| <!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: LTO object file layout</title> |
| |
| <meta name="description" content="GNU Compiler Collection (GCC) Internals: LTO object file layout"> |
| <meta name="keywords" content="GNU Compiler Collection (GCC) Internals: LTO object file layout"> |
| <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="LTO.html#LTO" rel="up" title="LTO"> |
| <link href="IPA.html#IPA" rel="next" title="IPA"> |
| <link href="LTO-Overview.html#LTO-Overview" rel="prev" title="LTO Overview"> |
| <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="LTO-object-file-layout"></a> |
| <div class="header"> |
| <p> |
| Next: <a href="IPA.html#IPA" accesskey="n" rel="next">IPA</a>, Previous: <a href="LTO-Overview.html#LTO-Overview" accesskey="p" rel="prev">LTO Overview</a>, Up: <a href="LTO.html#LTO" accesskey="u" rel="up">LTO</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="LTO-file-sections"></a> |
| <h3 class="section">24.2 LTO file sections</h3> |
| |
| <p>LTO information is stored in several ELF sections inside object files. |
| Data structures and enum codes for sections are defined in |
| <samp>lto-streamer.h</samp>. |
| </p> |
| <p>These sections are emitted from <samp>lto-streamer-out.c</samp> and mapped |
| in all at once from <samp>lto/lto.c</samp>:<code>lto_file_read</code>. The |
| individual functions dealing with the reading/writing of each section |
| are described below. |
| </p> |
| <ul> |
| <li> Command line options (<code>.gnu.lto_.opts</code>) |
| |
| <p>This section contains the command line options used to generate the |
| object files. This is used at link time to determine the optimization |
| level and other settings when they are not explicitly specified at the |
| linker command line. |
| </p> |
| <p>Currently, GCC does not support combining LTO object files compiled |
| with different set of the command line options into a single binary. |
| At link time, the options given on the command line and the options |
| saved on all the files in a link-time set are applied globally. No |
| attempt is made at validating the combination of flags (other than the |
| usual validation done by option processing). This is implemented in |
| <samp>lto/lto.c</samp>:<code>lto_read_all_file_options</code>. |
| </p> |
| |
| </li><li> Symbol table (<code>.gnu.lto_.symtab</code>) |
| |
| <p>This table replaces the ELF symbol table for functions and variables |
| represented in the LTO IL. Symbols used and exported by the optimized |
| assembly code of “fat” objects might not match the ones used and |
| exported by the intermediate code. This table is necessary because |
| the intermediate code is less optimized and thus requires a separate |
| symbol table. |
| </p> |
| <p>Additionally, the binary code in the “fat” object will lack a call |
| to a function, since the call was optimized out at compilation time |
| after the intermediate language was streamed out. In some special |
| cases, the same optimization may not happen during link-time |
| optimization. This would lead to an undefined symbol if only one |
| symbol table was used. |
| </p> |
| <p>The symbol table is emitted in |
| <samp>lto-streamer-out.c</samp>:<code>produce_symtab</code>. |
| </p> |
| |
| </li><li> Global declarations and types (<code>.gnu.lto_.decls</code>) |
| |
| <p>This section contains an intermediate language dump of all |
| declarations and types required to represent the callgraph, static |
| variables and top-level debug info. |
| </p> |
| <p>The contents of this section are emitted in |
| <samp>lto-streamer-out.c</samp>:<code>produce_asm_for_decls</code>. Types and |
| symbols are emitted in a topological order that preserves the sharing |
| of pointers when the file is read back in |
| (<samp>lto.c</samp>:<code>read_cgraph_and_symbols</code>). |
| </p> |
| |
| </li><li> The callgraph (<code>.gnu.lto_.cgraph</code>) |
| |
| <p>This section contains the basic data structure used by the GCC |
| inter-procedural optimization infrastructure. This section stores an |
| annotated multi-graph which represents the functions and call sites as |
| well as the variables, aliases and top-level <code>asm</code> statements. |
| </p> |
| <p>This section is emitted in |
| <samp>lto-streamer-out.c</samp>:<code>output_cgraph</code> and read in |
| <samp>lto-cgraph.c</samp>:<code>input_cgraph</code>. |
| </p> |
| |
| </li><li> IPA references (<code>.gnu.lto_.refs</code>) |
| |
| <p>This section contains references between function and static |
| variables. It is emitted by <samp>lto-cgraph.c</samp>:<code>output_refs</code> |
| and read by <samp>lto-cgraph.c</samp>:<code>input_refs</code>. |
| </p> |
| |
| </li><li> Function bodies (<code>.gnu.lto_.function_body.<name></code>) |
| |
| <p>This section contains function bodies in the intermediate language |
| representation. Every function body is in a separate section to allow |
| copying of the section independently to different object files or |
| reading the function on demand. |
| </p> |
| <p>Functions are emitted in |
| <samp>lto-streamer-out.c</samp>:<code>output_function</code> and read in |
| <samp>lto-streamer-in.c</samp>:<code>input_function</code>. |
| </p> |
| |
| </li><li> Static variable initializers (<code>.gnu.lto_.vars</code>) |
| |
| <p>This section contains all the symbols in the global variable pool. It |
| is emitted by <samp>lto-cgraph.c</samp>:<code>output_varpool</code> and read in |
| <samp>lto-cgraph.c</samp>:<code>input_cgraph</code>. |
| </p> |
| </li><li> Summaries and optimization summaries used by IPA passes |
| (<code>.gnu.lto_.<xxx></code>, where <code><xxx></code> is one of <code>jmpfuncs</code>, |
| <code>pureconst</code> or <code>reference</code>) |
| |
| <p>These sections are used by IPA passes that need to emit summary |
| information during LTO generation to be read and aggregated at |
| link time. Each pass is responsible for implementing two pass manager |
| hooks: one for writing the summary and another for reading it in. The |
| format of these sections is entirely up to each individual pass. The |
| only requirement is that the writer and reader hooks agree on the |
| format. |
| </p></li></ul> |
| |
| |
| <hr> |
| <div class="header"> |
| <p> |
| Next: <a href="IPA.html#IPA" accesskey="n" rel="next">IPA</a>, Previous: <a href="LTO-Overview.html#LTO-Overview" accesskey="p" rel="prev">LTO Overview</a>, Up: <a href="LTO.html#LTO" accesskey="u" rel="up">LTO</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> |