GNU Compiler Collection (GCC) Internals: LTO object file layout

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### 24.2 LTO file sections

LTO information is stored in several ELF sections inside object files. Data structures and enum codes for sections are defined in `lto-streamer.h`.

These sections are emitted from `lto-streamer-out.c` and mapped in all at once from `lto/lto.c`:`lto_file_read`. The individual functions dealing with the reading/writing of each section are described below.

- Command line options (`.gnu.lto_.opts`)

  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.

  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 `lto/lto.c`:`lto_read_all_file_options`.

- Symbol table (`.gnu.lto_.symtab`)

  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.

  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.

  The symbol table is emitted in `lto-streamer-out.c`:`produce_symtab`.

- Global declarations and types (`.gnu.lto_.decls`)

  This section contains an intermediate language dump of all declarations and types required to represent the callgraph, static variables and top-level debug info.

  The contents of this section are emitted in `lto-streamer-out.c`:`produce_asm_for_decls`. Types and symbols are emitted in a topological order that preserves the sharing of pointers when the file is read back in (`lto.c`:`read_cgraph_and_symbols`).

- The callgraph (`.gnu.lto_.cgraph`)

  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 `asm` statements.

  This section is emitted in `lto-streamer-out.c`:`output_cgraph` and read in `lto-cgraph.c`:`input_cgraph`.

- IPA references (`.gnu.lto_.refs`)

  This section contains references between function and static variables. It is emitted by `lto-cgraph.c`:`output_refs` and read by `lto-cgraph.c`:`input_refs`.

- Function bodies (`.gnu.lto_.function_body.<name>`)

  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.

  Functions are emitted in `lto-streamer-out.c`:`output_function` and read in `lto-streamer-in.c`:`input_function`.

- Static variable initializers (`.gnu.lto_.vars`)

  This section contains all the symbols in the global variable pool. It is emitted by `lto-cgraph.c`:`output_varpool` and read in `lto-cgraph.c`:`input_cgraph`.

- Summaries and optimization summaries used by IPA passes (`.gnu.lto_.<xxx>`, where `<xxx>` is one of `jmpfuncs`, `pureconst` or `reference`)

  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.

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