DS: Hack to work around ARM/Thumb relocation issues in the SCUMM engine

[ccawley2011]

Having all libc functions referenced in the main executable works around the issue described in Trac #14554. This allows the SCUMM engine to run correctly, instead of crashing when launching a game.

If this PR is accepted, I plan to create the v2.7.1 release build with this change applied. Hopefully a proper fix will be available before v2.8.0, but I don't know how to do this myself.

[lephilousophe]

There is the is the --whole-archive option which would include the full libc. This may not be efficient.
Instead of calling the functions, maybe just create something like that?

void *pluginHack[] = {
    atan2,
    modf,
};

I don't see something easy to force include missing symbols in libc.
Maybe we could add a post-build check for platforms using our ELF loader which checks that no symbol is missing from the main binary?

[sev-]

Instead of calling the functions, maybe just create something like that?

@lephilousophe what is the difference when the function is not called anyway?

[lephilousophe]

It avoids the compiler to emit a function with a sequence of call instructions.
Plus, it also avoids us to write the calls taking care of the number of arguments and cheating with volatile to fight call optimization.
By creating the array, we force the function inclusion because it's referenced and the result is only a sequence of pointers and they are never optimized.

[sev-]

Good points. @ccawley2011 what do you think?

[bluegr]

It avoids the compiler to emit a function with a sequence of call instructions. Plus, it also avoids us to write the calls taking care of the number of arguments and cheating with volatile to fight call optimization. By creating the array, we force the function inclusion because it's referenced and the result is only a sequence of pointers and they are never optimized.

@lephilousophe wait, but we want to have these functions optimized, right? Is there a benefit in not optimizing them?

[lephilousophe]

@lephilousophe wait, but we want to have these functions optimized, right? Is there a benefit in not optimizing them?

No, we just want them to be bundled inside the main binary so that plugins can make use of them.
What I meant by optimization, is that the compiler will always try to optimize the pluginHack function to avoid by all means to issue the calls to the functions we want.
That's why @ccawley2011 had to use volatile trick to force the compiler to do the call (and so to include the function in our binary).

[sev-]

Okay, merging this. Thank you!