Is there a way to get warned about unused functions?

Caolan Mc Namara, a LibreOffice developer, has made a small tool to detect this type of thing in LibreOffice source code. They had around thousands functions & methods unused in LibreOffice. His tool is a key element for removing them.

It's called callcatcher. It can

collect functions/methods defined and subtract called/referenced

It works directly on assembler output and so, it works only for x86 and x86_64 architecture. It can produce output like this. You can integrate it with your traditional compiling and linking call to gcc.

Caolan agrees that it should become a gcc plugin.

I know you asked for warnings and prefers not to use gcc option but it is really easy.

You can use linker optimization (--gc-sections) in order to remove the dead code from your application.

From gcc's man page:

--gc-sections --no-gc-sections Enable garbage collection of unused input sections. It is ignored on targets that do not support this option. The default behaviour (of not performing this garbage collection) can be restored by specifying --no-gc-sections on the command line.

--gc-sections decides which input sections are used by examining symbols and relocations. The section containing the entry symbol and all sections containing symbols undefined on the command-line will be kept, as will sections containing symbols referenced by dynamic objects. Note that when building shared libraries, the linker must assume that any visible symbol is referenced. Once this initial set of sections has been determined, the linker recursively marks as used any section referenced by their relocations. See --entry and --undefined.

This option can be set when doing a partial link (enabled with option -r). In this case the root of symbols kept must be explicitly specified either by an --entry or --undefined option or by a "ENTRY" command in the linker script.