171 lines
7.6 KiB
HTML
171 lines
7.6 KiB
HTML
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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) 1988-2016 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 5.2, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>GNU Compiler Collection (GCC) Internals: Sharing</title>
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<meta name="description" content="GNU Compiler Collection (GCC) Internals: Sharing">
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<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: Sharing">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content="makeinfo">
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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="RTL.html#RTL" rel="up" title="RTL">
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<link href="Reading-RTL.html#Reading-RTL" rel="next" title="Reading RTL">
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<link href="Calls.html#Calls" rel="prev" title="Calls">
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</style>
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</head>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Sharing"></a>
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<div class="header">
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<p>
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Next: <a href="Reading-RTL.html#Reading-RTL" accesskey="n" rel="next">Reading RTL</a>, Previous: <a href="Calls.html#Calls" accesskey="p" rel="prev">Calls</a>, Up: <a href="RTL.html#RTL" accesskey="u" rel="up">RTL</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="Structure-Sharing-Assumptions"></a>
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<h3 class="section">13.21 Structure Sharing Assumptions</h3>
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<a name="index-sharing-of-RTL-components"></a>
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<a name="index-RTL-structure-sharing-assumptions"></a>
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<p>The compiler assumes that certain kinds of RTL expressions are unique;
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there do not exist two distinct objects representing the same value.
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In other cases, it makes an opposite assumption: that no RTL expression
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object of a certain kind appears in more than one place in the
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containing structure.
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</p>
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<p>These assumptions refer to a single function; except for the RTL
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objects that describe global variables and external functions,
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and a few standard objects such as small integer constants,
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no RTL objects are common to two functions.
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</p>
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<ul>
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<li> <a name="index-reg_002c-RTL-sharing"></a>
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Each pseudo-register has only a single <code>reg</code> object to represent it,
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and therefore only a single machine mode.
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</li><li> <a name="index-symbolic-label"></a>
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<a name="index-symbol_005fref_002c-RTL-sharing"></a>
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For any symbolic label, there is only one <code>symbol_ref</code> object
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referring to it.
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</li><li> <a name="index-const_005fint_002c-RTL-sharing"></a>
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All <code>const_int</code> expressions with equal values are shared.
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</li><li> <a name="index-pc_002c-RTL-sharing"></a>
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There is only one <code>pc</code> expression.
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</li><li> <a name="index-cc0_002c-RTL-sharing"></a>
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There is only one <code>cc0</code> expression.
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</li><li> <a name="index-const_005fdouble_002c-RTL-sharing"></a>
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There is only one <code>const_double</code> expression with value 0 for
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each floating point mode. Likewise for values 1 and 2.
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</li><li> <a name="index-const_005fvector_002c-RTL-sharing"></a>
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There is only one <code>const_vector</code> expression with value 0 for
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each vector mode, be it an integer or a double constant vector.
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</li><li> <a name="index-label_005fref_002c-RTL-sharing"></a>
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<a name="index-scratch_002c-RTL-sharing"></a>
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No <code>label_ref</code> or <code>scratch</code> appears in more than one place in
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the RTL structure; in other words, it is safe to do a tree-walk of all
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the insns in the function and assume that each time a <code>label_ref</code>
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or <code>scratch</code> is seen it is distinct from all others that are seen.
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</li><li> <a name="index-mem_002c-RTL-sharing"></a>
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Only one <code>mem</code> object is normally created for each static
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variable or stack slot, so these objects are frequently shared in all
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the places they appear. However, separate but equal objects for these
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variables are occasionally made.
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</li><li> <a name="index-asm_005foperands_002c-RTL-sharing"></a>
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When a single <code>asm</code> statement has multiple output operands, a
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distinct <code>asm_operands</code> expression is made for each output operand.
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However, these all share the vector which contains the sequence of input
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operands. This sharing is used later on to test whether two
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<code>asm_operands</code> expressions come from the same statement, so all
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optimizations must carefully preserve the sharing if they copy the
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vector at all.
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</li><li> No RTL object appears in more than one place in the RTL structure
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except as described above. Many passes of the compiler rely on this
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by assuming that they can modify RTL objects in place without unwanted
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side-effects on other insns.
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</li><li> <a name="index-unshare_005fall_005frtl"></a>
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During initial RTL generation, shared structure is freely introduced.
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After all the RTL for a function has been generated, all shared
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structure is copied by <code>unshare_all_rtl</code> in <samp>emit-rtl.c</samp>,
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after which the above rules are guaranteed to be followed.
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</li><li> <a name="index-copy_005frtx_005fif_005fshared"></a>
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During the combiner pass, shared structure within an insn can exist
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temporarily. However, the shared structure is copied before the
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combiner is finished with the insn. This is done by calling
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<code>copy_rtx_if_shared</code>, which is a subroutine of
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<code>unshare_all_rtl</code>.
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</li></ul>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Reading-RTL.html#Reading-RTL" accesskey="n" rel="next">Reading RTL</a>, Previous: <a href="Calls.html#Calls" accesskey="p" rel="prev">Calls</a>, Up: <a href="RTL.html#RTL" accesskey="u" rel="up">RTL</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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