| <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> |
| <html> |
| <!-- Copyright (C) 1988-2015 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 5.2, 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="Calls.html#Calls" rel="prev" title="Calls"> |
| <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="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> |
| </div> |
| <hr> |
| <a name="Structure-Sharing-Assumptions"></a> |
| <h3 class="section">13.21 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-pc_002c-RTL-sharing"></a> |
| There is only one <code>pc</code> expression. |
| |
| </li><li> <a name="index-cc0_002c-RTL-sharing"></a> |
| There is only one <code>cc0</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.c</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="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> |
| </div> |
| |
| |
| |
| </body> |
| </html> |