Re: [Xen-devel] [PATCH 1/2] build/clang: remove the address-of-packed-member warning

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 06/03/17 15:16, Tim Deegan wrote:
> At 14:36 +0000 on 06 Mar (1488811016), George Dunlap wrote:
>> On 06/03/17 13:58, Jan Beulich wrote:
>>>>>> On 06.03.17 at 13:31, <roger.pau@xxxxxxxxxx> wrote:
>>>> --- a/Config.mk
>>>> +++ b/Config.mk
>>>> @@ -216,6 +216,7 @@ $(call 
>>>> cc-option-add,HOSTCFLAGS,HOSTCC,-Wdeclaration-after-statement)
>>>>  $(call cc-option-add,CFLAGS,CC,-Wdeclaration-after-statement)
>>>>  $(call cc-option-add,CFLAGS,CC,-Wno-unused-but-set-variable)
>>>>  $(call cc-option-add,CFLAGS,CC,-Wno-unused-local-typedefs)
>>>> +$(call cc-option-add,CFLAGS,CC,-Wno-address-of-packed-member)
>>> Actually, having thought some more about this, the warning
>>> should be suppressed only for x86 imo. ARM wants aligned
>>> accesses after all.
>> Looking at Roger's complaint, it appears that the warning is issued even
>> if the member actually is aligned, if *on some unknown system*, it might
>> someday be un-aligned.
> AIUI the complaint is (based on the simplified example from the ticket):
>
>     struct __attribute__((__packed__)) bar {
>         uint16_t x1;
>         uint16_t x2;
>     } b;
>     
>     &b.x2;
>
> Because the struct is packed, it has alignment 1, and so do its
> fields.   &b.x2 is a pointer to a uint16_t, but it _isn't_ 16-bit
> aligned (because the whole struct is only byte-aligned).
>
> So I guess that one fix would be to declare that the struct has
> appropriate alignment?  I don't know whether that would suppress the
> warning, but the clang devs might be more receptive to seeing it as
> a false positive.

The structs in question (segment_attributes and segment_register) have
proper natural alignment with no padding anyway, so don't need to be
__packed__ to be correct.

It would be better to remove the unnecessary __packed__.

~Andrew

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