make SCons compile everything in one gcc line?

Question

I have a rather complex SCons script that compiles a big C++ project.
This gcc manual page says:

The compiler performs optimization based on the knowledge it has of the program. Compiling multiple files at once to a single output file mode allows the compiler to use information gained from all of the files when compiling each of them.

So it's better to give all my files to a single g++ invocation and let it drive the compilation however it pleases.
But SCons does not do this. it calls g++ separately for every single C++ file in the project and then links them using ld

Is there a way to make SCons do this?

Answer 1

The main reason to have a build system with the ability to express dependencies is to support some kind of conditional/incremental build. Otherwise you might as well just use a script with the one command you need.

That being said, the result of having gcc/g++ optimize as the manual describe is substantial. In particular if you have C++ templates you use often. Good for run-time performance, bad for recompile performance.

I suggest you try and make your own builder doing what you need. Here is another question with an inspirational answer: SCons custom builder - build with multiple files and output one file

Answer 2

Currently the answer is no.

Logic similar to this was developed for MSVC only. You can see this in the man page ( http://scons.org/doc/production/HTML/scons-man.html ) as follows:

MSVC_BATCH When set to any true value, specifies that SCons should batch compilation of object files when calling the Microsoft Visual C/C++ compiler. All compilations of source files from the same source directory that generate target files in a same output directory and were configured in SCons using the same construction environment will be built in a single call to the compiler. Only source files that have changed since their object files were built will be passed to each compiler invocation (via the $CHANGED_SOURCES construction variable). Any compilations where the object (target) file base name (minus the .obj) does not match the source file base name will be compiled separately.

As always patches are welcome to add this in a more general fashion.

Answer 3

In general this should be left up to the program developer. Trying to compile all together in an amalgamation may introduce unintended behaviour to the program if it even compiles in the first place. Your best bet if you want this kind of optimisation without editing the source yourself is to use a compiler with inter-process optimisation like icc -ipo .

Example where an amalgamation of two .c files would not compile is for example if they use two identical static symbols with different functionality.