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

[Heinrich Schuchardt]

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.

Best regards

Heinrich