linux-kernel - Re: [PATCH v2] scripts/Makefile.clang: default to LLVM

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Date:   Tue, 10 Aug 2021 09:06:25 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Khem Raj <raj.khem@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Jonathan Corbet <corbet@....net>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:SIFIVE DRIVERS" <linux-riscv@...ts.infradead.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        linux-s390 <linux-s390@...r.kernel.org>
Subject: Re: [PATCH v2] scripts/Makefile.clang: default to LLVM_IAS=1

On Sat, Aug 7, 2021 at 4:53 AM Nathan Chancellor <nathan@...nel.org> wrote:
>
> On Fri, Aug 06, 2021 at 10:27:01AM -0700, Nick Desaulniers wrote:
> > LLVM_IAS=1 controls enabling clang's integrated assembler via
> > -integrated-as. This was an explicit opt in until we could enable
> > assembler support in Clang for more architecures. Now we have support
> > and CI coverage of LLVM_IAS=1 for all architecures except a few more
> > bugs affecting s390 and powerpc.
>
> The powerpc and s390 folks have been testing with clang, I think they
> should have been on CC for this change (done now).
>
> > This commit flips the default from opt in via LLVM_IAS=1 to opt out via
> > LLVM_IAS=0.  CI systems or developers that were previously doing builds
> > with CC=clang or LLVM=1 without explicitly setting LLVM_IAS must now
> > explicitly opt out via LLVM_IAS=0, otherwise they will be implicitly
> > opted-in.
> >
> > This finally shortens the command line invocation when cross compiling
> > with LLVM to simply:
> >
> > $ make ARCH=arm64 LLVM=1
> >
> > Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
>
> I am still not really sure how I feel about this. I would prefer not to
> break people's builds but I suppose this is inevitabile eventually.
>
> A little support matrix that I drafted up where based on ARCH and clang
> version for LLVM_IAS=1 support:
>
>              | 10.x | 11.x | 12.x | 13.x | 14.x |
> ARCH=arm     |  NO  |  NO  |  NO  |  YES |  YES |
> ARCH=arm64   |  NO  |  YES |  YES |  YES |  YES |
> ARCH=i386    |  YES |  YES |  YES |  YES |  YES |
> ARCH=mips*   |  YES |  YES |  YES |  YES |  YES |
> ARCH=powerpc |  NO  |  NO  |  NO  |  NO  |  NO  |
> ARCH=s390    |  NO  |  NO  |  NO  |  NO  |  NO  |
> ARCH=x86_64  |  NO  |  YES |  YES |  YES |  YES |
>
> The main issue that I have with this change is that all of these
> architectures work fine with CC=clang and their build commands that used
> to work fine will not with this change, as they will have to specify
> LLVM_IAS=0. I think that making this change for LLVM=1 makes sense but
> changing the default for just CC=clang feels like a bit much at this
> point in time. I would love to hear from others on this though, I am not
> going to object much further than this.
>
> Regardless of that concern, this patch does what it says so:
>
> Reviewed-by: Nathan Chancellor <nathan@...nel.org>


Applied to linux-kbuild.
Thanks.


-- 
Best Regards
Masahiro Yamada