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diff --git a/share/doc/gccint/Sharing.html b/share/doc/gccint/Sharing.html new file mode 100644 index 0000000..7cf9fe7 --- /dev/null +++ b/share/doc/gccint/Sharing.html @@ -0,0 +1,171 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- Copyright (C) 1988-2023 Free Software Foundation, Inc. + +Permission is granted to copy, distribute and/or modify this document +under the terms of the GNU Free Documentation License, Version 1.3 or +any later version published by the Free Software Foundation; with the +Invariant Sections being "Funding Free Software", the Front-Cover +Texts being (a) (see below), and with the Back-Cover Texts being (b) +(see below). A copy of the license is included in the section entitled +"GNU Free Documentation License". + +(a) The FSF's Front-Cover Text is: + +A GNU Manual + +(b) The FSF's Back-Cover Text is: + +You have freedom to copy and modify this GNU Manual, like GNU + software. Copies published by the Free Software Foundation raise + funds for GNU development. --> +<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>GNU Compiler Collection (GCC) Internals: Sharing</title> + +<meta name="description" content="GNU Compiler Collection (GCC) Internals: Sharing"> +<meta name="keywords" content="GNU Compiler Collection (GCC) Internals: Sharing"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="index.html#Top" rel="start" title="Top"> +<link href="Option-Index.html#Option-Index" rel="index" title="Option Index"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="RTL.html#RTL" rel="up" title="RTL"> +<link href="Reading-RTL.html#Reading-RTL" rel="next" title="Reading RTL"> +<link href="Changing-Multiple-RTL-SSA-Instructions.html#Changing-Multiple-RTL-SSA-Instructions" rel="previous" title="Changing Multiple RTL SSA Instructions"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +div.smalllisp {margin-left: 3.2em} +kbd {font-style:oblique} +pre.display {font-family: inherit} +pre.format {font-family: inherit} +pre.menu-comment {font-family: serif} +pre.menu-preformatted {font-family: serif} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +span.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Sharing"></a> +<div class="header"> +<p> +Next: <a href="Reading-RTL.html#Reading-RTL" accesskey="n" rel="next">Reading RTL</a>, Previous: <a href="RTL-SSA.html#RTL-SSA" accesskey="p" rel="previous">RTL SSA</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> +</div> +<hr> +<a name="Structure-Sharing-Assumptions"></a> +<h3 class="section">14.22 Structure Sharing Assumptions</h3> +<a name="index-sharing-of-RTL-components"></a> +<a name="index-RTL-structure-sharing-assumptions"></a> + +<p>The compiler assumes that certain kinds of RTL expressions are unique; +there do not exist two distinct objects representing the same value. +In other cases, it makes an opposite assumption: that no RTL expression +object of a certain kind appears in more than one place in the +containing structure. +</p> +<p>These assumptions refer to a single function; except for the RTL +objects that describe global variables and external functions, +and a few standard objects such as small integer constants, +no RTL objects are common to two functions. +</p> +<ul> +<li> <a name="index-reg_002c-RTL-sharing"></a> +Each pseudo-register has only a single <code>reg</code> object to represent it, +and therefore only a single machine mode. + +</li><li> <a name="index-symbolic-label"></a> +<a name="index-symbol_005fref_002c-RTL-sharing"></a> +For any symbolic label, there is only one <code>symbol_ref</code> object +referring to it. + +</li><li> <a name="index-const_005fint_002c-RTL-sharing"></a> +All <code>const_int</code> expressions with equal values are shared. + +</li><li> <a name="index-const_005fpoly_005fint_002c-RTL-sharing"></a> +All <code>const_poly_int</code> expressions with equal modes and values +are shared. + +</li><li> <a name="index-pc_002c-RTL-sharing"></a> +There is only one <code>pc</code> expression. + +</li><li> <a name="index-const_005fdouble_002c-RTL-sharing"></a> +There is only one <code>const_double</code> expression with value 0 for +each floating point mode. Likewise for values 1 and 2. + +</li><li> <a name="index-const_005fvector_002c-RTL-sharing"></a> +There is only one <code>const_vector</code> expression with value 0 for +each vector mode, be it an integer or a double constant vector. + +</li><li> <a name="index-label_005fref_002c-RTL-sharing"></a> +<a name="index-scratch_002c-RTL-sharing"></a> +No <code>label_ref</code> or <code>scratch</code> appears in more than one place in +the RTL structure; in other words, it is safe to do a tree-walk of all +the insns in the function and assume that each time a <code>label_ref</code> +or <code>scratch</code> is seen it is distinct from all others that are seen. + +</li><li> <a name="index-mem_002c-RTL-sharing"></a> +Only one <code>mem</code> object is normally created for each static +variable or stack slot, so these objects are frequently shared in all +the places they appear. However, separate but equal objects for these +variables are occasionally made. + +</li><li> <a name="index-asm_005foperands_002c-RTL-sharing"></a> +When a single <code>asm</code> statement has multiple output operands, a +distinct <code>asm_operands</code> expression is made for each output operand. +However, these all share the vector which contains the sequence of input +operands. This sharing is used later on to test whether two +<code>asm_operands</code> expressions come from the same statement, so all +optimizations must carefully preserve the sharing if they copy the +vector at all. + +</li><li> No RTL object appears in more than one place in the RTL structure +except as described above. Many passes of the compiler rely on this +by assuming that they can modify RTL objects in place without unwanted +side-effects on other insns. + +</li><li> <a name="index-unshare_005fall_005frtl"></a> +During initial RTL generation, shared structure is freely introduced. +After all the RTL for a function has been generated, all shared +structure is copied by <code>unshare_all_rtl</code> in <samp>emit-rtl.cc</samp>, +after which the above rules are guaranteed to be followed. + +</li><li> <a name="index-copy_005frtx_005fif_005fshared"></a> +During the combiner pass, shared structure within an insn can exist +temporarily. However, the shared structure is copied before the +combiner is finished with the insn. This is done by calling +<code>copy_rtx_if_shared</code>, which is a subroutine of +<code>unshare_all_rtl</code>. +</li></ul> + +<hr> +<div class="header"> +<p> +Next: <a href="Reading-RTL.html#Reading-RTL" accesskey="n" rel="next">Reading RTL</a>, Previous: <a href="RTL-SSA.html#RTL-SSA" accesskey="p" rel="previous">RTL SSA</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> +</div> + + + +</body> +</html> |