> 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. -cary _______________________________________________ Dwarf-Discuss mailing list Dwarf-Discuss@lists.dwarfstd.org http://lists.dwarfstd.org/listinfo.cgi/dwarf-discuss-dwarfstd.org
