Re:Abstraction without overhead: traits in Rust
by adrusi on Monday May 11, 2015 @06:08PM
(#9528180)
I don't try think that's feasible. The vtable is stored in the binary's data block, so that's usually a pretty compact region of memory, maybe even referenceable by 16 bits. Except that's no longer true if the type's impl for the trait is defined in a shared library.
It should be possible as a compiler option if you are compiling every object that you link to and statically linking them into a single binary. Even then there's be a performance hit.
Would there be a hit? C++ has to do this:
object pointer -> load vtable -> load function pointer
while a compressed fat pointer would be like:
fat pointer -> shift to extract vtable index -> shift to make into a global table offset -> load global table base from a global variable -> load function pointer
Two loads in both cases, but in the compressed case the first one will almost always be cached; more code bloat, but not that much, and you save a bit of memory in the objects themselves.
Though the added complexity could offset the 16-to-8 byte size reduction.