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| <title>GNU Compiler Collection (GCC) Internals: LTO object file layout</title> |
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| 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> |
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| <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> |
| |
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| 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> |
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