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73 lines
3.5 KiB
HTML
73 lines
3.5 KiB
HTML
4 years ago
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<title>Class Preferences - GNU Compiler Collection (GCC) Internals</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Constraints.html#Constraints" title="Constraints">
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<link rel="prev" href="Multi_002dAlternative.html#Multi_002dAlternative" title="Multi-Alternative">
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<link rel="next" href="Modifiers.html#Modifiers" title="Modifiers">
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Copyright (C) 1988-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation; with the
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Invariant Sections being ``Funding Free Software'', the Front-Cover
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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<body>
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<div class="node">
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<a name="Class-Preferences"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Modifiers.html#Modifiers">Modifiers</a>,
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Previous: <a rel="previous" accesskey="p" href="Multi_002dAlternative.html#Multi_002dAlternative">Multi-Alternative</a>,
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Up: <a rel="up" accesskey="u" href="Constraints.html#Constraints">Constraints</a>
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<hr>
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</div>
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<h4 class="subsection">16.8.3 Register Class Preferences</h4>
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<p><a name="index-class-preference-constraints-3350"></a><a name="index-register-class-preference-constraints-3351"></a>
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<a name="index-voting-between-constraint-alternatives-3352"></a>The operand constraints have another function: they enable the compiler
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to decide which kind of hardware register a pseudo register is best
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allocated to. The compiler examines the constraints that apply to the
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insns that use the pseudo register, looking for the machine-dependent
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letters such as ‘<samp><span class="samp">d</span></samp>’ and ‘<samp><span class="samp">a</span></samp>’ that specify classes of registers.
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The pseudo register is put in whichever class gets the most “votes”.
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The constraint letters ‘<samp><span class="samp">g</span></samp>’ and ‘<samp><span class="samp">r</span></samp>’ also vote: they vote in
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favor of a general register. The machine description says which registers
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are considered general.
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<p>Of course, on some machines all registers are equivalent, and no register
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classes are defined. Then none of this complexity is relevant.
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</body></html>
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