PIC.html 6.3 KB

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  68. <a name="Position-Independent-Code"></a>
  69. <h3 class="section">17.19 Position Independent Code</h3>
  70. <a name="index-position-independent-code"></a>
  71. <a name="index-PIC"></a>
  72. <p>This section describes macros that help implement generation of position
  73. independent code. Simply defining these macros is not enough to
  74. generate valid PIC; you must also add support to the hook
  75. <code>TARGET_LEGITIMATE_ADDRESS_P</code> and to the macro
  76. <code>PRINT_OPERAND_ADDRESS</code>, as well as <code>LEGITIMIZE_ADDRESS</code>. You
  77. must modify the definition of &lsquo;<samp>movsi</samp>&rsquo; to do something appropriate
  78. when the source operand contains a symbolic address. You may also
  79. need to alter the handling of switch statements so that they use
  80. relative addresses.
  81. </p>
  82. <dl>
  83. <dt><a name="index-PIC_005fOFFSET_005fTABLE_005fREGNUM"></a>Macro: <strong>PIC_OFFSET_TABLE_REGNUM</strong></dt>
  84. <dd><p>The register number of the register used to address a table of static
  85. data addresses in memory. In some cases this register is defined by a
  86. processor&rsquo;s &ldquo;application binary interface&rdquo; (ABI). When this macro
  87. is defined, RTL is generated for this register once, as with the stack
  88. pointer and frame pointer registers. If this macro is not defined, it
  89. is up to the machine-dependent files to allocate such a register (if
  90. necessary). Note that this register must be fixed when in use (e.g.
  91. when <code>flag_pic</code> is true).
  92. </p></dd></dl>
  93. <dl>
  94. <dt><a name="index-PIC_005fOFFSET_005fTABLE_005fREG_005fCALL_005fCLOBBERED"></a>Macro: <strong>PIC_OFFSET_TABLE_REG_CALL_CLOBBERED</strong></dt>
  95. <dd><p>A C expression that is nonzero if the register defined by
  96. <code>PIC_OFFSET_TABLE_REGNUM</code> is clobbered by calls. If not defined,
  97. the default is zero. Do not define
  98. this macro if <code>PIC_OFFSET_TABLE_REGNUM</code> is not defined.
  99. </p></dd></dl>
  100. <dl>
  101. <dt><a name="index-LEGITIMATE_005fPIC_005fOPERAND_005fP"></a>Macro: <strong>LEGITIMATE_PIC_OPERAND_P</strong> <em>(<var>x</var>)</em></dt>
  102. <dd><p>A C expression that is nonzero if <var>x</var> is a legitimate immediate
  103. operand on the target machine when generating position independent code.
  104. You can assume that <var>x</var> satisfies <code>CONSTANT_P</code>, so you need not
  105. check this. You can also assume <var>flag_pic</var> is true, so you need not
  106. check it either. You need not define this macro if all constants
  107. (including <code>SYMBOL_REF</code>) can be immediate operands when generating
  108. position independent code.
  109. </p></dd></dl>
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