Re: [PATCH] invoke: RISC-V's -march doesn't take ISA strings

["Christoph Müllner" <christoph.muellner@vrull.eu>]

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On Mon, Nov 7, 2022 at 8:01 PM Palmer Dabbelt <palmer@rivosinc.com> wrote:

> The docs say we take ISA strings, but that's never really been the case:
> at a bare minimum we've required lower case strings, but there's
> generally been some subtle differences as well in things like version
> handling and such.  We talked about removing the lower case requirement
> in the last GNU toolchain meeting and we've always called other
> differences just bugs.  We don't have profile support yet, but based on
> the discussions on the RISC-V lists it looks like we're going to have
> some differences there as well.


> So let's just stop pretending these are ISA strings.  That's been a
> headache for years now, if we're meant to just be ISA-string-like here
> then we don't have to worry about all these long-tail ISA string parsing
> issues.
>

You are right, we should first properly specify the -march string,
before we talk about the implementation details of the parser.

I tried to collect all the recent change requests and undocumented
properties of the -march string and worked on a first draft specification.
As the -march flag should share a common behavior across different
compilers and tools, I've made a PR to the RISC-V toolchain-conventions
repo:
  https://github.com/riscv-non-isa/riscv-toolchain-conventions/pull/26

Do you mind if we continue the discussion there?



>
> Link: https://lists.riscv.org/g/sig-toolchains/message/486
>
> gcc/ChangeLog
>
>         doc/invoke.texi (RISC-V): -march doesn't take ISA strings.
>
> ---
>
> This is now woefully under-documented, as we can't even fall back on the
> "it's just an ISA string" excuse any more.  I'm happy to go document
> that, but figured I'd just send this along now so we can have the
> discussion.
> ---
>  gcc/doc/invoke.texi | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index 94a2e20cfc1..780b0364c52 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -28617,11 +28617,11 @@ Produce code conforming to version 20191213.
>  The default is @option{-misa-spec=20191213} unless GCC has been configured
>  with @option{--with-isa-spec=} specifying a different default version.
>
> -@item -march=@var{ISA-string}
> +@item -march=@var{target-string}
>  @opindex march
> -Generate code for given RISC-V ISA (e.g.@: @samp{rv64im}).  ISA strings
> must be
> -lower-case.  Examples include @samp{rv64i}, @samp{rv32g}, @samp{rv32e},
> and
> -@samp{rv32imaf}.
> +Generate code for given target (e.g.@: @samp{rv64im}).  Target strings
> are
> +similar to ISA strings, but must be lower-case.  Examples include
> @samp{rv64i},
> +@samp{rv32g}, @samp{rv32e}, and @samp{rv32imaf}.
>
>  When @option{-march=} is not specified, use the setting from
> @option{-mcpu}.
>
> --
> 2.38.1
>
>