[Bug debug/48232] New: Excessive DW_AT_low_pc+DW_AT_high_pc for no code

[jan.kratochvil at redhat dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48232

           Summary: Excessive DW_AT_low_pc+DW_AT_high_pc for no code
           Product: gcc
           Version: 4.5.3
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: debug
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: jan.kratochvil@redhat.com


It affects any tested gcc:
gcc (GCC) 4.4.6 20110322 (prerelease)
gcc (GCC) 4.5.3 20110322 (prerelease)
gcc (GCC) 4.6.0 20110322 (prerelease)
gcc (GCC) 4.7.0 20110322 (experimental)

echo 'int i;'|gcc -x c - -c -o 1.o -g;echo 'main(){}'|gcc -o 1 1.o -x c -
 <0><b>: Abbrev Number: 1 (DW_TAG_compile_unit)
    <15>   DW_AT_low_pc      : 0x400474 
    <1d>   DW_AT_high_pc     : 0x400474 

DWARF-4:
# Compilation unit entries may have the following attributes:
# 1. Either a DW_AT_low_pc and DW_AT_high_pc pair of attributes
# or a DW_AT_ranges attribute whose values encode the contiguous
# or non-contiguous address ranges, respectively, of the machine instructions
# generated for the compilation unit (see Section 2.17).

Also according to my mail
http://sourceware.org/ml/gdb-patches/2011-03/msg00739.html
DWARF-4 does not say explicitly what does mean DW_AT_low_pc == DW_AT_high_pc
but for DW_AT_ranges it says
    A range list entry (but not a base address selection or end of list
    entry) whose beginning and ending addresses are equal has no effect
    because the size of the range covered by such an entry is zero.
therefore I believe DW_AT_low_pc == DW_AT_high_pc means such DIE should be
ignored and such DIE is IMO invalid.

elfutils dwarflint complains now (upon my request)
on DW_AT_low_pc==DW_AT_high_pc, with Jakub Jelinek's reaction:
# warnings about low_pc not below high_pc (e.g. for CUs which generated no
# code, but only some vars),

I believe neither DW_AT_low_pc nor DW_AT_high_pc should be produced in such
cases.  If it is not agreed upon it may be worth bringing it up on
dwarf-discuss.