Re: Linux 6.3-rc3

From: Sedat Dilek <sedat.dilek@gmail.com>
To: Kalle Valo <kvalo@kernel.org>
Cc: Nathan Chancellor <nathan@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	David Airlie <airlied@gmail.com>, Daniel Vetter <daniel@ffwll.ch>,
	dri-devel@lists.freedesktop.org,
	linux-toolchains@vger.kernel.org, llvm@lists.linux.dev
Subject: Re: Linux 6.3-rc3
Date: Wed, 22 Mar 2023 17:40:15 +0100	[thread overview]
Message-ID: <CA+icZUUYyqhV2HFzVtpi_KjBoYxjk7OB0UBVd2mX6abjmYhDjg@mail.gmail.com> (raw)
In-Reply-To: <87pm91uf9c.fsf@kernel.org>

On Wed, Mar 22, 2023 at 1:49 PM Kalle Valo <kvalo@kernel.org> wrote:
>
> Nathan Chancellor <nathan@kernel.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@kernel.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