Re: [PATCH 0/3] DWARF type signature lookup fallback.

[Dominik Mascherbauer <dominik.mascherbauer@gmail.com>]

>>>>>> dominikmascherbauer  <dominik.mascherbauer@gmail.com> writes:
> 
>> I am working on a patch that adds parameters to allow type signature
>> fallback for DWARF type units to fallback to other objfiles.  This
>> allows to only create type units once and reuse them by their type
>> signature, reducing duplication of type units.  It builds on the
>> uniqueness of type signatures, so a type signature always references
>> the same type unit.
> 
> I am not sure this can really work due to lifetime constraints.
> 
> GDB has some rules about type ownership:
> 
> 1. Arch-owned types must only refer to other arch-owned types
> 
> 2. Objfile-owned types must only refer to other types coming from the
>    same objfile, or to arch-owned types
> 
> These rules exist so that if an objfile is removed, there will not be
> any dangling pointers.
> 
> I think this series violates rule 2.
> 
> There's a follow-on rule to this that isn't as frequently discussed, but
> a symbol in an objfile has to follow rule 2 as well: it can't refer to
> types from another objfile.  And I think this series also violates this.
> 
> I think you can see this in action for opaque types, where the lookup is
> done over and over, because caching the result would violate the rules.
> (This brings up the question of why you want this feature at all as
> opposed to just using opaque type resolution.)
> 
> 
> Anyway, this can't be easily remedied.  One idea we have discussed in
> the past is to have a "type GC".  What this would mean is removing type
> ownership -- just allocate all types globally.  Then, have a garbage
> collector that removes type objects when things change, for instance
> after an objfile is destroyed.
> 
> This is difficult to implement though.  For one thing types can refer
> back to symbols.  Though perhaps this particular special case could be
> handled by the GC.
> 
> Maybe other approaches are possible too, I don't know.
> 
> Tom

Thanks for the response.  I agree that this series violates rule 2 as you 
mentioned.  I was not aware of those rules at the time of implementing
this patch.

Thank you very much for pointing out opaque types.  I did not come across
those when trying to figure out how to reference existing types from other
objfiles.  For checking available options I mainly used the DWARF specification
which does not mention them.

My initial idea was to use type signatures as they should be unique even
across multiple objfiles.  Based on that I thought it would be possible
to use type signatures for finding type units in other not linked 
objfiles such as objfiles coming from the JIT compilation interface.

I tested opaque types for my use-case, and it did work out.  So, I guess the
best option for me is to adapt the generated debug info for opaque types.

Thanks again,
Dominik