallel</code> RTXs containing various <code>reg</code>, <code>mem</code> or
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constants which are just the operands. The RTL structure of these
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insns is not sufficient to indicate what the insns actually do. What
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<code>NOTICE_UPDATE_CC</code> should do when it sees one is just to run
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<code>CC_STATUS_INIT</code>.
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<p>A possible definition of <code>NOTICE_UPDATE_CC</code> is to call a function
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that looks at an attribute (see <a href="Insn-Attributes.html#Insn-Attributes">Insn Attributes</a>) named, for example,
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‘<samp><span class="samp">cc</span></samp>’. This avoids having detailed information about patterns in
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two places, the <samp><span class="file">md</span></samp> file and in <code>NOTICE_UPDATE_CC</code>.
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</p></blockquote></div>
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</body></html>
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