[PATCH 1/1] usb: avoid -Werror=address-of-packed-member

[Masahiro Yamada]

On Tue, Dec 31, 2019 at 12:41 AM Tom Rini <trini at konsulko.com> wrote:
>
> On Tue, Dec 17, 2019 at 08:52:01PM +0100, Marek Vasut wrote:
> > On 12/17/19 8:32 PM, Heinrich Schuchardt wrote:
> > > On 12/17/19 1:19 PM, Marek Vasut wrote:
> > >> On 12/17/19 12:59 PM, Heinrich Schuchardt wrote:
> > >>> On 12/17/19 12:19 PM, Marek Vasut wrote:
> > >>>> On 12/17/19 12:14 PM, Heinrich Schuchardt wrote:
> > >>>>> On 12/17/19 11:00 AM, Marek Vasut wrote:
> > >>>>>> On 12/17/19 10:27 AM, Heinrich Schuchardt wrote:
> > >>>>>>> With GCC 9.2.1 errors of type -Werror=address-of-packed-member occur
> > >>>>>>> when
> > >>>>>>> passing a member of a packed structure to le16_to_cpus() on a big
> > >>>>>>> endian
> > >>>>>>> system (e.g. P2041RDB_defconfig).
> > >>>>>>>
> > >>>>>>> Replace le16_to_cpus() by get_unaligned_le16(). Check
> > >>>>>>> defined(__BIG_ENDIAN)
> > >>>>>>> to avoid the introduction of unnecessary instructions on little
> > >>>>>>> endian
> > >>>>>>> systems as seen on aarch64.
> > >>>>>>
> > >>>>>> I would expect the compiler would optimize such stuff out ?
> > >>>>>> Can we do without the ifdef ?
> > >>>>>
> > >>>>> When compiling qemu_arm64_defconfig without the #ifdef the GCC 9.2.1
> > >>>>> adds assembler instructions:
> > >>>>
> > >>>> Why ?
> > >>>>
> > >>>
> > >>> I am not a GCC developer. I simply observed that GCC currently cannot
> > >>> optimize this away on its own. That is why I added the #ifdef.
> > >>
> > >> Are we now adding workarounds instead of solving issues properly?
> > >>
> > >>> Identifying that bit operations like << 8 in __get_unaligned_le16() have
> > >>> zero effect is not an easy task when developing a compiler. You would
> > >>> have to follow the flow of every bit.
> > >>
> > >> Maybe the fix is then to somehow optimize the get_unaligned_le16() to
> > >> help the compiler ?
> > >
> > > Inside get_unaligned_le16() it is not known that we will be reassigning
> > > to the same memory location. So I cannot imagine what to improve here.
> > >
> > > You could invent new functions for in place byte swapping. But that
> > > would only move the #ifdef to a different place.
> >
> > Isn't there already such a function in Linux ?
> >
> > Also, why would you need the ifdef if the compiler would now know that
> > the operation is noop ?
>
> This particular patch looks to me exactly why we want to follow the
> Linux Kernel and disable this particular warning for GCC like we already
> do for LLVM.

Agree. I think we can follow
Linux commit 6f303d60534c46aa1a239f29c321f95c83dda748



-- 
Best Regards
Masahiro Yamada