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toolchain-gcc-arm-embedded/share/doc/gcc-arm-none-eabi/html/gccint/Class-Preferences.html

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  25. <title>Class Preferences (GNU Compiler Collection (GCC) Internals)</title>

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  33. <link href="Option-Index.html#Option-Index" rel="index" title="Option Index">

  34. <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">

  35. <link href="Constraints.html#Constraints" rel="up" title="Constraints">

  36. <link href="Modifiers.html#Modifiers" rel="next" title="Modifiers">

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  70. <a name="Class-Preferences"></a>

  71. <div class="header">

  72. <p>

  73. Next: <a href="Modifiers.html#Modifiers" accesskey="n" rel="next">Modifiers</a>, Previous: <a href="Multi_002dAlternative.html#Multi_002dAlternative" accesskey="p" rel="prev">Multi-Alternative</a>, Up: <a href="Constraints.html#Constraints" accesskey="u" rel="up">Constraints</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>

  74. </div>

  75. <hr>

  76. <a name="Register-Class-Preferences"></a>

  77. <h4 class="subsection">17.8.3 Register Class Preferences</h4>

  78. <a name="index-class-preference-constraints"></a>

  79. <a name="index-register-class-preference-constraints"></a>

  80. 

  81. <a name="index-voting-between-constraint-alternatives"></a>

  82. <p>The operand constraints have another function: they enable the compiler

  83. to decide which kind of hardware register a pseudo register is best

  84. allocated to.  The compiler examines the constraints that apply to the

  85. insns that use the pseudo register, looking for the machine-dependent

  86. letters such as &lsquo;<samp>d</samp>&rsquo; and &lsquo;<samp>a</samp>&rsquo; that specify classes of registers.

  87. The pseudo register is put in whichever class gets the most &ldquo;votes&rdquo;.

  88. The constraint letters &lsquo;<samp>g</samp>&rsquo; and &lsquo;<samp>r</samp>&rsquo; also vote: they vote in

  89. favor of a general register.  The machine description says which registers

  90. are considered general.

  91. </p>

  92. <p>Of course, on some machines all registers are equivalent, and no register

  93. classes are defined.  Then none of this complexity is relevant.

  94. </p>

  95. 

  96. 

  97. 

  98. </body>

  99. </html>