[Bug default/26679] New: Perform dead DIE removal in dwz

["jakub at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=26679

            Bug ID: 26679
           Summary: Perform dead DIE removal in dwz
           Product: dwz
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: default
          Assignee: nobody at sourceware dot org
          Reporter: jakub at redhat dot com
                CC: dwz at sourceware dot org
  Target Milestone: ---

E.g. on:
test1.C:
struct S { virtual ~S () {} int s; };
S s;
int main () {}
test2.C:
struct S { virtual ~S () {} int s; };
S t;
without -flto both the S::~S() dtors will be emitted with DW_AT_low_pc etc. and
all the details, but as the definition is comdat, only one of them will be
chosen by the linker and therefore the other one will end up with DW_AT_low_pc
of 0.
It would be nice if dwz could perform some form of dead DIE removal, it should
be very easy to discover them (subroutine or variable DIEs with zero
addresses), but the question is what to do with those exactly.  Just drop those
DIEs and their children, or e.g. first check if there is a real definition DIE
for that entity elsewhere and drop only then?  Or just prune some sub-DIEs and
some attributes?
Dropping DIEs or parts of DIEs shouldn't be that hard, but I guess dropping
stuff from .debug_loc/.debug_loclists if they are never referenced because of
such removal would be harder, there is no code for that right now.
On the bright side, this is something that would need to be done only at the
single executable/DSO mode, at multifile processing which is more memory
intensive this would be already done.
And, haven't checked, but probably it isn't relevant to -flto, because then the
compiler emits or should emit only one definition.

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