[Dwarf-Discuss] DWARF2 - Endianity issues for DW_AT_bit_offsetfields
Thu Sep 8 10:27:35 GMT 2011
> Hello David,
> Thanks for the explanation.
> There is one more little-endian architecture - AVR on which I work. I get
> this output for 'l' of test. (gcc 4.5.1)
> <2><5e>: Abbrev Number: 4 (DW_TAG_member)
> <5f> DW_AT_name : l
> <61> DW_AT_decl_file : 1
> <62> DW_AT_decl_line : 6
> <63> DW_AT_type : <0x75>
> <67> DW_AT_byte_size : 1
> <68> DW_AT_bit_size : 2
> <69> DW_AT_bit_offset : 0xffffffff 0xffffffff
> <71> DW_AT_data_member_location: 2 byte block: 23 0
> (DW_OP_plus_uconst: 0)
Yes, it's valid.
If I use gcc 4.4.4 (x86 mingw tdm) with '-g -fpack-struct' arguments
firstly, the compiler reports
dwarfbits.c:6: note: Offset of packed bit-field 'l' has changed in GCC 4.4
- since an earlier gcc version I guess -
then secondly, 'objdump -W' of the produced Dwarf2 also says
<2><80>: Abbrev Number: 4 (DW_TAG_member)
<81> DW_AT_name : l
<83> DW_AT_decl_file : 1
<84> DW_AT_decl_line : 5
<85> DW_AT_type : <0x93>
<89> DW_AT_byte_size : 1
<8a> DW_AT_bit_size : 2
<8b> DW_AT_bit_offset : 0xffffffff
<8f> DW_AT_data_member_location: 2 byte block: 23 0 (DW_OP_plus_uconst: 0)
The debug_abbrev for DW_AT_bit_offset is DW_FORM_data4, I assume it is DW_FORM_data8 for you. (I didn't actually know AVR's could address that much memory, or just maybe there's a problem around here in GCC4.5.3 or the AVR port.)
It'd be easier to understand if the compiler were to use DW_FORM_sdata for 'signed constant' class of DW_AT_bit_offset then it is clearly signed for the DWARF consumer.
However if you interpret that DW_AT_bit_offset value as signed the 2 bit-field 'l' straddles addr+1 and addr+0. It all makes sense.
In Dwarf v4 the new DW_AT_data_bit_offset (alternative to Dwarf2 or Dwarf3 style DW_AT_data_member_location +or- DW_AT_bit_offset)
"must be greater than or equal to zero" so it is unsigned.
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