[Freemint-discuss] New m68k-atari-mintelf toolchain with PRG/ELF format

[Vincent R. <vin...@fr...>]

Dear FreeMiNT users,

I've designed a new executable format named PRG/ELF (a.k.a 
elf32-atariprg). It is 100% PRG and 99% ELF. Basically, it is a standard 
ELF executable embedded into a PRG. It has a PRG header at the 
beginning, followed by ELF data, then a PRG relocation table as usual. 
As it is a valid PRG, it can be loaded as usual by TOS or FreeMiNT. 
Because of the PRG header, standard ELF tools can't recognize such 
binary as ELF. But after a tiny patch to skip the PRG header, they just 
work. So such executables provide the best of the 2 worlds: full PRG 
compatibility, and almost full ELF compatibility after minor patch of 
the ELF tools.

Nowadays, ELF is the standard format for .o files and executables. It is 
used by Linux and many other systems. Key point is the ability to 
organize data in arbitrary "sections" stored in the files. Some sections 
are automatically loaded into memory by the operating system. Other 
sections can contain additional data, not loaded, such as debug information.

The trick: in an ELF executable, loaded sections are always stored 
before non-load sections. So I just arranged things to fit loaded 
sections into the PRG TEXT/DATA segments, and non-loaded sections into 
the PRG symbols table. As the symbols table is never loaded by the OS, 
it just works: such extra data doesn't waste any RAM (except the small 
ELF headers at the beginning). I'll give more detailed technical 
information later.

So I'm glad to announce a brand-new toolchain with GCC 13:
m68k-atari-mintelf toolchain with PRG/ELF format

It is available on that new page:
http://vincent.riviere.free.fr/soft/m68k-atari-mintelf/

As usual, Cygwin and Ubuntu binaries are provided. You can safely 
install them side-by-side with the old m68k-atari-mint a.out toolchain, 
they won't interfere.

This toolchain features:

- binutils 2.41
gas is configured to produce ELF object files. And I have patched the 
linker to produce PRG/ELF executables. This is a brand-new patch for the 
ELF linker, I have completely reworked it. There is no more a.out stuff 
in that patch, so we don't have to fear obsolescence anyore. 
Fortunately, there are some similarities between the a.out and ELF 
targets, so I've been able to reuse some code from Guido's original 
patch. The tricky part was to convince ld to write a valid ELF header 
after the beginning of the file. But after some efforts, it finally worked.
Note that I haven't enabled any a.out stuff. So you can't reuse old 
a.out libraries (but you can rebuild them).

- MiNTBin
I have patched the tools such as m68k-atari-mintelf-stack and 
m68k-atari-mintelf-flags to support the new PRG/ELF file format. This 
was a piece of cake. I also kept support for the old a.out files, so 
those tools can still be used on any PRG.

- GCC 13.2.0
The GCC patch has always been rather thin (as most of the magic resides 
in the binutils). Thorsten has regularly tested new GCC versions, and 
added many features on his own fork. In the version I provide here, I 
have stripped the GCC patch to a maximum, just to enable support for the 
new m68k-atari-mintelf target. I thank Thorsten for his efforts, 
specially on the C++ library support. So I've been able to quickly use 
his GCC before stripping it down. Of course features could easily be 
reintroduced.

- FDLIBM
Oh, I have finally dropped PML. Many thanks to Thorsten for his hard 
work on this math library. It works out of the box.

- There is also gemlib, cflib, gemma, as usual.

So, how to try this new toolchain? Install it. Then use 
m68k-atari-mintelf-gcc (instead of the old m68k-atari-mint-gcc). Nothing 
more, it just works. I've been able to build a few programs, as well as 
EmuTOS and FreeMiNT (*). I only made limited testing. Specially, I 
haven't tested ColdFire binaries. Nor corner cases such as cpx, ldg, or 
slb. But we will see shortly.

With the obsolescence of a.out in binutils and GCC, we had to find a 
solution. ELF was the obvious choice. I always thought it was possible 
to build hybrid PRG/ELF binaries. Now, this is done. We may find a few 
bugs, but I consider the binutils patch as mature. I expected it to 
become the new standard for next decades.

(*) I encountered minor issues when building FreeMiNT with GCC 13. I 
will report them in a separate message.

Another note: back to 2009, I already made experiments with a mintelf 
target. It used ELF object files, but a.out PRG files. This was possible 
as the GNU linker is able to mix different object file formats. But this 
was only half-ELF. Thorsten still builds that target. I kept the same 
m68k-atari-mintelf triplet to identify the new PRG/ELF target, because 
this is really the sequel of that old one, with full ELF support.

Enjoy 😃

-- 
Vincent Rivière