On Mon, May 14, 2018 at 10:40 AM, H.J. Lu <hjl.to...@gmail.com> wrote:
>>>> $ cat c.i
>>>> struct B { int i; };
>>>> struct C { struct B b; } __attribute__ ((packed));
>>>>
>>>> long* g8 (struct C *p) { return p; }
>>>> $ gcc -O2 -S c.i -Wno-incompatible-pointer-types
>>>> c.i: In function ‘g8’:
>>>> c.i:4:33: warning: taking value of packed 'struct C *' may result in an
>>>> unaligned pointer value [-Waddress-of-packed-member]
>>
>> ^^^^^
>> That should read "taking address" (not value) but...
>
> The value of 'struct C *' is an address. There is no address taken here.
>
>> ...to help explain the problem I would suggest to mention the expected
>> and actual alignment in the warning message. E.g.,
>>
>> storing the address of a packed 'struct C' in 'struct C *' increases the
>> alignment of the pointer from 1 to 4.
>
> I will take a look.
>
>> (IIUC, the source type and destination type need not be the same so
>> including both should be helpful in those cases.)
>>
>> Adding a note pointing to the declaration of either the struct or
>> the member would help users find it if it's a header far removed
>> from the point of use.
>
> I will see what I can do.
How about this
[hjl@gnu-skx-1 pr51628]$ cat n9.i
struct B { int i; };
struct C { struct B b; } __attribute__ ((packed));
long* g8 (struct C *p) { return p; }
[hjl@gnu-skx-1 pr51628]$
/export/build/gnu/gcc-test/build-x86_64-linux/gcc/xgcc
-B/export/build/gnu/gcc-test/build-x86_64-linux/gcc/ -O2 -S n9.i
n9.i: In function ‘g8’:
n9.i:4:33: warning: returning ‘struct C *’ from a function with
incompatible return type ‘long int *’ [-Wincompatible-pointer-types]
long* g8 (struct C *p) { return p; }
^
n9.i:4:33: warning: taking value of packed ‘struct C *’ increases the
alignment of the pointer from 1 to 8 [-Waddress-of-packed-member]
n9.i:2:8: note: defined here
struct C { struct B b; } __attribute__ ((packed));
^
[hjl@gnu-skx-1 pr51628]$
--
H.J.
