[GSoC - x86_64 BSP] Using fPIC to compile RTEMS as a shared library

[Amaan Cheval]

Hi!

I figured I'd quickly confirm the direction I'm taking towards
compiling RTEMS as a dynamic/shared library.

Problems I've run into in setting up amd64.cfg to compile all of RTEMS
as a shared library:

- In the x86_64 tools, gcc's "-shared" flag has a different effect
than the "-Wl,-shared" flag used to pass the flag to the linker - the
former silently seems to compile a static library. The latter leads us
to our next issue

- Newlib seems to be compiled as a static libc.a. This leads to errors
such as the following:

https://gist.github.com/AmaanC/475bc0298697d22b944577ac80ec2736#file-rtems-make-fpic-log-L178

I believe this _should_ be solvable by compiling newlib as a shared
library, and _then_ linking the shared libc with RTEMS together. See
[1][2][3] for more.

Please let me know if that approach doesn't make sense - given that
there is no dynamic loader in the RTEMS kernel as far as I know, what
we really want _is_ a static file, but for it to be a relocatable PE,
we need to convince GCC to spit out a relocatable but fully resolved
shared library.

- Similarly, the gcc-compiled crtbegin and crtend also include static
relocations which are invalid when compiling with fPIC.

I think we can use crtbeginS.o and crtendS.o[4] in place of those -
that way, we might still be able to have GCC handle their inclusion,
without needing our bsp_specs. I'll look into this after figuring
newlib out.

-----------------------------------

I'd just like some confirmation on this being the correct path to
follow. I'm not quite sure, because if newlib is a shared library, I
think we'll need to divide the current build stage up to add stages
like:
- Compile librtemscpu.a and librtemsbsp.a with -fPIC
- Compile test_xyz.c while linking it with libc, librtems*, lib*efi,
etc. to create a fully resolved test_xyz.so
- Convert .so to PE using objcopy

Does that make sense to you all?

-----------------------------------

[1] https://cygwin.com/git/gitweb.cgi?p=newlib-cygwin.git;a=blob;f=newlib/README;h=e793d57ce75e56d1eb044e2c0325631e9eeef1af;hb=HEAD#l498
[2] https://sourceware.org/ml/newlib/2016/msg01106.html
[3] https://forum.osdev.org/viewtopic.php?p=276046&sid=c7798911615bef866354e92a64125b1c#p276046
[4] https://dev.gentoo.org/~vapier/crt.txtz