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

[Heinrich Schuchardt]

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.

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.

Best regards

Heinrich