test_objects_lto.sh

#!/bin/bash

set -x

GCC=gcc
OBJDUMP=objdump

# Create three source files: test1.c, test2.c, test3.c
cat <<EOT > test1.c
int func1();
int func2();
int func3();

int func1() {
  return 1;
}
EOT

cat <<EOT > test2.c
int func1();
int func2();
int func3();

int func2() {
  return 2 + func1();
}
EOT

cat <<EOT > test3.c
int func1();
int func2();
int func3();

int func3() {
  return 3 + func2();
}
EOT

OPTIM_LEVEL=2

# Compile them with LTO
$GCC -c test1.c -flto -O$OPTIM_LEVEL -o test1.o
$GCC -c test2.c -flto -O$OPTIM_LEVEL -o test2.o
$GCC -c test3.c -flto -O$OPTIM_LEVEL -o test3.o

# Get list of sections (notice that .text is empty and there are many .gnu.lto_ sections that contain IL)
$OBJDUMP -h test1.o test2.o test3.o

# Link a relocatable object, get 1 .o file instead of 3 different ones, also compile LTO IL into x86 machine code
# NOTE:
#   for GCC<9, -flinker-output does not support `nolto-rel`, `rel` should be used instead
#   for GCC>=9, -flinker-output=nolto-rel is REQUIRED to compile LTO IL into machine code
$GCC -r -nostdlib -O$OPTIM_LEVEL -flinker-output=nolto-rel test1.o test2.o test3.o -o reloc.o

# Get list of sections (notice that .gnu.lto_ sections are gone and we have normal .text filled with machine code)
$OBJDUMP -h reloc.o

# A small test program
cat <<EOT > main.c
#include <stdio.h>

int func3();

int main() {
  printf("%d\n", func3());
  return 0;
}
EOT

# Compile and link app with relocatable object
$GCC main.c reloc.o -o main

# Run app
./main