RE: [PATCH] Makefile.extrawarn: Move -Wunaligned-access to W=2

[David Laight <David.Laight@ACULAB.COM>]

From: Nathan Chancellor
> Sent: 02 February 2022 16:26
> 
> On Wed, Feb 02, 2022 at 09:12:06AM +0100, Arnd Bergmann wrote:
> > On Wed, Feb 2, 2022 at 12:22 AM Nathan Chancellor <nathan@kernel.org> wrote:
> > >
> > > -Wunaligned-access is a new warning in clang that is default enabled for
> > > arm and arm64 under certain circumstances within the clang frontend (see
> > > LLVM commit below). Under an ARCH=arm allmodconfig, there are
> > > 1284 total/70 unique instances of this warning (most of the instances
> > > are in header files), which is quite noisy.
> > >
...
> >
> > The warning seems important enough to be considered for W=1 on 32-bit arm,
> > otherwise the chances of anyone actually fixing drivers for it is
> > relatively slim.
> 
> Fair point, I suppose barely anyone does W=2 builds, which means we
> might as well just disable it outright.
> 
> > Can you point post the (sufficiently trimmed) output that you get with
> > the warning
> > enabled in an allmodconfig build?
> 
> Sure thing.
> 
> Here is the trimmed version:
> 
> https://gist.github.com/nathanchance/6682e6894f75790059ca698c29212c71/raw/f63d54819afeb96f3ea0bb055096
> 849912ac0185/trimmed.log

So it is objecting to non-packed structures inside packed ones?
I'd have thought it ought to generate the byte accesses for code
that references the data from the __packed structure.

Recent gcc enable -Waddress-of-packed-member (even on x86) if you
take the address to pass somewhere else.
That is a bug if CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS=n.

...
> There is this comment above this block in the source code:
> 
>     // Assume pre-ARMv6 doesn't support unaligned accesses.
>     //
>     // ARMv6 may or may not support unaligned accesses depending on the
>     // SCTLR.U bit, which is architecture-specific. We assume ARMv6
>     // Darwin and NetBSD targets support unaligned accesses, and others don't.
>     //
>     // ARMv7 always has SCTLR.U set to 1, but it has a new SCTLR.A bit
>     // which raises an alignment fault on unaligned accesses. Linux
>     // defaults this bit to 0 and handles it as a system-wide (not
>     // per-process) setting. It is therefore safe to assume that ARMv7+
>     // Linux targets support unaligned accesses. The same goes for NaCl
>     // and Windows.
>     //
>     // The above behavior is consistent with GCC.
> 
> I notice that CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS under certain
> conditions in arch/arm/Kconfig. Would it be worth telling clang that it
> can generate unaligned accesses in those cases via -munaligned-access or
> would that be too expensive? If we did, these warnings would be
> eliminated for configs with CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS=y,
> then it could be safely placed under W=1.

You really never want to do the fixup.
Except, maybe, if it is a very common path and the data is actually aligned
99.9999% of the time.

Part of the problem is almost certainly structures that are marked __packed
but can never be misaligned.

	David

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