[PATCH] D12455: [IR] Teach `llvm::User` to co-allocate a descriptor.

[Sanjoy Das via llvm-commits]

sanjoy added a comment.

In http://reviews.llvm.org/D12455#238526, @pete wrote:

> This is just a question, not intended to block the commit if someone else gives a LGTM.
>
> Have you considered making use of 'hung off uses' instead of adding DescBytes to new?
>
> It would take a little work to change 'hung off uses' to actually be 'hung off bytes', and i haven't looked to see how possible that is, but it feels to me like it should be considered.
>
> This also has the advantage of moving the desc bytes handling out of new so that all User's don't pay the price for checking them in new, and instead the rare case of meeting extra bytes can get them only on request.


Is the only motivation for making the descriptor bytes be hung off
that we won't have to check `DescriptorBytes == 0` in `operator new`
(I cannot think of anything else that hung off descriptor bytes will
help)?

If so, I think there is an easier way to fix this -- I can just
overload `operator new` into a version that takes the
`DescriptorBytes` argument, and a version that does not.  Most of
`User`s will call the latter, and won't have to pay the penalty of
checking `DescriptorBytes == 0`.  There will still be the penalty of
setting `Obj->HasDescriptor = 0` in the latter, but I'd expect that to
be folded into whatever `operator new` does to set
`Obj->HasHungOffUses = false`.  Does this sound reasonable to you?


http://reviews.llvm.org/D12455