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.

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)

$< 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

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.

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

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

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

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.)

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

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.