The document describes the GCC Link Time Optimization (LTO) compilation process. It shows how GCC and the linker communicate via a transfer vector in the liblto_plugin.so library. GCC generates LTO objects and passes symbol information to the linker. The lto-wrapper driver then performs the LTO and code generation steps, outputting new object files for the linker to incorporate into the final executable.

1. GCC LTO

2. f1.c
f2.c
test.c
f1.o
f2.o
test.o
compile&
assemble
compile&
assemble
compile&
assemble
linker test
gcc -flto -c f1.c
gcc -flto -c f2.c
gcc -flto -c test.c
gcc -flto f1.o f2.o test.o -o test

3. f1.c
f2.c
test.c
LGEN
LGEN
LGEN
f1.o
f2.o
test.o
WPA
test.ltrans0.o
test.ltrans1.o
cc1 -flto -c f1.c
cc1 -flto -c f2.c
cc1 -flto -c test.c
lto1 -fwpa -fltrans-output-list=test.out
f1.o f2.o test.o
test.out:
test.ltrans0.o
test.ltrans1.o
LTO objects

4. test.ltrans0.o
test.ltrans1.o
LTRANS
LTRANS
test.ltrans0.l
trans.o
test.ltrans1.l
trans.o
ld test
lto1 -fltrans test.ltrans0.o -o test.ltrans0.s
lto1 -fltrans test.ltrans1.o -o test.ltrans1.s
as test.ltrans0.s -o test.ltrans0.ltrans.o
as test.ltrans1.s -o test.ltrans1.ltrans.o
target objects

5. $ gcc -v --save-temps f1.o f2.o test.o -O3 -flto -o test
/home/users/kai/gcc-x86/libexec/gcc/x86_64-pc-linux-gnu/6.4.1/collect2 -
plugin /home/users/kai/gcc-x86/libexec/gcc/x86_64-pc-linux-gnu/6.4.1/
liblto_plugin.so -plugin-opt=/home/users/kai/gcc-x86/libexec/gcc/x86_64-pc-
linux-gnu/6.4.1/lto-wrapper -plugin-opt=-fresolution=test.res …

6. liblto_plugin.so
• Linker will pass transfer vector by onload() in
liblto_plugin.so provided by GCC.
struct ld_plugin_tv {
enum ld_plugin_tag tv_tag;
union {
int tv_val;
const char *tv_string;
ld_plugin_register_claim_file tv_register_claim_file;
…
} tv_u;
};

7. tv_tag tv_u used by
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 (claim_file_handler) LD O
LDPT_REGISTER_ALL_SYMBOLS_READ_HOOK register_all_symbols_read (all_symbols_read_handler) LD O
LDPT_REGISTER_CLEANUP_HOOK register_cleanup (cleanup_handler) LD O
LDPT_ADD_SYMBOLS add_symbols GCC O
LDPT_GET_INPUT_FILE get_input_file GCC
LDPT_GET_VIEW get_view GCC
LDPT_RELEASE_INPUT_FILE release_input_file GCC
LDPT_GET_SYMBOLS get_symbols_v1 GCC O
LDPT_GET_SYMBOLS_V2 get_symbols_v2 GCC O
LDPT_ADD_INPUT_FILE add_input_file GCC O
LDPT_ADD_INPUT_LIBRARY add_input_library GCC O
LDPT_SET_EXTRA_LIBRARY_PATH set_extra_library_path GCC
LDPT_OPTION $PREFIX/libexec/gcc/x86_64-unknown-linux-gnu/5.4.1/lto-wrapper
LDPT_OPTION -fresolution=/tmp/ccCVtp8y.res
LDPT_NULL 0
Transfer Vector

8. claim_file_handler
• called by open_input_bfds () in linker
• process .gnu.lto_.symtab
• call add_symbols
open_input_bfds
claim_file_handler
(extract .gnu.lto_.symtab)
add_symbols
(add symbols to bfd symtab)

9. all_symbols_read_handler
• called after all symbols read in linker
• write_resolution ()
• get_symbols_v2: get symbol resolution information
• print to the resolution file
• prepare lto arguments
• lto-wrapper -fresolution=test.res -flinker-output=exec f1.o f2.o
test.o
• execute lto-wrapper
• call add_input_file () to add generated target objects.

10. cleanup_handler
• remove argument files.
• remove generated target objects after LTRANS

11. lto-wrapper
f1.o
f2.o
test.o
WPA
test.ltrans0.o
test.ltrans1.o
LTO objects
LTRANS
LTRANS
test.ltrans0.l
trans.o
test.ltrans1.l
trans.o
target objects

12. lto-wrapper
• process options in COLLECT_GCC_OPTIONS
• options given by command line
• decoded_options
• process options, .gnu.lto_.opts, in LTO objects
• options stored in LTO objects (.gnu.lto_.opts)
• merge_and_complain ()
• fdecoded_options
• fork gcc -xlto -c $fdecoded_options $decoded_options -fwpa …
• fork gcc -xlto -c $fdecoded_options $decoded_options -fltrans …
• print output_names to stdout
• GCC will read in output_names and pass to Linker to continue linking process.

13. Linker liblto_plugin.so lto-wrapper GCC
all_symbols_read_handler
gcc -fwpa
gcc -fltrans
print output_names
add_input_file
exec lto-wrapper