The QT-Chooser package on a system I use refused to compile for the longest time. Checking the make.conf file I realized I had both -mtune=atom and -march=atom set for the CFLAGS variable which was causing problems with the package when I attempted to compile it. After changing -mtune to generic (-mtune=generic), I removed -march and emerged @world with only a few minor hiccups.

###-mtune vs. -march

What are these options and how to they effect things?

-mtune and -march are options that can be passed to the GCC compiler at run time in order to specific generate instructions for the machine’s CPU.

-mtune “tunes the generated code for the specified cpu-type.” mtune is magical because it has a “generic” option, which allows the system to compile code that can work on a verity of modern CPUs (IA32/AMD64/EM64T).

-march “allows GCC to generate code that may not run at all on processors other than the one indicated.”

In summary use -mtune=generic if:

  • You want your compiled package to work on machines other than your machine;
  • To choose a setting that doesn't break on any packages;
  • An easy, no hassle compile solution to most packages.
  • If you are planning on using distcc. Use **-march=** if:
    • You have a specific CPU that code should be compiled for;
    • To have the smallest, fastest, lightest binaries specific to the system's CPU.
    • If you are not concerned about using Distcc. Both the flags can actually be used together. For the list of all available configuration options (and better explanations than I can provide) see GCC's [i386 and x86-64 Options]( page. Maffblaster