[fpc-devel] Question: ref counts / strings, smart pointers...

[Martin Frb]

I know how refcount internally work on strings and dyn array.

I am thinking about formulating a request to the DWARF specs. So I want 
to check the details, and also get information how they
- will work for smart pointers,
- where else they may occur?

Background, refcounts may be of interest for the debugger in at least 
the following cases.
- A user modifies a variable (or an indexed value) via the debugger.
   ~ The debugger may additionally need info, if it should "copy on 
write" / though that can be derived....
   ~ The debugger could inform the user about "side effects on other 
references to the value"
- The user may want to explicitly see the refcount.
   ~ A user may have "pchar" access to  a string, and the user wants to 
ensure the string was correctly "uniquestring" treated.
- A user wants to call a function that has a string/array as 
argument/result.
   ~ The debugger must deal with that, without introducing mem leaks.

If I am correct, then the following dwarf tags may need information

DW_TAG_Array (string and array)
DW_TAG_string_type
DW_TAG_structure // or class / interface => for smart pointers
// other types? base type, e.g. a refcounted char or integer???

The last assumes that the info can be encoded to the "smart pointed to" 
structure?
- Are there non-structure values that could be affected?
- Should maybe the DW_TAG_Pointer hold the refcount info instead?

I would expect the ref-count to be in the target value? Unless you can 
have a non-ref-counted structure, but have a ref-counted pointer to it????


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The "copy on write" can be known by checking if it is a string. That is, 
as long as smart pointers will not look like a string.

The debugger likely will need some knowledge on the producer (and then 
such type based decisions can be allowed)>
This is because the debugger must also know how to call 
fpc_ansistring_decref etc...

And trying to encode what functions to call (including where to pass 
which data as parameter =>probably way to complex)

So my current idea is to request a simple
   DW_AT_refcount  // location, reference, constant // maybe flag
maybe
   DW_AT_refcount_size / bit_size


(And maybe similar for the encoding field... any idea on what other 
fields may be of interest, or any that are going to be added soon-ish?)

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I am not sure yet which methods the debugger may have to call on smart 
pointers, or what else it would need to know.
Any feedback on that part would be welcome