Implementation of a native context switch for Fiber on RISC-V

[denizzzka]

Same code can be tested using LDC: ldc-developers/ldc#4867

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[kinke]

Hey Denis, first of all sorry for this PR being ignored for so long. I can't really comment on the asm itself, but I take it you've tested it, so I think that's a start at the very least.

I don't think this change here works. $< in the recipe is the first input, so the new asm file would be ignored. You'd most likely either need a 2nd rule, or simply merge the new asm into the existing file, as for the existing archs needing asm. I know you don't like it ;) - but this is code hardly anyone will have to look at, so the file doesn't need to be super-pretty, and keeping it a single file means no build systems need to be adapted.

[denizzzka]

What if just don't change this line at all?

Actually, DMD won't use this file because it doesn't compile for RISC-V, LDC uses its own CMake scripts and GDC uses another approach at all for context switching code organisation (https://github.com/gcc-mirror/gcc/tree/master/libphobos/libdruntime/config)

[denizzzka]

$< in the recipe is the first input, so the new asm file would be ignored.

I checked target right now just by swapping these two .S files and druntime tests passed.
So, target $(ROOT)/threadasm$(DOTOBJ) works with all these files

[kinke]

No it doesn't work at all, as I said: https://github.com/dlang/dmd/actions/runs/14878562144/job/41780986635?pr=21353#step:10:78:

cc -c -m64 -fPIC -DHAVE_UNISTD_H -O3 --target=x86_64-darwin-apple src/core/thread/fiber/switch_context_riscv.S -o../generated/osx/release/64/threadasm.o

That 2nd PR of yours just proves that DMD doesn't need the current existing file at all.

[denizzzka]

it is possible to add a separate target for RISC-V context switch, but there is no point in this: DMD does not use this code anyway

I suggest then canceling this PR

[denizzzka]

Just leave ASM and .d files here, without changing the Makefile?

[kinke]

Again, if you just add the RISC-V asm to the existing file, no compilers will have to change anything.

[denizzzka]

GCC will do: they approach assumes splitting this ASM file into pieces for each architecture: https://github.com/gcc-mirror/gcc/tree/master/libphobos/libdruntime/config (Or maybe they wrote context switching from scratch, I don't really know.)

[kinke]

Oh I see! @ibuclaw: Have you split up this switch_context_asm.S for GDC? Or are your asm files any different?

[ibuclaw]

Some have likely diverged to support more platforms and assemblers.

Not everything in assembly is for fiber support. SystemZ has an asm implementation of get_tls_offset for example.