On Fri, Nov 13, 2020 at 03:18:36PM +0100, Johan Hovold wrote:
On Wed, Nov 11, 2020 at 04:47:16PM +0100, Jessica Yu wrote:
Thanks for providing the links and references. Your explanation and
this reply from Jakub [1] clarified things for me. I was not aware of
the distinction gcc made between aligned attributes on types vs. on
variables. So from what I understand now, gcc suppresses the
optimization when the alignment is specified in the variable
declaration, but not necessarily when the aligned attribute is just on
the type.
Even though it's been in use for a long time, I think it would be
really helpful if this gcc quirk was explained just a bit more in the
patch changelogs, especially since this is undocumented behavior.
I found the explanation in [1] (as well as in your cover letter) to be
sufficient. Maybe something like "GCC suppresses any optimizations
increasing alignment when the alignment is specified in the variable
declaration, as opposed to just on the type definition. Therefore,
explicitly specify type alignment when declaring entries to prevent
gcc from increasing alignment."
Sure, I can try to expand the commit messages a bit.
I've amended the commit messages of the relevant patches to make it more
clear that the optimisation can be suppressed by specifying alignment
when declaring variables, but without making additional claims about the
type attribute. I hope the result is acceptable to you.
Perhaps you can include a lore link to the patches when applying so that
this thread can be found easily if needed.
Johan
