[PATCH 0/2] Fully support standalone Clang/LLVM toolchains

[Bin Meng]

On Tue, Jul 6, 2021 at 2:47 PM Anup Patel <anup at brainfault.org> wrote:
>
> On Mon, Jul 5, 2021 at 6:45 PM Bin Meng <bmeng.cn at gmail.com> wrote:
> >
> > On Sun, Jul 4, 2021 at 1:56 AM Jessica Clarke <jrtc27 at jrtc27.com> wrote:
> > >
> > > This patch series is comprised of two parts.
> > >
> > > The first patch adds build system support for using Clang and LLVM
> > > binutils, provided your Clang is able to locate a libgcc.a in its search
> > > path.
> > >
> > > However, pure LLVM toolchains do not use libgcc, they use compiler-rt
> > > (libclang_rt.builtins.<arch>.a). We could change the Makefile to not
> > > hard-code -lgcc and instead use -print-libgcc-file-name, but that still
> > > requires a bare metal compiler-rt built for the right -march/-mabi to be
> > > present, which is often not the case. Moreover, we need very little from
> > > libgcc/compiler-rt; RV64 needs nothing, and RV32 only needs 64-bit
> > > division. Thus, the second patch vendors part of FreeBSD's libquad and
> > > stops linking against libgcc entirely, allowing OpenSBI to be built with
> > > just a cross-compiler. This means that building with any distro-provided
> > > LLVM just works, as does compiling with the system Clang compiler and
> > > LLD linker on FreeBSD without any external packages needed (beyond GNU
> > > make).
> >
> > I am concerned about removing libgcc. If we want to support running
> > OpenSBI on a processor without M extension, it would require us to
> > link with libgcc, or compiler-rt.
>
> We have mandated rv32ima or rv64ima as minimal requirement in
> docs/platform_requirements.md. The 'I', 'M' and 'A' are very fundamental
> extensions and we don't want to emulate these extensions in OpenSBI.
>

I don't understand why we have to emulate 'M' or 'A' in OpenSBI. It's
perfectly okay to run OpenSBI in a uniprocessor environment, all the
way up to the operating system which is configured to UP mode. No 'A'
extension will be used in the software stack hence no need to emulate
'A'.

> Further, a platform must support rv32gc or rv64gc to have platform
> firmware binaries as-part of OpenSBI release tarball. If a platform
> supports rv32ima or rv64ima but does not support rv32gc or rv64gc
> then users have to compile platform firmware binaries from sources.
>
> I think we are okay removing dependency on libgcc. since it allows
> us to have llvm toolchain support in OpenSBI. Overall, I am quite
> happy with this series.
>
> Is there a pre-built llvm risc-v toolchain ?? or steps to create one ??
>
> The BSD-2-clause is compatible with the BSD-3-clause used by
> PATCH2. Am I correct ?
>
> I hope we don't need to specifically mention anything in the
> "Copyright and License" section of the top-level README.md
> for BSD-3-clause licensed code ??

Regards,
Bin