How can I use g++ to get assembly code for a C++ header (.hpp) file?

Question

I'm interested in trying to see the assembly code for my header file. Playing around with a simple .cpp file with a dummy main method and compiling it is easy: g++ -S -o prog.exe main.cpp

However, I'm going into header files now. I have a dummy header file func.hpp that just contains a dummy method and now I want to be able to compile it and see its assembly in a .S file. (The main reason for this is so that if I have a more complicated function, maybe I can do some manual optimization in assembly and just build the final executable through g++ -o prog.exe func.S main.cpp.

However, I am not able to find sources that explain this. If I try g++ -S -o func.S func.hpp I get an error message output filename specified twice. Can anyone help me with this?

For reference, here is func.hpp:

int number()
{
    return 0;
}

I'm also trying it out with another hpp file named struct.hpp:

struct Coord
{
    int x;
    int y;
};

Same error for both, but I want to be able to see the assembly for both headers without converting them to .cpp files (because that breaks the meaning of a header file.)

does it start working if you create a .cpp file which does _nothing_ but `#include` the header, and run `g++ -S` on that? — Useless, Sep 20 '12 at 13:30

score 6 · Accepted Answer · edited May 23 '17 at 12:15

You could force g++ to treat the .hpp file as a C++ source file.

g++ -x c++ -S -o func.S func.hpp

The reason you get that "output filename specified twice" message is because, when passing a header file to gcc, it will assume you want to compile it into a pre-compiled header. Just as named, this will "compile" the headers, so a *.s file will be generated in the middle. If you pass the -v flag with the original command, you will see that cc1plus is called like:

 /usr/lib/gcc/x86_64-unknown-linux-gnu/4.7.1/cc1plus
     -quiet
     -v 
     -D_GNU_SOURCE 
     func.hpp
     -quiet 
     -dumpbase func.hpp 
     -mtune=generic 
     -march=x86-64 
     -auxbase-strip func.S
     -version 
     -o func.S                # <---
     -o /tmp/ccxEg3J7.s       # <---
     --output-pch= func.S

So indeed -o is specified twice, once from your -o and once from the compiler.

(Of course the compiler could avoid adding the -o /tmp/ccxEg3J7.s, but the added complexity for this edge case isn't quite worth it.)

That's right. Otherwise, it thinks you're asking it to create a precompiled header file, so it behaves as if you'd specified `-x c++-header`. I think this question still indicates a bug in how g++ generates cc1plus commands for header files, but at least there's a workaround. — Rob Kennedy, Sep 20 '12 at 13:39

How can I use g++ to get assembly code for a C++ header (.hpp) file?

1 Answers1