Re: [PATCH] ARM: do not assemble iwmmxt.S with LLVM toolchain

[Nathan Chancellor]

On Thu, Apr 09, 2020 at 05:01:33PM -0700, 'Nick Desaulniers' via Clang Built Linux wrote:
> On Thu, Apr 9, 2020 at 4:28 PM Jian Cai <caij2003@xxxxxxxxx> wrote:
> >
> > iwmmxt.S contains XScale instructions LLVM ARM backend does not support.
> > Skip this file if LLVM integrated assemmbler or LLD is used to build ARM
> 
> Hi Jian, thank you for the patch.  It's pretty much spot on for what I
> was looking for.  Some notes below:
> 
> s/assemmbler/assembler
> 
> :set spell
> 
> ;)
> 
> Also, could use a link tag, ie.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/975
> 
> (Always include the link tag to help us track when and where patches land).
> 
> Finally, I think the hunks for the two different files should be
> split; the init/Kconfig change should be it's own dedicated change
> that goes to Masahiro, the maintainer of the Kbuild tree.  Then when
> we have that, the dependent configs should go separately.  Would you
> mind splitting this patch in two, and re-sending just the Kbuild patch
> for now?  Since you used Sami's patch for inspiration
> (https://github.com/ClangBuiltLinux/linux/issues/975#issuecomment-611694153,
> https://github.com/samitolvanen/linux/commit/fe9786cb52a0100273c75117dcea8b8e07006fde),
> it would be polite to Sami to add a tag like:
> 
> Suggested-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
> 
> or maybe better yet, take Sami's patch, add your signed off by tag
> (maintaining him as the git author, see `git log --pretty=fuller`),
> then rebase your AS_IS_CLANG hunk on top, make that a second patch,
> then finally have the arm change as a third patch.
> 
> This change is exactly what I'm looking for, so these are just process concerns.

I think that they can be sent as a series that is picked up by Masahiro
with an ack from an ARM maintainer.

> > kernel.
> >
> > Signed-off-by: Jian Cai <caij2003@xxxxxxxxx>
> > ---
> >  arch/arm/Kconfig | 2 +-
> >  init/Kconfig     | 6 ++++++
> >  2 files changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> > index 66a04f6f4775..39de8fc64a73 100644
> > --- a/arch/arm/Kconfig
> > +++ b/arch/arm/Kconfig
> > @@ -804,7 +804,7 @@ source "arch/arm/mm/Kconfig"
> >
> >  config IWMMXT
> >         bool "Enable iWMMXt support"
> > -       depends on CPU_XSCALE || CPU_XSC3 || CPU_MOHAWK || CPU_PJ4 || CPU_PJ4B
> > +       depends on !AS_IS_CLANG && !LD_IS_LLD && (CPU_XSCALE || CPU_XSC3 || CPU_MOHAWK || CPU_PJ4 || CPU_PJ4B)
> >         default y if PXA27x || PXA3xx || ARCH_MMP || CPU_PJ4 || CPU_PJ4B
> >         help
> >           Enable support for iWMMXt context switching at run time if
> > diff --git a/init/Kconfig b/init/Kconfig
> > index 1c12059e0f7e..b0ab3271e900 100644
> > --- a/init/Kconfig
> > +++ b/init/Kconfig
> > @@ -19,6 +19,12 @@ config GCC_VERSION
> >  config CC_IS_CLANG
> >         def_bool $(success,$(CC) --version | head -n 1 | grep -q clang)
> >
> > +config AS_IS_CLANG
> > +       def_bool $(success,$(AS) --version | head -n 1 | grep -q clang)

$(AS) is being replaced with $(LLVM_IAS). That line should be:

    def_bool $(success,test $(LLVM_IAS) -eq 1)

I think. That depends on a commit in Masahiro's for-next branch [1] so
it should be based/tested against that.

Otherwise, I agree with Nick's comment about being split into two
patches (one for the init/Kconfig change and one for the ARM change) and
adding the Link tag.

I ran about 75 randconfigs with LD=ld.lld and LLVM_IAS=1 and I did not
see any Kconfig warnings from this and CONFIG_IWMMXT was unset in every
one. Should prevent the errors that you encountered. Feel free to apply
the following tags to any follow up revisions.

Tested-by: Nathan Chancellor <natechancellor@xxxxxxxxx> # randconfig
Reviewed-by: Nathan Chancellor <natechancellor@xxxxxxxxx>

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/masahiroy/linux-kbuild.git

> > +config LD_IS_LLD
> > +       def_bool $(success,$(LD) --version | head -n 1 | grep -q LLD)
> > +
> >  config CLANG_VERSION
> >         int
> >         default $(shell,$(srctree)/scripts/clang-version.sh $(CC))
> > --
> 
> -- 
> Thanks,
> ~Nick Desaulniers
> 

Cheers,
Nathan