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From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Sun, 6 Dec 2020 12:09:31 -0800
Message-ID: <CABCJKue9TJnhge6TVPj9vfZXPGD4RW2JYiN3kNwVKNovTCq8ZA@mail.gmail.com>
Subject: Re: [PATCH v8 00/16] Add support for Clang LTO
To:     Nathan Chancellor <natechancellor@gmail.com>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Will Deacon <will@kernel.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        linux-arch <linux-arch@vger.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@lists.infradead.org>,
        linux-kbuild <linux-kbuild@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        PCI <linux-pci@vger.kernel.org>, Jian Cai <jiancai@google.com>,
        Kristof Beyls <Kristof.Beyls@arm.com>
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On Sat, Dec 5, 2020 at 10:50 PM Nathan Chancellor
<natechancellor@gmail.com> wrote:
>
> On Fri, Dec 04, 2020 at 02:52:41PM -0800, Sami Tolvanen wrote:
> > On Thu, Dec 3, 2020 at 2:32 PM Nick Desaulniers <ndesaulniers@google.com> wrote:
> > >
> > > So I'd recommend to Sami to simply make the Kconfig also depend on
> > > clang's integrated assembler (not just llvm-nm and llvm-ar).
> >
> > Sure, sounds good to me. What's the preferred way to test for this in Kconfig?
> >
> > It looks like actually trying to test if we have an LLVM assembler
> > (e.g. using $(as-instr,.section
> > ".linker-options","e",@llvm_linker_options)) doesn't work as Kconfig
> > doesn't pass -no-integrated-as to clang here.

After a closer look, that's actually not correct, this seems to work
with Clang+LLD no matter which assembler is used. I suppose we could
test for .gasversion. to detect GNU as, but that's hardly ideal.

> >I could do something
> > simple like $(success,echo $(LLVM) $(LLVM_IAS) | grep -q "1 1").
> >
> > Thoughts?
> >
> > Sami
>
> I think
>
>     depends on $(success,test $(LLVM_IAS) -eq 1)
>
> should work, at least according to my brief test.

Sure, looks good to me. However, I think we should also test for
LLVM=1 to avoid possible further issues with mismatched toolchains
instead of only checking for llvm-nm and llvm-ar.

Sami