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