Hi! When GDB and LLDB perform virtual unwinding, they subtract one byte from the return addresses of the outer frames. This is for example necessary when unwinding from a non-returning call that is placed last in the function, as the return address then can point to a different function. I assume that this is also necessary to get the variables that were in scope at the time of the call, and the right location expressions for the variables, etc.
As far as I have understood it, GCC utilizes this fact for location list entries that are expressed in caller-saved registers, by subtracting one from the (exclusive) ending address of the entries. This means that variables in outer frames that are located in caller- saved registers will be printed out as <optimized out> by GDB. I have not been able to find anything in the DWARF standard that describes this. Is this something that is defined by standard, or is it established praxis between GCC and GDB? If the latter, do you know of other producers and consumers that behave like this? The reason why I ask this is because Clang/LLVM at the moment ends location list entries expressed in caller-saved registers at the first instruction after the call. This means that variables in outer frames using such location list entries will incorrectly be evaluated using the inner-most frame's register values when debugging in GDB. (As a side note, as far as I have understood it, LLDB has fallback knowledge of the ABI, so caller-saved registers will be considered unavailable in outer frames, meaning that such location list entries are not an issue when combining Clang/LLVM and LLDB.) Best regards, David _______________________________________________ Dwarf-Discuss mailing list Dwarf-Discuss@lists.dwarfstd.org http://lists.dwarfstd.org/listinfo.cgi/dwarf-discuss-dwarfstd.org
