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. -- Tom
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