Xtensa-Call-Relaxation.html 5.3 KB

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  63. <a name="Function-Call-Relaxation"></a>
  64. <h4 class="subsubsection">9.53.4.2 Function Call Relaxation</h4>
  65. <a name="index-relaxation-of-call-instructions"></a>
  66. <a name="index-call-instructions_002c-relaxation"></a>
  67. <p>Function calls may require relaxation because the Xtensa immediate call
  68. instructions (<code>CALL0</code>, <code>CALL4</code>, <code>CALL8</code> and
  69. <code>CALL12</code>) provide a PC-relative offset of only 512 Kbytes in either
  70. direction. For larger programs, it may be necessary to use indirect
  71. calls (<code>CALLX0</code>, <code>CALLX4</code>, <code>CALLX8</code> and <code>CALLX12</code>)
  72. where the target address is specified in a register. The Xtensa
  73. assembler can automatically relax immediate call instructions into
  74. indirect call instructions. This relaxation is done by loading the
  75. address of the called function into the callee&rsquo;s return address register
  76. and then using a <code>CALLX</code> instruction. So, for example:
  77. </p>
  78. <div class="smallexample">
  79. <pre class="smallexample"> call8 func
  80. </pre></div>
  81. <p>might be relaxed to:
  82. </p>
  83. <div class="smallexample">
  84. <pre class="smallexample"> .literal .L1, func
  85. l32r a8, .L1
  86. callx8 a8
  87. </pre></div>
  88. <p>Because the addresses of targets of function calls are not generally
  89. known until link-time, the assembler must assume the worst and relax all
  90. the calls to functions in other source files, not just those that really
  91. will be out of range. The linker can recognize calls that were
  92. unnecessarily relaxed, and it will remove the overhead introduced by the
  93. assembler for those cases where direct calls are sufficient.
  94. </p>
  95. <p>Call relaxation is disabled by default because it can have a negative
  96. effect on both code size and performance, although the linker can
  97. usually eliminate the unnecessary overhead. If a program is too large
  98. and some of the calls are out of range, function call relaxation can be
  99. enabled using the &lsquo;<samp>--longcalls</samp>&rsquo; command-line option or the
  100. <code>longcalls</code> directive (see <a href="Longcalls-Directive.html#Longcalls-Directive">longcalls</a>).
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