On August 9, 2016 4:36:24 PM GMT+02:00, Jakub Jelinek <ja...@redhat.com> wrote: >On Mon, Aug 01, 2016 at 12:44:30PM +0200, Richard Biener wrote: >> Note that reading the dwarf standard, it looks like it accepts a >location >> which means we could do an implicit location description using >> DW_OP_stack_value which gives us access to arbitrary dwarf >> expressions (and thus the possibility to handle it similar to VLAs). > >Sure. But for the DW_AT_string_length, the issue is that with early >debug, >we want the attribute early, but locations within a function obviously >can be provided late, after the function is compiled. > >Which is why the patch uses DW_OP_call4 as the holder of the >information >that the string length is provided by the value of some variable. >In certain cases it can stay that way even during late debug, if the >var >disappears or has no location info etc., then DW_AT_string_length is >dropped, or in other cases the location of the var is copied into the >attribute etc. > >This is different from the VLA bounds in DW_AT_upper_bound etc., where >it is allowed to just refer to a DIE that holds the value (so we can >stick >it into the attribute early and just supply the var's location later), >for >DW_AT_string_length it is not allowed. > >> But maybe I am missing something? (now running into the issue >> with LTO debug and gfortran.dg/save_5.f90 where during early debug >> we emit a location that ends up refering to a symbol that might be >> optimized away later - early debug cannot sanitize referenced symbols >> via resolv_addr obviously). Annotating the DIE late is also not >> what I want to do as I'd need to pull in all type DIEs into the late >CU >> that way (well, at least selected ones, but I'm really trying to >avoid >> going down that route). > >In some cases like location of file scope vars, or this >DW_AT_string_length, >you really need to adjust late the debug info created early, there is >no >workaround for that.
I suppose the workaround is to fix/extend DWARF... (DW_AT_GNU_string_length that allows us to refer to another DIE?) Richard. > Jakub
