Re: Source info in discontiguous hand asm files

[Daniel Jacobowitz <drow at false dot org>]

On Thu, May 04, 2006 at 02:06:08AM -0700, Ross Morley wrote:
> Yes, it is correct, though not as extensive as it could be.
> In the case where there are multiple ranges of text in a compilation
> unit, DA_AT_{low,high}_pc is omitted from the CU per DWARF spec.
> Where there is only one range of text, those are present and correct.
> The assembler doesn't add function-specific info.
> There is info in the aranges table to associate the ranges with the
> compilation unit, so a scan of that table could provide it.

The section .debug_info contains:

  Compilation Unit @ offset 0x0:
   Length:        52
   Version:       2
   Abbrev Offset: 0
   Pointer Size:  8
 <0><b>: Abbrev Number: 1 (DW_TAG_compile_unit)
     DW_AT_stmt_list   : 0      
     DW_AT_name        : text.S 
     DW_AT_comp_dir    : /home/drow/text        
     DW_AT_producer    : GNU AS 2.16.91 
     DW_AT_language    : 32769  (MIPS assembler)

>From the DWARF standard:

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 2.16).

   A DW_AT_low_pc attribute may also be specified in combination with
   DW_AT_ranges to specify the default base address for use in location
   lists (see Section 2.5.4) and range lists (see Section 2.16.3).

Now, there's a "may" there, so the compilation unit clearly isn't
required to have either.  But GDB definitely expects a compilation unit
to have one or the other.  It does not implement .debug_aranges
support.

So, your options are to use .debug_aranges to look up textlow/texthigh
for compilation units that don't have a PC range, or to modify gas to
emit DW_AT_ranges.  The latter is likely to be easier.

-- 
Daniel Jacobowitz
CodeSourcery