Date:   Mon, 20 Mar 2023 11:53:37 -0700
From:   Nathan Chancellor <nathan@kernel.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     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
Message-ID: <20230320185337.GA615556@dev-arch.thelio-3990X>
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 <20230320180501.GA598084@dev-arch.thelio-3990X>
 <CAHk-=wgSqpdkeJBb92M37JNTdRQJRnRUApraHKE8uGHTqQuu2Q@mail.gmail.com>
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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/

> That syncpt_irq thing isn't written to anywhere, so that's pretty egregious.
> 
> We use -Wno-maybe-uninitialized because gcc gets it so wrong, but
> that's different from the "-Wuninitialized" thing (without the
> "maybe").
> 
> I've seen gcc mess this up when there is one single assignment,
> because then the SSA format makes it *so* easy to just use that
> assignment out-of-order (or unconditionally), but this case looks
> unusually clear-cut.
> 
> So the fact that gcc doesn't warn about it is outright odd.
> 
> > If that does not come to you through other means before -rc4, could you
> > just apply it directly so that I can stop applying it to our CI? :)
> 
> Bah. I took it now, there's no excuse for that thing.

Thanks!

> Do we have any gcc people around that could explain why gcc failed so
> miserably at this trivial case?

Cc'ing linux-toolchains. The start of the thread is here:

https://lore.kernel.org/CAHk-=wgSqpdkeJBb92M37JNTdRQJRnRUApraHKE8uGHTqQuu2Q@mail.gmail.com/

The problematic function before the fix is here:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/gpu/host1x/dev.c?id=3d3699bde4b043eea17993e4e76804a8128f0fdb#n487

I will see if I have some cycles to try and reduce something out for the
GCC folks.

Cheers,
Nathan