[Dwarf-Discuss] DWARF2 - Endianity issues for DW_AT_bit_offsetfields

[David Earlam]

> 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)
> 
Hi Anitha,

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.

David 






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