[Dwarf-discuss] Proposal: Describe prologue and epilogue ranges

[Robinson, Paul]

Andrew Cagney wrote:

> > A single location description (which can be either simple or composite
> > location descriptions) has the lifetime of its closest containing scope.
> > The case we care about here is when that scope is a subprogram, and
> > therefore the lifetime spans the entire subprogram. Pedantically, that
> > lifetime includes prologue and epilogue ranges.
> >
> > It is common practice for unoptimized code to allocate local variables
> > to a stack frame, and use that stack location in the single location
> > description. Because the stack frame is not necessarily in a valid state
> > during prologue or epilogue code, in practice, debuggers typically
> assume
> > that a single location description is not valid during a prologue or
> > epilogue, although the DWARF spec does not explicitly say so (AFAIK).
> 
> Does this problem extend to instructions within a statement where a
> simple location can also be invalid?  For instance, given:
> 
>     load r1 from i        # i++
>     inc r1
>  -> store r1 in i
> 
> an attempt to modify "i" would be trashed when the store instruction is
> executed
> 
> I'm not sure if this should be mentioned in the standard though.
> Perhaps this is covered by "... and it does not move during its
> lifetime."

I don't see this case as any different from any other assignment.
"i" hasn't moved, it has been copied in order to do some computation.
The assignment doesn't actually occur until the store is executed.
In typical unoptimized code, you wouldn't stop between the "inc"
and the "store."

--paulr