Jan Schaumann <jschauma@netmeister.org> writes:

> Under i386, it appears that gcc if passed '-g' generates the wrong
> debugging symbols per default:
[...]
> When compiled with '-g', running it through gdb it will correctly print
> 0.184000, but if you break and inspect x, it will give:
> 
> (gdb) p x
> $1 = {c = 97 'a', d = 5.2867121265460283e-315}
> (gdb) 
> 
> Now using '-gstabs' or '-gstabs+' or '-gstabs3', it will behave
> correctly.  So... what am I doing wrong, or why is gcc not creating the
> correct debugging symbols per default?

I can confirm that the debugging information is wrong; it describes
the 'd' field as being 8 bytes from the start of the structure, when
the generated code puts it 4 bytes from the start of the structure
(and which is correct according to the ABI).

From "readelf --debug-dump":

 <1><3a8>: Abbrev Number: 10 (DW_TAG_structure_type)
     DW_AT_sibling     : <3cd>  
     DW_AT_name        : (indirect string, offset: 0x153): astruct      
     DW_AT_byte_size   : 12     
     DW_AT_decl_file   : 1      
     DW_AT_decl_line   : 3      
 <2><3b4>: Abbrev Number: 12 (DW_TAG_member)
     DW_AT_name        : c      
     DW_AT_decl_file   : 1      
     DW_AT_decl_line   : 4      
     DW_AT_type        : <42>   
     DW_AT_data_member_location: 2 byte block: 23 0     (DW_OP_plus_uconst: 0)
 <2><3c0>: Abbrev Number: 12 (DW_TAG_member)
     DW_AT_name        : d      
     DW_AT_decl_file   : 1      
     DW_AT_decl_line   : 5      
     DW_AT_type        : <3cd>  
     DW_AT_data_member_location: 2 byte block: 23 8     (DW_OP_plus_uconst: 8)

I couldn't say *why* it's doing this, though.

        - Nathan