[Dwarf-Discuss] variable locations - safe use as lvalues
Tue Jan 21 00:05:34 GMT 2020
> A debugger cannot currently be told that any particular variable
> location expression is safe to use as an lvalue, something roughly
> "exclusive, exhaustive, -O0-equivalent". I believe most debuggers
> don't even handle the multiple-locations case for writes at all. I
> don't know why - assume complications are rare? or we have kind of
> papered over the problem?
> As a DWARF standard level matter, descriptive rather than prescriptive
> as it is, what might be the minimum extension required to communicate
> lvalue-safety to a debugger? A DW_OP_lvalue_safe assertive wrapper
> around a real expression?
In DWARF 5, we added the concepts of bounded and default location
descriptions. I think it would generally be safe to modify a value
under the following conditions:
(a) The variable's location is covered by a single location
description, or by a location list with a default location description
but where no bounded location description matches the current pc.
(b) Such location description is a simple memory or register location.
(c) You're stopped at a suggested breakpoint location.
I'm sure there are circumstances where you could still get into
trouble (e.g., your third example), but I think it would be unusual,
and there we'd probably be talking about a quality of implementation
issue. The courteous thing for a compiler to do would be to at least
try to make this ok at suggested breakpoint locations, though I
wouldn't expect a compiler at -O2 to deliberately avoid an
optimization solely for the sake of being able to modify a variable
while stopped at a breakpoint.
In your third example, once the compiler has loaded a copy of the
variable into a register to use in a computation, it could (should?)
create a bounded location description, thus marking it unsafe under
the rules above. So, rather than mark lvalue-safe regions, a compiler
could instead use a bounded location descriptions to mark a region
unsafe that wouldn't already be deemed unsafe.
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