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<h4 class="subsection">16.8.3 Register Class Preferences</h4>
<p><a name="index-class-preference-constraints-3350"></a><a name="index-register-class-preference-constraints-3351"></a>
<a name="index-voting-between-constraint-alternatives-3352"></a>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 ‘<samp><span class="samp">d</span></samp>’ and ‘<samp><span class="samp">a</span></samp>’ that specify classes of registers.
The pseudo register is put in whichever class gets the most “votes”.
The constraint letters ‘<samp><span class="samp">g</span></samp>’ and ‘<samp><span class="samp">r</span></samp>’ also vote: they vote in
favor of a general register. The machine description says which registers
are considered general.
<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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