Typed locals in functions and probes? Feasibility of language enhancement?

[Frank Ch. Eigler]

craig wrote:

> I find that I write things like this a lot:
>
> if (foo && (@cast(foo)->bar)
>     return @cast(foo)->bar->baz
>
> Because systemtap type-erases locals to long or string, code for member
> access gets repetitive fast. I can @define macros for shorthand, but that
> only goes so far, and it obfuscates the code a bit.
> Is there an accepted pattern for this that I'm missing?

Systemtap actually tries to associate some dwarf-based typing with local
variables.  For example, this works:

  probe kernel.function("work_busy") { f($work) }
  function f(w) { ww = & @cast(w,"work_struct");
                  println(ww->data->counter) }

This is due to the "auto-casting" facility added to systemtap
back in 2014.  I ... can't seem to find the facility actually
documented in the stap or stapprobes man page tho.  See this
bit of the systemtap 2.6 release notes:

https://sourceware.org/legacy-ml/systemtap/2014-q3/msg00231.html

We could probably start tracking these types across function call
parameter-passing boundaries also.  Via PR25841, functions called from a
probe handler also have some connection to the dwarf context.


- FChE