devkitPro

I want to mod a game, but I'm having trouble recompiling after decompiling. I've decompiled my rom with luvdis (https://github.com/aarant/luvdis), which outputs a .s file. Luvdis claims that the decompiled output should recompile identically to the original rom. However, I was unable to recompile my rom (without any modifications). The new rom and the old rom do not match and the new rom doesn't boot in any emulator.

How do I compile a .s file to a useable .gba rom? Am I even approaching this the right way?

I tried luvdis with some devkitARM compiled .gba files and managed to build identical ROMs OK. These are the commands I used to recompile the source.
There's a warning: cannot find entry symbol _start but that's benign.