Re: P.S. Follow up to my last post

[Jan Beulich <jbeulich@suse.com>]

On 25.08.2023 21:44, jacob navia wrote:
> I am just invoking the assembler with its default settings:
> 
> ~/star64 $ as -o test.o test.s
> 
> The assembler is testing the default architecture here:
> Function: riscv_ip
> 
> 	for (; insn && insn->name && strcmp(insn->name,str) == 0; insn++) {
> #if 0
> 		if ((insn->xlen_requirement != 0) && (xlen != insn->xlen_requirement))
> 			continue;
> 
> 		if (!riscv_multi_subset_supports(&riscv_rps_as,insn->insn_class)) {
> 			error.missing_ext = riscv_multi_subset_supports_ext(&riscv_rps_as,
> 							    insn->insn_class);
> 			continue;
> 		}
> #endif
> 
> The #if 0/#endif were added by me. I think those tests do not belong here. The assembler should assemble and not test if the instruction is legal. If the instruction is illegal the program will crash at run-time, something not that unusual if you are programming in assembler!

Having a program crash at runtime when this can be avoided at build time is
deemed undesirable by many people, including me. I did point out before that
there are two approaches for assemblers - default-enabling everything and
default-disabling everything except the base architecture insns. To a large
part this is a decision to be taken by the people writing an assembler. But
there are technical constraints as well: As soon as you have two conflicting
extensions (which RISC-V was always allowing for, and which it now actively
has), the former model won't work very well anymore.

Of course you're free to propose a patch allowing the alternative model (via
command line option and/or directive). Obviously whether that'll be approved
is unknown up front.

> And if you are assembling compiler output, it is up to the compiler to furnish the correct instructions.

Yes. Plus, with an assembler following the former model, the necessary
directives. (As an aside, compilers aren't bug-free, and hence the assembler
still applying some sanity checks makes some sense.)

> Tiny-asm doesn’t test any instruction compliance and that is why it works out of the box!

As said, which model to use is up to the writers of any particular assembler.

Jan