Question

I have many source files in source directory. For example a.c, b.c, c.c and want to compile it by gcc with -combine option.

set(CMAKE_C_FLAGS "-combine")
set(SRC a.c b.c c.c)
add_executable(a.out ${SRC})

Cmake compiles each *.c file in object file, but I want to compile all sources to one object. How can I get this.

In terms of gcc:

gcc -combine a.c b.c c.c -o module.o
ar rcs library.a module.o

But cmake uses per-module translation of sources.

gcc a.c -o a.o
gcc b.c -o b.o
gcc c.c -o c.o
ar rcs library.a a.o b.o c.o

Translation with combine could speed up performance of program. I couldn’t use LTO which could resolve the same problem in cmake due to old version of gcc.

Thanks.

Answer 1

Use add_custom_target/add_custom_command.

In any way it is non-portable construction, so here simple example

[project root] with two folders in it [src] – here N files, [build] for binary, and CMakeLists.txt

cmake_minimum_required(VERSION 2.6.0 FATAL_ERROR)
set(TARGET_NAME whole)
project(${TARGET_NAME} C)

file(GLOB SRC_FILES src/*.c)

add_custom_target(${TARGET_NAME} ${CMAKE_C_COMPILER} -flto -fwhole-program ${SRC_FILES} -o ${TARGET_NAME})

In build folder run cmake ..; make VERBOSE=1 whole

This will make the work for you.

But, -fwhole-program work only with executable, as per documentation.

-fwhole-program
Assume that the current compilation unit represents the whole program being compiled. All public functions and variables with the exception of main and those merged by attribute externally_visible become static functions and in effect are optimized more aggressively by interprocedural optimizers.

So you mast have main defined anywhere in your source files.