On 10/04/18 13:12 -0400, Jason Merrill wrote:
In Joseph's patch for bug 52023, making _Alignof(double) 4 rather than 8 on x86, he deliberately didn't change the C++ behavior, based on this wording in the C++ standard:-- The alignment required for a type might be different when it is used as the type of a complete object and when it is used as the type of a subobject. [ Example: struct B { long double d; }; struct D : virtual B { char c; }; When D is the type of a complete object, it will have a subobject of type B, so it must be aligned appropriately for a long double. If D appears as a subobject of another object that also has B as a virtual base class, the B subobject might be part of a different subobject, reducing the alignment requirements on the D subobject. — end example ] The result of the alignof operator reflects the alignment requirement of the type in the complete-object case. -- This suggests that alignof shouldn't consider alignment of non-static data members. I think that this wording is wrong, that "subobject" was intended only to refer to base subobjects. But really this is beside the point: the x86 ABI says that the alignment of double is 4, so alignof(double) should be 4 regardless of what GCC wants to do internally. And I think the same is true of __alignof__. Thoughts?
I don't have a view on what's more correct, but better alignment (pun intended) with C's _Alignof seems good to me.
