[PATCH] D20561: Warn when taking address of packed member

[Evgenii Stepanov via cfe-commits]

On Mon, Jun 13, 2016 at 4:12 PM, Aaron Ballman <aaron.ballman at gmail.com> wrote:
> On Mon, Jun 13, 2016 at 6:30 PM, Evgeniy Stepanov <eugenis at google.com> wrote:
>> eugenis added a subscriber: eugenis.
>> eugenis added a comment.
>>
>> In http://reviews.llvm.org/D20561#446031, @aaron.ballman wrote:
>>
>>> In http://reviews.llvm.org/D20561#445824, @rogfer01 wrote:
>>>
>>> > I think I wasn't clear with the purpose of the fix-it: there are a few cases where getting the address of an unaligned pointer is safe (i.e. false positives).
>>> >
>>> > For instance, when I checked Firefox and Chromium there are cases where getting the address of an unaligned pointer is fine. For the particular case of these two browsers, they both use a library (usrsctp) that represents protocol data as packed structs. That library passes addresses of packed fields to `memcpy` and `memmove` which is OK.
>>>
>>>
>>> I think this is a false-positive that should be fixed.
>>
>>
>> This patch was committed without fixing the false positive case, why?
>
> Can you give some more code context, like perhaps a test case we can
> add to the suite? I believe the current behavior should be essentially
> false-positive-free because it only triggers the diagnostic when the
> alignments actually differ, so if there are other false-positives, I
> agree that we should determine a disposition for them.

This is actually the same library the comment above is talking about:
https://bugs.chromium.org/p/chromium/issues/detail?id=619640

>> Could this warning be excluded from -Wall?
>
> Would you like the warning to be excluded from -Wall even if the
> false-positive is fixed?

No, the warning seems useful if the false positive is fixed.