[PATCH] RISC-V: Check the Zca extension when disabling the C extension using .option norvc.

[Nelson Chu]

Hi,

According to the newest ISA spec,
https://github.com/riscv/riscv-isa-manual/blob/main/src/zc.adoc, the C is
actually same as Zca, Zcf and Zcd, so this change looks conflict.  The
original code should be correct, the RVC flag means enable the C extension,
so NORVC means disable C, and it makes sense to disable the RVC flag.  But
this patch also reminds us that the current toolchain needs to be updated
to the newest spec.

1. C needs imply Zca, Zcf and Zcd, so the implicit table needs to be
updated.

2. Removing extensions using ".option arch" is more complicated than the
first implemented.  For this case, C is the set of Zca, Zcf and Zcd, so we
need to imply those extensions when adding C, and should also need to
remove those extensions when removing C.  That is - riscv_update_subset
(&riscv_rps_as, "-c,-zca,-zcf,-zcd") for .option norvc.  But unfortunately
it's not always work to directly remove the extension from the implicit
table in reverse.  For example, zcd implies d and zca, but remove zcd
doesn't mean remove d since zcd isn't a super set of d and zca.

cc Andrew in case I am wrong ;)

Thanks
Nelson


On Fri, Oct 4, 2024 at 3:12 PM Ethan Yu-Cheng Liang(梁淯程) <
ycl669@andestech.com> wrote:

> .option norvc disables only the C extension, not all 16-bit instructions.
>
> Signed-off-by: Ethan Yu-Cheng Liang <ycl669@andestech.com>
> ---
>  gas/config/tc-riscv.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/gas/config/tc-riscv.c b/gas/config/tc-riscv.c
> index ef455e449b9..4ef7b8e689e 100644
> --- a/gas/config/tc-riscv.c
> +++ b/gas/config/tc-riscv.c
> @@ -4917,7 +4917,7 @@ s_riscv_option (int x ATTRIBUTE_UNUSED)
>      {
>        riscv_update_subset (&riscv_rps_as, "-c");
>        riscv_set_arch_str (&riscv_rps_as.subset_list->arch_str);
> -      riscv_set_rvc (false);
> +      riscv_set_rvc (riscv_subset_supports (&riscv_rps_as, "zca"));
>      }
>    else if (strcmp (name, "pic") == 0)
>      riscv_opts.pic = true;
> --
> 2.45.2
>
> CONFIDENTIALITY NOTICE:
>
> This e-mail (and its attachments) may contain confidential and legally
> privileged information or information protected from disclosure. If you are
> not the intended recipient, you are hereby notified that any disclosure,
> copying, distribution, or use of the information contained herein is
> strictly prohibited. In this case, please immediately notify the sender by
> return e-mail, delete the message (and any accompanying documents) and
> destroy all printed hard copies. Thank you for your cooperation.
>
> Copyright ANDES TECHNOLOGY CORPORATION - All Rights Reserved.
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://sourceware.org/pipermail/binutils/attachments/20241008/3a4f6733/attachment-0001.htm>