This document describes the LTO plugin interface between GCC and the linker. It explains that the GCC plugin collects options and registers callbacks with the linker via a transfer vector. The linker then calls these callbacks during the link process to handle claim files, get symbol resolutions, and clean up files. The plugin adds symbols, gets symbol information, and adds input files to integrate LTO optimization into the linking stage.

1. LTO Plugin
Kai

2. LTO Plugin Interface
• lto-plugin/lto-plugin.c
• Exported interface: void onload(struct ld_plugin_tv *tv)

3. GCC plugin onload
• process transfer vector (tv)
• collect plugin options to lto-wrapper
• call register_claim_file (MUST have)
• call register_cleanup (optional)
• call register_all_symbols_read (optional)
• getenv (“COLLECT_GCC_OPTIONS”)
• check “-fno-use-linker-plugin”

4. Transfer Vector
tv_tag tv_val / tv_ptr
LDPT_MESSAGE message O
LDPT_API_VERSION 1
LDPT_GNU_LD_VERSION major * 100 + minor
LDPT_LINKER_OUTPUT LDPO_REL / LDPO_EXEC / LDPO_PIE / LDPO_DYN
LDPT_OUTPUT_NAME output_filename
LDPT_REGISTER_CLAIM_FILE_HOOK register_claim_file O
LDPT_REGISTER_ALL_SYMBOLS_READ_HOOK register_all_symbols_read O
LDPT_REGISTER_CLEANUP_HOOK register_cleanup O
LDPT_ADD_SYMBOLS add_symbols O
LDPT_GET_INPUT_FILE get_input_file
LDPT_GET_VIEW get_view
LDPT_RELEASE_INPUT_FILE release_input_file
LDPT_GET_SYMBOLS get_symbols_v1 O
LDPT_GET_SYMBOLS_V2 get_symbols_v2 O
LDPT_ADD_INPUT_FILE add_input_file O
LDPT_ADD_INPUT_LIBRARY add_input_library O
LDPT_SET_EXTRA_LIBRARY_PATH set_extra_library_path
LDPT_OPTION $PREFIX/libexec/gcc/x86_64-unknown-linux-gnu/5.4.1/lto-wrapper
LDPT_OPTION -fresolution=/tmp/ccCVtp8y.res
LDPT_NULL 0

5. GCC Plugin Callbacks from
Linker
GCC Plugin
Callbacks
LD Functions
message message
add_symbols add_symbols
get_symbols_v2 get_symbols_v2
get_symbols get_symbols_v1
add_input_file add_input_file
add_input_library add_input_library

6. Linker Callbacks from GCC
Plugin
LD Callbacks GCC Functions
called_plugin->claim_file_handler claim_file_handler
called_plugin->all_symbols_read_handler all_symbols_read_handler
called_plugin->cleanup_handler cleanup_handler

8. claim_file_handler
• process symtab
• read out gnu.lto_.symtab
• use add_symbols to add symbols to linker
symbol table
• decide if the opened file is LTO object or not

9. all_symbols_read_handler
• write_resolution
• use get_symbols_v2 to find out symbol resolution
• prepare arguments for lto-wrapper
• put arguments into a file and pass the file to lto-wrapper
• execute lto-wrapper
• get result filenames after lto-wrapper
• use add_input_file to pass to the linker

10. cleanup_handler
• remove arguments file
• remove output files