[Bug c/46354] attribute((aligned(...))) can incorrectly decrease structure field alignment

[lukebenes at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=46354

Luke  changed:

   What|Removed |Added

 CC||lukebenes at hotmail dot com

--- Comment #5 from Luke  ---
For the LLVM/Clang version of this bug, See:

https://bugs.llvm.org/show_bug.cgi?id=9253

[Bug c/46354] attribute((aligned(...))) can incorrectly decrease structure field alignment

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=46354

Martin Sebor  changed:

   What|Removed |Added

 Status|UNCONFIRMED |NEW
   Last reconfirmed||2016-01-26
 CC||msebor at gcc dot gnu.org
 Ever confirmed|0   |1
  Known to fail||4.5.3, 4.8.3, 4.9.3, 5.3.0,
   ||6.0

--- Comment #4 from Martin Sebor  ---
Confirmed. The documentation contradicts the implementation.  See also the
duplicate bug 65672 and bug 69502.

[Bug c/46354] attribute((aligned(...))) can incorrectly decrease structure field alignment

[pageexec at freemail dot hu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46354

--- Comment #1 from pageexec at freemail dot hu 2010-11-07 22:43:33 UTC ---
Created attachment 22314
  -- http://gcc.gnu.org/bugzilla/attachment.cgi?id=22314
sample code to demonstrate structure field offsets

[Bug c/46354] attribute((aligned(...))) can incorrectly decrease structure field alignment

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46354

--- Comment #2 from Richard Guenther rguenth at gcc dot gnu.org 2010-11-07 
23:00:25 UTC ---
Generally GCC lays out structures based on the types of the elemets, not
based on the alignment specified on fields.  Which is why I think what you
see is correct and intended.

[Bug c/46354] attribute((aligned(...))) can incorrectly decrease structure field alignment

[pageexec at freemail dot hu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46354

--- Comment #3 from pageexec at freemail dot hu 2010-11-07 23:33:54 UTC ---
(In reply to comment #2)
 Generally GCC lays out structures based on the types of the elemets, not
 based on the alignment specified on fields.

according to the gcc docs, explicit alignment on structure fields *is* taken
into account in that one can *increase* the natural alignment associated with a
given type:

 The `aligned' attribute can only increase the alignment; but you
 can decrease it by specifying `packed' as well.  See below.

in this bug you can see that even without the packed attribute gcc can decrease
the alignment. so either the docs or the implementation is buggy ;).

the second issue is that when one does use the packed attribute on a structure,
the resulting field alignment seems inconsistent depending on where the aligned
attribute is (typedef vs. structure field). i don't see where the docs specify
or imply this behaviour.