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100 lines
4.5 KiB
HTML
100 lines
4.5 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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any later version published by the Free Software Foundation; with the
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"GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Class Preferences (GNU Compiler Collection (GCC) Internals)</title>
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<meta name="description" content="Class Preferences (GNU Compiler Collection (GCC) Internals)">
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<meta name="keywords" content="Class Preferences (GNU Compiler Collection (GCC) Internals)">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Constraints.html#Constraints" rel="up" title="Constraints">
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<link href="Modifiers.html#Modifiers" rel="next" title="Modifiers">
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<link href="Multi_002dAlternative.html#Multi_002dAlternative" rel="prev" title="Multi-Alternative">
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<a name="Class-Preferences"></a>
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<div class="header">
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<p>
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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> [<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>
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</div>
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<hr>
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<a name="Register-Class-Preferences"></a>
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<h4 class="subsection">17.8.3 Register Class Preferences</h4>
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<a name="index-class-preference-constraints"></a>
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<a name="index-register-class-preference-constraints"></a>
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<a name="index-voting-between-constraint-alternatives"></a>
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<p>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>d</samp>’ and ‘<samp>a</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>g</samp>’ and ‘<samp>r</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>
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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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</p>
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</body>
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