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  25. <title>Jump Patterns (GNU Compiler Collection (GCC) Internals)</title>

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  35. <link href="Machine-Desc.html#Machine-Desc" rel="up" title="Machine Desc">

  36. <link href="Looping-Patterns.html#Looping-Patterns" rel="next" title="Looping Patterns">

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  70. <a name="Jump-Patterns"></a>

  71. <div class="header">

  72. <p>

  73. Next: <a href="Looping-Patterns.html#Looping-Patterns" accesskey="n" rel="next">Looping Patterns</a>, Previous: <a href="Dependent-Patterns.html#Dependent-Patterns" accesskey="p" rel="prev">Dependent Patterns</a>, Up: <a href="Machine-Desc.html#Machine-Desc" accesskey="u" rel="up">Machine Desc</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="Defining-Jump-Instruction-Patterns"></a>

  77. <h3 class="section">17.12 Defining Jump Instruction Patterns</h3>

  78. <a name="index-jump-instruction-patterns"></a>

  79. <a name="index-defining-jump-instruction-patterns"></a>

  80. 

  81. <p>GCC does not assume anything about how the machine realizes jumps.

  82. The machine description should define a single pattern, usually

  83. a <code>define_expand</code>, which expands to all the required insns.

  84. </p>

  85. <p>Usually, this would be a comparison insn to set the condition code

  86. and a separate branch insn testing the condition code and branching

  87. or not according to its value.  For many machines, however,

  88. separating compares and branches is limiting, which is why the

  89. more flexible approach with one <code>define_expand</code> is used in GCC.

  90. The machine description becomes clearer for architectures that

  91. have compare-and-branch instructions but no condition code.  It also

  92. works better when different sets of comparison operators are supported

  93. by different kinds of conditional branches (e.g. integer vs.

  94. floating-point), or by conditional branches with respect to conditional stores.

  95. </p>

  96. <p>Two separate insns are always used if the machine description represents

  97. a condition code register using the legacy RTL expression <code>(cc0)</code>,

  98. and on most machines that use a separate condition code register

  99. (see <a href="Condition-Code.html#Condition-Code">Condition Code</a>).  For machines that use <code>(cc0)</code>, in

  100. fact, the set and use of the condition code must be separate and

  101. adjacent<a name="DOCF4" href="#FOOT4"><sup>4</sup></a>, thus

  102. allowing flags in <code>cc_status</code> to be used (see <a href="Condition-Code.html#Condition-Code">Condition Code</a>) and

  103. so that the comparison and branch insns could be located from each other

  104. by using the functions <code>prev_cc0_setter</code> and <code>next_cc0_user</code>.

  105. </p>

  106. <p>Even in this case having a single entry point for conditional branches

  107. is advantageous, because it handles equally well the case where a single

  108. comparison instruction records the results of both signed and unsigned

  109. comparison of the given operands (with the branch insns coming in distinct

  110. signed and unsigned flavors) as in the x86 or SPARC, and the case where

  111. there are distinct signed and unsigned compare instructions and only

  112. one set of conditional branch instructions as in the PowerPC.

  113. </p>

  114. <div class="footnote">

  115. <hr>

  116. <h4 class="footnotes-heading">Footnotes</h4>

  117. 

  118. <h3><a name="FOOT4" href="#DOCF4">(4)</a></h3>

  119. <p><code>note</code> insns can separate them, though.</p>

  120. </div>

  121. <hr>

  122. <div class="header">

  123. <p>

  124. Next: <a href="Looping-Patterns.html#Looping-Patterns" accesskey="n" rel="next">Looping Patterns</a>, Previous: <a href="Dependent-Patterns.html#Dependent-Patterns" accesskey="p" rel="prev">Dependent Patterns</a>, Up: <a href="Machine-Desc.html#Machine-Desc" accesskey="u" rel="up">Machine Desc</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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