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153 lines
7.1 KiB
HTML
153 lines
7.1 KiB
HTML
4 years ago
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<!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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<!-- Copyright (C) 1988-2018 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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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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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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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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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>Leaf Functions (GNU Compiler Collection (GCC) Internals)</title>
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<meta name="description" content="Leaf Functions (GNU Compiler Collection (GCC) Internals)">
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<meta name="keywords" content="Leaf Functions (GNU Compiler Collection (GCC) Internals)">
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<meta name="resource-type" content="document">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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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="Registers.html#Registers" rel="up" title="Registers">
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<link href="Stack-Registers.html#Stack-Registers" rel="next" title="Stack Registers">
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<link href="Values-in-Registers.html#Values-in-Registers" rel="prev" title="Values in Registers">
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<style type="text/css">
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</style>
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</head>
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<body lang="en">
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<a name="Leaf-Functions"></a>
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<div class="header">
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<p>
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Next: <a href="Stack-Registers.html#Stack-Registers" accesskey="n" rel="next">Stack Registers</a>, Previous: <a href="Values-in-Registers.html#Values-in-Registers" accesskey="p" rel="prev">Values in Registers</a>, Up: <a href="Registers.html#Registers" accesskey="u" rel="up">Registers</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="Handling-Leaf-Functions"></a>
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<h4 class="subsection">18.7.4 Handling Leaf Functions</h4>
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<a name="index-leaf-functions"></a>
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<a name="index-functions_002c-leaf"></a>
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<p>On some machines, a leaf function (i.e., one which makes no calls) can run
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more efficiently if it does not make its own register window. Often this
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means it is required to receive its arguments in the registers where they
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are passed by the caller, instead of the registers where they would
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normally arrive.
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</p>
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<p>The special treatment for leaf functions generally applies only when
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other conditions are met; for example, often they may use only those
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registers for its own variables and temporaries. We use the term “leaf
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function” to mean a function that is suitable for this special
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handling, so that functions with no calls are not necessarily “leaf
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functions”.
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</p>
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<p>GCC assigns register numbers before it knows whether the function is
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suitable for leaf function treatment. So it needs to renumber the
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registers in order to output a leaf function. The following macros
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accomplish this.
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</p>
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<dl>
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<dt><a name="index-LEAF_005fREGISTERS"></a>Macro: <strong>LEAF_REGISTERS</strong></dt>
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<dd><p>Name of a char vector, indexed by hard register number, which
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contains 1 for a register that is allowable in a candidate for leaf
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function treatment.
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</p>
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<p>If leaf function treatment involves renumbering the registers, then the
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registers marked here should be the ones before renumbering—those that
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GCC would ordinarily allocate. The registers which will actually be
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used in the assembler code, after renumbering, should not be marked with 1
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in this vector.
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</p>
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<p>Define this macro only if the target machine offers a way to optimize
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the treatment of leaf functions.
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</p></dd></dl>
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<dl>
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<dt><a name="index-LEAF_005fREG_005fREMAP"></a>Macro: <strong>LEAF_REG_REMAP</strong> <em>(<var>regno</var>)</em></dt>
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<dd><p>A C expression whose value is the register number to which <var>regno</var>
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should be renumbered, when a function is treated as a leaf function.
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</p>
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<p>If <var>regno</var> is a register number which should not appear in a leaf
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function before renumbering, then the expression should yield -1, which
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will cause the compiler to abort.
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</p>
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<p>Define this macro only if the target machine offers a way to optimize the
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treatment of leaf functions, and registers need to be renumbered to do
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this.
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</p></dd></dl>
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<a name="index-current_005ffunction_005fis_005fleaf"></a>
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<a name="index-current_005ffunction_005fuses_005fonly_005fleaf_005fregs"></a>
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<p><code>TARGET_ASM_FUNCTION_PROLOGUE</code> and
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<code>TARGET_ASM_FUNCTION_EPILOGUE</code> must usually treat leaf functions
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specially. They can test the C variable <code>current_function_is_leaf</code>
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which is nonzero for leaf functions. <code>current_function_is_leaf</code> is
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set prior to local register allocation and is valid for the remaining
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compiler passes. They can also test the C variable
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<code>current_function_uses_only_leaf_regs</code> which is nonzero for leaf
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functions which only use leaf registers.
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<code>current_function_uses_only_leaf_regs</code> is valid after all passes
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that modify the instructions have been run and is only useful if
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<code>LEAF_REGISTERS</code> is defined.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Stack-Registers.html#Stack-Registers" accesskey="n" rel="next">Stack Registers</a>, Previous: <a href="Values-in-Registers.html#Values-in-Registers" accesskey="p" rel="prev">Values in Registers</a>, Up: <a href="Registers.html#Registers" accesskey="u" rel="up">Registers</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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</body>
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</html>
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