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

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

[-- Attachment #1: Type: text/plain, Size: 4403 bytes --]

On Wed, Nov 9, 2022 at 4:00 AM Palmer Dabbelt <palmer@rivosinc.com> wrote:

> On Tue, 08 Nov 2022 05:40:10 PST (-0800), christoph.muellner@vrull.eu
> wrote:
> > 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?
>
> IMO trying to handle this with another RISC-V spec is a waste of time:
> we've spent many years trying to follow the specs here, it's pretty
> clear they're just not meant to be read in that level of detail.  This
> sort of problem is all over the place in RISC-V land, moving to a
> different spec doesn't fix the problem.
>

I created the documentation as a response of your comment in your patch
about
the flag being "woefully under-documented".
You can call my attempt to address this a "waste of time", but a more
constructive
approach would be appreciated.

The reason I created a PR over there in the riscv-toolchain-conventions
repo is,
that it is the agreed place to document the common behavior of RISC-V
compilers/tools (e.g. command line flags).
I.e. to ensure that LLVM developers can also contribute to a common
solution.

If I understand correctly, you want something between the documentation that
you wrote as part of this patch and the PR that I created.
If so, then please let me know the details you don't want to have documented
in my proposal.

Anyway, thanks for your feedback.
I'll quote/reference it in the PR so it won't get lost.


>
> >> 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
> >>
> >>
>