[PATCH -next v21 23/27] riscv: detect assembler support for .option arch

[Andy Chiu]

On Sun, Jan 21, 2024 at 10:55 AM Palmer Dabbelt <palmer at dabbelt.com> wrote:
>
> On Sat, 20 Jan 2024 17:13:41 PST (-0800), ebiggers at kernel.org wrote:
> > Hi Andy,
> >
> > On Mon, Jun 05, 2023 at 11:07:20AM +0000, Andy Chiu wrote:
> >> +config AS_HAS_OPTION_ARCH
> >> +    # https://reviews.llvm.org/D123515
> >> +    def_bool y
> >> +    depends on $(as-instr, .option arch$(comma) +m)
> >> +    depends on !$(as-instr, .option arch$(comma) -i)
> >
> > With tip-of-tree clang (llvm-project commit 85a8e5c3e0586e85), I'm seeing
> > AS_HAS_OPTION_ARCH be set to n.  It's the second "depends on" that makes it be
> > set to n, so apparently clang started accepting ".option arch -i".  What was
> > your intent here for checking that ".option arch -i" is not supported?  I'd
> > think that just the first "depends on" would be sufficient.

The reason why I added the second check is because clang and gcc only
return an assembler warning when it does not support ".option arch" at
all. No errors were reported. So, it ended up passing the first
condition check for old toolchains that shouldn't be passing.

>
> I'm not sure what Andy's rationale was, but de3a913df6e ("RISC-V:
> Clarify the behavior of .option arch directive.") in binutils-gdb
> stopped accepting `.option arch, -i` along with fixing a handful of
> other oddities in our `.option arch` handling.
>
> If that's all this is testing for then we should probably add some sort
> of version check for old binutils (or maybe just ignore it, looks like
> it was a bugfix and the old version was never released).
>
> +Nelson, as he probably knows better than I do.
>
> That said: what does LLVM do if you ask it to turn the "I" base ISA off?
> I'd argue there's no instructions left at that point...

Maybe what we really should do is to upgrade the condition check to a
one liner shell script and grep if "Warning" is being printed. Sadly
this warning is not failing the compilation with -Werror.

I can try forming a patch on this if it feels alright to people.

Thanks,
Andy