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  25. <title>Condition Code (GNU Compiler Collection (GCC) Internals)</title>

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  35. <link href="Target-Macros.html#Target-Macros" rel="up" title="Target Macros">

  36. <link href="CC0-Condition-Codes.html#CC0-Condition-Codes" rel="next" title="CC0 Condition Codes">

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  70. <a name="Condition-Code"></a>

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  72. <p>

  73. 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> &nbsp; [<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>

  74. </div>

  75. <hr>

  76. <a name="Condition-Code-Status"></a>

  77. <h3 class="section">18.15 Condition Code Status</h3>

  78. <a name="index-condition-code-status"></a>

  79. 

  80. <p>The macros in this section can be split in two families, according to the

  81. two ways of representing condition codes in GCC.

  82. </p>

  83. <p>The first representation is the so called <code>(cc0)</code> representation

  84. (see <a href="Jump-Patterns.html#Jump-Patterns">Jump Patterns</a>), where all instructions can have an implicit

  85. clobber of the condition codes.  The second is the condition code

  86. register representation, which provides better schedulability for

  87. architectures that do have a condition code register, but on which

  88. most instructions do not affect it.  The latter category includes

  89. most RISC machines.

  90. </p>

  91. <p>The implicit clobbering poses a strong restriction on the placement of

  92. the definition and use of the condition code.  In the past the definition

  93. and use were always adjacent.  However, recent changes to support trapping

  94. arithmatic may result in the definition and user being in different blocks.

  95. Thus, there may be a <code>NOTE_INSN_BASIC_BLOCK</code> between them.  Additionally,

  96. the definition may be the source of exception handling edges.

  97. </p>

  98. <p>These restrictions can prevent important

  99. optimizations on some machines.  For example, on the IBM RS/6000, there

  100. is a delay for taken branches unless the condition code register is set

  101. three instructions earlier than the conditional branch.  The instruction

  102. scheduler cannot perform this optimization if it is not permitted to

  103. separate the definition and use of the condition code register.

  104. </p>

  105. <p>For this reason, it is possible and suggested to use a register to

  106. represent the condition code for new ports.  If there is a specific

  107. condition code register in the machine, use a hard register.  If the

  108. condition code or comparison result can be placed in any general register,

  109. or if there are multiple condition registers, use a pseudo register.

  110. Registers used to store the condition code value will usually have a mode

  111. that is in class <code>MODE_CC</code>.

  112. </p>

  113. <p>Alternatively, you can use <code>BImode</code> if the comparison operator is

  114. specified already in the compare instruction.  In this case, you are not

  115. interested in most macros in this section.

  116. </p>

  117. <table class="menu" border="0" cellspacing="0">

  118. <tr><td align="left" valign="top">&bull; <a href="CC0-Condition-Codes.html#CC0-Condition-Codes" accesskey="1">CC0 Condition Codes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Old style representation of condition codes.

  119. </td></tr>

  120. <tr><td align="left" valign="top">&bull; <a href="MODE_005fCC-Condition-Codes.html#MODE_005fCC-Condition-Codes" accesskey="2">MODE_CC Condition Codes</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">Modern representation of condition codes.

  121. </td></tr>

  122. </table>

  123. 

  124. <hr>

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  127. 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> &nbsp; [<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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