linux-kernel - Re: Linux 6.3-rc3

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Message-ID: <CA+icZUUYyqhV2HFzVtpi_KjBoYxjk7OB0UBVd2mX6abjmYhDjg@mail.gmail.com>
Date:   Wed, 22 Mar 2023 17:40:15 +0100
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Kalle Valo <kvalo@...nel.org>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org, linux-toolchains@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: Linux 6.3-rc3

On Wed, Mar 22, 2023 at 1:49 PM Kalle Valo <kvalo@...nel.org> wrote:
>
> Nathan Chancellor <nathan@...nel.org> writes:
>
> > On Mon, Mar 20, 2023 at 11:26:17AM -0700, Linus Torvalds wrote:
> >> On Mon, Mar 20, 2023 at 11:05 AM Nathan Chancellor <nathan@...nel.org> wrote:
> >> >
> >> > On the clang front, I am still seeing the following warning turned error
> >> > for arm64 allmodconfig at least:
> >> >
> >> >   drivers/gpu/host1x/dev.c:520:6: error: variable 'syncpt_irq' is uninitialized when used here [-Werror,-Wuninitialized]
> >> >           if (syncpt_irq < 0)
> >> >               ^~~~~~~~~~
> >>
> >> Hmm. I do my arm64 allmodconfig builds with gcc, and I'm surprised
> >> that gcc doesn't warn about this.
> >
> > Perhaps these would make doing allmodconfig builds with clang more
> > frequently less painful for you?
> >
> > https://lore.kernel.org/llvm/20230319235619.GA18547@dev-arch.thelio-3990X/
>
> Thank you, at least for me this is really helpful. I tried now clang for
> the first time but seeing a strange problem.
>
> I prefer to define the compiler in GNUmakefile so it's easy to change
> compilers and I don't need to remember the exact command line. So I have
> this in the top level GNUmakefile (all the rest commented out):
>
> LLVM=/opt/clang/llvm-16.0.0/bin/
>

Welcome to the LLVM/Clang world!

First try - First Cry...

In my build-environment I add (export) /path/to/llvm/bin to $PATH and
pass single CC LD AR etc. (what is substituted by LLVM=1):

make CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \
 OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump READELF=llvm-readelf \
 HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar HOSTLD=ld.lld

Equivalent to:

make LLVM=1

I cannot comment on `make LLVM=/path/to/llvm/` and/or combinations
with `LLVM=1` as I have never used it

> If I run 'make oldconfig' it seems to use clang but after I run just
> 'make' it seems to switch back to the host GCC compiler and ask for GCC
> specific config questions again. Workaround for this seems to be adding
> 'export LLVM' to GNUmakefile, after that also 'make' uses clang as
> expected.
>

You have to pass `make LLVM=1` in any case... to `oldconfig` or when
adding any MAKEFLAGS like -j${number-of-available-cpus}.

Hope that helps.

Best regards,
-Sedat-

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/kbuild/llvm.rst
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/kbuild/llvm.rst#n52