Fragile ABI

[Michel Fortin]

On 2012-08-18 14:50:54 +0000, "R Grocott" <rgrocottbugzilla at gmail.com> said:

> Hey Michel, posted this question earlier in the thread, but I think you 
> might have missed it:
> 
>> The blog post I linked only talks about rewriting vtables at 
>> dynamic-link-time. Would it be possible to implement something similar 
>> for the sizes of structs and scope-classes, and the relative address of 
>> member variables? dsimcha's post has led me to realize that without 
>> these additional features, the ABI would still be quite fragile.
> 
> Long story short: Could your proposal realistically be extended to 
> support flexible sizeofs and member-variable offsets? I figure that 
> such a system might as well stabilize the entire class ABI, rather than 
> just vtables.
> 
> I would guess that flexible sizeofs would increase the runtime cost 
> only very slightly, but flexible member-variables might be more of an 
> issue. The cost could be minimized by looking up offsets only when 
> crossing a library boundary (say, when calling a template method on an 
> extern (D_FlexibleABI) class, or accessing a protected member variable 
> of same), but would that actually be possible?

Apple's Modern Objective-C runtime has a non-fragile ABI that extends 
to member variables which works very well. It could be implemented 
pretty much the same way in D. There would a very small runtime 
overhead for accessing member variables though: each time you access a 
member you need need first to read a runtime-initialized global 
variable giving you its offset.

As for structs, you could do it the same way, but I don't think you'd 
get enough benefit to compensate the drawback in performance and 
elsewhere. To truly have non-fragile structs you'd need to disable 
almost all compile-time introspection too. That would be very 
disruptive.

So I don't think it makes sense to have non-fragile structs.

By the way, did you take a look at the benchmarks?
<http://michelf.ca/blog/2009/vtable-benchmarking/>

-- 
Michel Fortin
michel.fortin at michelf.ca
http://michelf.ca/