https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79671
--- Comment #75 from rguenther at suse dot de <rguenther at suse dot de> --- On Tue, 28 Mar 2017, bernd.edlinger at hotmail dot de wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79671 > > --- Comment #74 from Bernd Edlinger <bernd.edlinger at hotmail dot de> --- > (In reply to rguent...@suse.de from comment #73) > > > > Ah, I remember seeing this. So this introduces object size as a way > > to allow some TBAA to happen. Specifically it forbids creating > > a series of float sub-objects inside a char[sizeof(float)*4] sub-object > > due to "there is no smaller array object that satisfies these > > constraints"(?). One would need to use sth like > > > > struct { unsigned char storage[sizeof(float)]; } st[4]; > > > > I think that is meant differently, but I am not sure if I > understand this kind of english good enough..., the sample uses: > > template<typename ...T> > struct AlignedUnion { > alignas(T...) unsigned char data[max(sizeof(T)...)]; > }; > > so that data array is max of sizeof(int) and sizeof(char) > > ... the more they write the less clear it becomes :) I was thinking about how to arrange for _multiple_ objects to be constructed inside a memory area, the interesting size clause seems to require workarounds like above. Otherwise the "array element" choice doesn't make much sense I guess.
