On Tue, Mar 27, 2018 at 02:22:31PM -1000, Linus Torvalds wrote: > On Tue, Mar 27, 2018 at 1:03 PM, Andrew Morton > <[email protected]> wrote: > > > > Why? What caused this padding? It happens in all configs? > > I assume what happens is that the anonymous struct ends up containing > fields that are cacheline-aligned, and then the whole anonymous struct > is cacheline-aligned.
Yes, structures inherit the alignment requirements of their constituent members. > Which is all kinds of stupid, since the anonymous struct itself does > not exist outside of the outer struct. So it would be entirely > sufficient to just make the outer struct cacheline aligned (like it > used to be), but not align the inner anonymous one - just the fields > in it. > > But there may be "reasons" why the inner anonymous one needs to be > aligned. Maybe it's some standards requirement, or maybe it's just an > internal gcc implementation detail. Last time I read the standard there wasn't a distinction between anonymous and regular structures for this. So in that regards a strict reading of the standard would mandate this behaviour. > Regardless, it's a bit sad. It also means that when randomization is > on, that unnecessary padding will be there. The other complaint is that the anonymous structure makes the pahole output (which is what I showed) unnecessarily ugly.
