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

[Marek Vasut]

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 ?