Xtensa-Branch-Relaxation.html 4.8 KB

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  14. <title>Using as: Xtensa Branch Relaxation</title>
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  60. Next: <a href="Xtensa-Call-Relaxation.html#Xtensa-Call-Relaxation" accesskey="n" rel="next">Xtensa Call Relaxation</a>, Up: <a href="Xtensa-Relaxation.html#Xtensa-Relaxation" accesskey="u" rel="up">Xtensa Relaxation</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="AS-Index.html#AS-Index" title="Index" rel="index">Index</a>]</p>
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  63. <a name="Conditional-Branch-Relaxation"></a>
  64. <h4 class="subsubsection">9.53.4.1 Conditional Branch Relaxation</h4>
  65. <a name="index-relaxation-of-branch-instructions"></a>
  66. <a name="index-branch-instructions_002c-relaxation"></a>
  67. <p>When the target of a branch is too far away from the branch itself,
  68. i.e., when the offset from the branch to the target is too large to fit
  69. in the immediate field of the branch instruction, it may be necessary to
  70. replace the branch with a branch around a jump. For example,
  71. </p>
  72. <div class="smallexample">
  73. <pre class="smallexample"> beqz a2, L
  74. </pre></div>
  75. <p>may result in:
  76. </p>
  77. <div class="smallexample">
  78. <pre class="smallexample"> bnez.n a2, M
  79. j L
  80. M:
  81. </pre></div>
  82. <p>(The <code>BNEZ.N</code> instruction would be used in this example only if the
  83. density option is available. Otherwise, <code>BNEZ</code> would be used.)
  84. </p>
  85. <p>This relaxation works well because the unconditional jump instruction
  86. has a much larger offset range than the various conditional branches.
  87. However, an error will occur if a branch target is beyond the range of a
  88. jump instruction. <code>as</code> cannot relax unconditional jumps.
  89. Similarly, an error will occur if the original input contains an
  90. unconditional jump to a target that is out of range.
  91. </p>
  92. <p>Branch relaxation is enabled by default. It can be disabled by using
  93. underscore prefixes (see <a href="Xtensa-Opcodes.html#Xtensa-Opcodes">Opcode Names</a>), the
  94. &lsquo;<samp>--no-transform</samp>&rsquo; command-line option (see <a href="Xtensa-Options.html#Xtensa-Options">Command Line Options</a>), or the <code>no-transform</code> directive
  95. (see <a href="Transform-Directive.html#Transform-Directive">transform</a>).
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