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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> |
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| <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 ‘<samp>d</samp>’ and ‘<samp>a</samp>’ that specify classes of registers. |
| The pseudo register is put in whichever class gets the most “votes”. |
| The constraint letters ‘<samp>g</samp>’ and ‘<samp>r</samp>’ 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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