Xtensa-Jump-Relaxation.html 6.1 KB

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  60. Next: <a href="Xtensa-Immediate-Relaxation.html#Xtensa-Immediate-Relaxation" accesskey="n" rel="next">Xtensa Immediate Relaxation</a>, Previous: <a href="Xtensa-Call-Relaxation.html#Xtensa-Call-Relaxation" accesskey="p" rel="prev">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="Jump-Relaxation"></a>
  64. <h4 class="subsubsection">9.53.4.3 Jump Relaxation</h4>
  65. <a name="index-relaxation-of-jump-instructions"></a>
  66. <a name="index-jump-instructions_002c-relaxation"></a>
  67. <p>Jump instruction may require relaxation because the Xtensa jump instruction
  68. (<code>J</code>) provide a PC-relative offset of only 128 Kbytes in either
  69. direction. One option is to use jump long (<code>J.L</code>) instruction, which
  70. depending on jump distance may be assembled as jump (<code>J</code>) or indirect
  71. jump (<code>JX</code>). However it needs a free register. When there&rsquo;s no spare
  72. register it is possible to plant intermediate jump sites (trampolines)
  73. between the jump instruction and its target. These sites may be located in
  74. areas unreachable by normal code execution flow, in that case they only
  75. contain intermediate jumps, or they may be inserted in the middle of code
  76. block, in which case there&rsquo;s an additional jump from the beginning of the
  77. trampoline to the instruction past its end. So, for example:
  78. </p>
  79. <div class="smallexample">
  80. <pre class="smallexample"> j 1f
  81. ...
  82. retw
  83. ...
  84. mov a10, a2
  85. call8 func
  86. ...
  87. 1:
  88. ...
  89. </pre></div>
  90. <p>might be relaxed to:
  91. </p>
  92. <div class="smallexample">
  93. <pre class="smallexample"> j .L0_TR_1
  94. ...
  95. retw
  96. .L0_TR_1:
  97. j 1f
  98. ...
  99. mov a10, a2
  100. call8 func
  101. ...
  102. 1:
  103. ...
  104. </pre></div>
  105. <p>or to:
  106. </p>
  107. <div class="smallexample">
  108. <pre class="smallexample"> j .L0_TR_1
  109. ...
  110. retw
  111. ...
  112. mov a10, a2
  113. j .L0_TR_0
  114. .L0_TR_1:
  115. j 1f
  116. .L0_TR_0:
  117. call8 func
  118. ...
  119. 1:
  120. ...
  121. </pre></div>
  122. <p>The Xtensa assempler uses trampolines with jump around only when it cannot
  123. find suitable unreachable trampoline. There may be multiple trampolines
  124. between the jump instruction and its target.
  125. </p>
  126. <p>This relaxation does not apply to jumps to undefined symbols, assuming they
  127. will reach their targets once resolved.
  128. </p>
  129. <p>Jump relaxation is enabled by default because it does not affect code size
  130. or performance while the code itself is small. This relaxation may be
  131. disabled completely with &lsquo;<samp>--no-trampolines</samp>&rsquo; or &lsquo;<samp>--no-transform</samp>&rsquo;
  132. command-line options (see <a href="Xtensa-Options.html#Xtensa-Options">Command Line Options</a>).
  133. </p>
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