Re: Introduction + interest in a riscv64 port of GNU Mach/Hurd

On Tue, Aug 25, 2026 at 4:47 PM Hakan Candar <[email protected]> wrote:
> That's funny, I actually never used 'la' myself. Does GCC/Clang
> even use that at all? I thought everyone does auipc+addi, as that's what
> the compilers output in general. When I'm writing hand-written assembly,
> I try to match how the compiler does these, as using the assembler
> directives can generate relocations that do not work on non-PIC code.
> Explicitly writing the auipc+addi sequence also is clearer imo and
> clearly distinguishes whether something is a GOT or a direct load.

GCC appears to emit 'lla' and 'la' (but not 'lga', which as I read is
a more recent addition to binutils).

> The current toolchains that target GNU/Hurd (Clang and GCC) use
> whatever ABI GNU/Linux uses, so that'd be the gp-as-reserved-register
> case, I think? If we were to change that, we need to send patches
> to GCC and LLVM, changing the default Mach ABI.

Well, we generally try to stick close to the ABI that Linux uses
(calling convention-wise), because why wouldn't we. But it's not
automatic, the *-gnu target has to be defined in GCC & binutils, even
if it's defined similarly to the *-linux-gnu and the BSD targets. See
gcc:gcc/config/aarch64/aarch64-gnu.h for example. So we need to submit
our target upstream either way. (And there's no default Mach ABI, but
there are targets for GNU/Hurd systems on various architectures.)

In case of gp relaxation, I believe LLVM already defaults to not doing
it, but binutils (bfd) defaults the other way around
(riscv_elf_params.relax_gp = 1), we could maybe switch that around for
riscv-gnu, but still pass --relax-gp when building gnumach.

But really, this should be a decision that somebody actually familiar
with RISC-V (and toolchains and ABIs) makes, I'm just floating the
idea.

> I think we should
> keep the current behaviour as it'd be consistent with the GNU/Linux ABI,
> but that is just my opinion.
>
> Hakan

Sergey