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

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<a name="Register-Class-Preferences"></a>
<h4 class="subsection">16.8.3 Register Class Preferences</h4>
<a name="index-class-preference-constraints"></a>
<a name="index-register-class-preference-constraints"></a>

<a name="index-voting-between-constraint-alternatives"></a>
<p>The operand constraints have another function: they enable the compiler
to decide which kind of hardware register a pseudo register is best
allocated to.  The compiler examines the constraints that apply to the
insns that use the pseudo register, looking for the machine-dependent
letters such as &lsquo;<samp>d</samp>&rsquo; and &lsquo;<samp>a</samp>&rsquo; that specify classes of registers.
The pseudo register is put in whichever class gets the most &ldquo;votes&rdquo;.
The constraint letters &lsquo;<samp>g</samp>&rsquo; and &lsquo;<samp>r</samp>&rsquo; also vote: they vote in
favor of a general register.  The machine description says which registers
are considered general.
</p>
<p>Of course, on some machines all registers are equivalent, and no register
classes are defined.  Then none of this complexity is relevant.
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