MIPS-Symbol-Sizes.html 5.1 KB

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  63. <a name="Directives-to-override-the-size-of-symbols"></a>
  64. <h4 class="subsection">9.27.3 Directives to override the size of symbols</h4>
  65. <a name="index-_002eset-sym32"></a>
  66. <a name="index-_002eset-nosym32"></a>
  67. <p>The n64 ABI allows symbols to have any 64-bit value. Although this
  68. provides a great deal of flexibility, it means that some macros have
  69. much longer expansions than their 32-bit counterparts. For example,
  70. the non-PIC expansion of &lsquo;<samp>dla $4,sym</samp>&rsquo; is usually:
  71. </p>
  72. <div class="smallexample">
  73. <pre class="smallexample">lui $4,%highest(sym)
  74. lui $1,%hi(sym)
  75. daddiu $4,$4,%higher(sym)
  76. daddiu $1,$1,%lo(sym)
  77. dsll32 $4,$4,0
  78. daddu $4,$4,$1
  79. </pre></div>
  80. <p>whereas the 32-bit expansion is simply:
  81. </p>
  82. <div class="smallexample">
  83. <pre class="smallexample">lui $4,%hi(sym)
  84. daddiu $4,$4,%lo(sym)
  85. </pre></div>
  86. <p>n64 code is sometimes constructed in such a way that all symbolic
  87. constants are known to have 32-bit values, and in such cases, it&rsquo;s
  88. preferable to use the 32-bit expansion instead of the 64-bit
  89. expansion.
  90. </p>
  91. <p>You can use the <code>.set sym32</code> directive to tell the assembler
  92. that, from this point on, all expressions of the form
  93. &lsquo;<samp><var>symbol</var></samp>&rsquo; or &lsquo;<samp><var>symbol</var> + <var>offset</var></samp>&rsquo;
  94. have 32-bit values. For example:
  95. </p>
  96. <div class="smallexample">
  97. <pre class="smallexample">.set sym32
  98. dla $4,sym
  99. lw $4,sym+16
  100. sw $4,sym+0x8000($4)
  101. </pre></div>
  102. <p>will cause the assembler to treat &lsquo;<samp>sym</samp>&rsquo;, <code>sym+16</code> and
  103. <code>sym+0x8000</code> as 32-bit values. The handling of non-symbolic
  104. addresses is not affected.
  105. </p>
  106. <p>The directive <code>.set nosym32</code> ends a <code>.set sym32</code> block and
  107. reverts to the normal behavior. It is also possible to change the
  108. symbol size using the command-line options <samp>-msym32</samp> and
  109. <samp>-mno-sym32</samp>.
  110. </p>
  111. <p>These options and directives are always accepted, but at present,
  112. they have no effect for anything other than n64.
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