Re: [RFC] Partial support for dwarf3 DW_AT_ranges

[Elena Zannoni <ezannoni at redhat dot com>]

Daniel Jacobowitz writes:
 > On Fri, Jan 04, 2002 at 12:00:07PM -0500, Elena Zannoni wrote:
 > > Richard Henderson writes:
 > >  > GCC began emitting DW_AT_ranges back in September to deal with
 > >  > lexical scopes made discontiguous by basic block reordering.
 > >  > 
 > >  > As of today, it may also create discontiguous lexical scopes
 > >  > due to scheduling.  (Before today under the same circumstances
 > >  > we'd lose track of which instructions belonged to which scope
 > >  > and fail to emit any debug information whatsoever.)
 > >  > 
 > > 
 > > Richard, there was some initial effort to deal with this problem
 > > in gdb's symbol tables structures back in October.
 > > See the thread at:
 > > http://sources.redhat.com/ml/gdb-patches/2001-10/msg00304.html
 > > However, no real changes have been made to the symbol tables yet.
 > > 
 > >  > However, GDB doesn't recognize DW_AT_ranges as a valid way of
 > >  > marking a lexical scope, which causes it to discard the scope
 > >  > entirely.  Which is probably the least useful thing that could
 > >  > be done.
 > >  > 
 > >  > The following does not add proper support for discontiguous
 > >  > address ranges.  I couldn't figure out how to do that in any
 > >  > way that wasn't horribly invasive.  I'm willing to expend a
 > >  > significant amount of effort on this if someone is willing to
 > >  > provide some direction.
 > >  > 
 > > 
 > > The thread mentioned above has some initial implementation of an
 > > address set for partial symbol tables which would be used in case of
 > > non contiguous addr ranges. You should coordinate with Jim.
 > > 
 > >  > What this does do is find the "bounding box" of the discontiguous
 > >  > range and use that.  Yes, that will do the wrong thing in some
 > >  > circumstances, but the current behaviour is wrong under all
 > >  > circumstances, so it may be a net improvement.
 > >  > 
 > > 
 > > I am OK with committing some initial support. At least now gdb can
 > > read the info. 
 > > 
 > > I have a few comments on your changes.  Maybe I am missing it, but is
 > > the -1 value returned by dwarf2_get_pc_bounds used anywhere yet? If
 > > not, I would suggest we don't bother with it right now, given that the
 > > final solution will probably have to restructure that function again.
 > > Also, I would not want to introduce more goto's in gdb. I think that
 > > that sequence can be rewritten w/o goto's pretty easily.  Also there
 > > is some formatting problem (white spaces around operators, full names
 > > for variables).  Adding some comments on how the .debug_ranges list is
 > > organized would be helpful too.
 > > 
 > > I think JimB is the one which has the last say on this however, since
 > > he has a patch in the works.
 > > 
 > 
 > Ping out there, folks.
 > 
 > This lack of support for DW_AT_ranges is responsible for at least one,
 > probably two or more of the current PRs about "missing local
 > variables".  Affected GCC versions have been shipping for some time; at
 > least 3.2 generates DW_AT_ranges, maybe earlier.

Yes, I am looking at this patch at the moment, too. The lack of this
feature is creating lots of troubles. 

 > 
 > Since nothing ever came of the grand plans to support ranges in the
 > symbol table directly, can we at least move forwards on this year-old
 > patch?  If so, I'll update it for current GDB.
 > 

If you have time, I'd appreciate it. I have adapted the patch to the
current sources, but even with it, I wasn't able to make Jakub's
example work.  The example is in PR 833.
I can send you the diffs I have, if it helps.


Elena


 > > thanks 
 > > Elena
 > > 
 > > 
 > >  > My test case for this was
 > >  > 
 > >  > 	static int foo(int *);
 > >  > 	static void bar(int *);
 > >  > 
 > >  > 	int main()
 > >  > 	{
 > >  > 	  {
 > >  > 	    int x = 0, r;
 > >  > 	    r = foo(&x);
 > >  > 	    if (__builtin_expect (r, 1))
 > >  > 	      return 0;
 > >  > 	    bar (&x);
 > >  > 	    return 1;
 > >  > 	  }
 > >  > 	}
 > >  > 
 > >  > 	static int foo(int *p)
 > >  > 	{
 > >  > 	  *p = 1;
 > >  > 	  return 0;
 > >  > 	}
 > >  > 
 > >  > 	static void bar(int *p)
 > >  > 	{
 > >  > 	  *p = 2;
 > >  > 	}
 > >  > 
 > >  > For any GCC target that can emit epilogues as rtl, we will
 > >  > arrange the code here as
 > >  > 
 > >  > 	x = 0
 > >  > 	call foo
 > >  > 	if r == 0 goto L1
 > >  > 	ret = 0
 > >  >     L0:
 > >  > 	return ret
 > >  >     L1:
 > >  > 	call bar
 > >  > 	ret = 1
 > >  > 	goto L0
 > >  > 
 > >  > The lexical scope created by the extra set of braces doesn't
 > >  > cover the epilogue, so the scope's range is the block before
 > >  > L0, plus the block after L1.
 > >  > 
 > >  > 
 > >  > r~
 > > 
 > >  > -/* Get low and high pc attributes from a die.
 > >  > -   Return 1 if the attributes are present and valid, otherwise, return 0.  */
 > >  > +/* Get low and high pc attributes from a die.  Return 1 if the attributes
 > >  > +   are present and valid, otherwise, return 0.  Return -1 if the range is
 > >  > +   discontinuous, i.e. derived from DW_AT_ranges information.  */
 > >  >  
 > >  >  static int
 > >  > -dwarf2_get_pc_bounds (struct die_info *die, CORE_ADDR *lowpc, CORE_ADDR *highpc,
 > >  > -		      struct objfile *objfile)
 > >  > +dwarf2_get_pc_bounds (struct die_info *die, CORE_ADDR *lowpc,
 > >  > +		      CORE_ADDR *highpc, struct objfile *objfile,
 > >  > +		      const struct comp_unit_head *cu_header)
 > >  >  {
 > >  > +  bfd *obfd = objfile->obfd;
 > >  >    struct attribute *attr;
 > >  >    CORE_ADDR low;
 > >  >    CORE_ADDR high;
 > >  > +  int ret;
 > >  >  
 > >  > -  attr = dwarf_attr (die, DW_AT_low_pc);
 > >  > -  if (attr)
 > >  > -    low = DW_ADDR (attr);
 > >  > -  else
 > >  > -    return 0;
 > >  >    attr = dwarf_attr (die, DW_AT_high_pc);
 > >  >    if (attr)
 > >  > -    high = DW_ADDR (attr);
 > >  > -  else
 > >  > -    return 0;
 > >  > +    {
 > >  > +      high = DW_ADDR (attr);
 > >  > +      attr = dwarf_attr (die, DW_AT_low_pc);
 > >  > +      if (attr)
 > >  > +	low = DW_ADDR (attr);
 > >  > +      else
 > >  > +	return 0;
 > >  > +      ret = 1;
 > >  > +    }
 > >  > +  else if ((attr = dwarf_attr (die, DW_AT_ranges)) != NULL)
 > >  > +    {
 > >  > +      unsigned int addr_size = cu_header->addr_size;
 > >  > +      CORE_ADDR mask = ~(~(CORE_ADDR)1 << (addr_size * 8 - 1));
 > >  > +      unsigned int offset = DW_UNSND (attr);
 > >  > +      CORE_ADDR base;
 > >  > +      int dummy;
 > >  > +      unsigned int i;
 > >  > +      char *buffer;
 > >  >  
 > >  > +      /* The applicable base address is determined by (1) the closest
 > >  > +         preceding base address selection entry in the range list or
 > >  > +	 (2) the DW_AT_low_pc of the compilation unit.  */
 > >  > +      /* ??? We definitely need some sort of indexed data structure here.
 > >  > +	 At minimum we should recognize the common case of there being
 > >  > +	 no base address selection entries.  */
 > >  > +
 > >  > +      buffer = dwarf_ranges_buffer + offset;
 > >  > +      for (i = offset; i > 2 * addr_size; )
 > >  > +	{
 > >  > +	  CORE_ADDR marker;
 > >  > +
 > >  > +	  i -= 2 * addr_size;
 > >  > +	  buffer -= 2 * addr_size;
 > >  > +
 > >  > +	  marker = read_address (obfd, buffer, cu_header, &dummy);
 > >  > +	  if ((marker & mask) == mask)
 > >  > +	    {
 > >  > +	      base = read_address (obfd, buffer+addr_size, cu_header, &dummy);
 > >  > +	      goto found_base;
 > >  > +	    }
 > >  > +	}
 > >  > +
 > >  > +      /* ??? Was in dwarf3 draft4, and has since been removed.
 > >  > +	 GCC still uses it though.  */
 > >  > +      attr = dwarf_attr (cu_header->die, DW_AT_entry_pc);
 > >  > +      if (attr)
 > >  > +	{
 > >  > +	  base = DW_ADDR (attr);
 > >  > +	  goto found_base;
 > >  > +	}
 > >  > +
 > >  > +      attr = dwarf_attr (cu_header->die, DW_AT_low_pc);
 > >  > +      if (attr)
 > >  > +	{
 > >  > +	  base = DW_ADDR (attr);
 > >  > +	  goto found_base;
 > >  > +	}
 > >  > +
 > >  > +      /* We have no valid base address for the ranges data.  */
 > >  > +      return 0;
 > >  > +
 > >  > +    found_base:
 > >  > +      buffer = dwarf_ranges_buffer + offset;
 > >  > +      low = read_address (obfd, buffer, cu_header, &dummy);
 > >  > +      buffer += addr_size;
 > >  > +      high = read_address (obfd, buffer, cu_header, &dummy);
 > >  > +      buffer += addr_size;
 > >  > +      if (low == 0 && high == 0)
 > >  > +	/* If the first entry is an end-of-list marker, the range
 > >  > +	   describes an empty scope, i.e. no instructions.  */
 > >  > +	return 0;
 > >  > +
 > >  > +      while (1)
 > >  > +	{
 > >  > +	  CORE_ADDR b, e;
 > >  > +	  offset += 2 * addr_size;
 > >  > +
 > >  > +	  b = read_address (obfd, buffer, cu_header, &dummy);
 > >  > +	  buffer += addr_size;
 > >  > +	  e = read_address (obfd, buffer, cu_header, &dummy);
 > >  > +	  buffer += addr_size;
 > >  > +	  if (b == 0 && e == 0)
 > >  > +	    break;
 > >  > +	  if (b < low)
 > >  > +	    low = b;
 > >  > +	  if (e > high)
 > >  > +	    high = e;
 > >  > +	}
 > >  > +
 > >  > +      low += base;
 > >  > +      high += base;
 > >  > +      ret = -1;
 > >  > +    }
 > >  > +
 > >  >    if (high < low)
 > >  >      return 0;
 > >  >  
 > >  > @@ -1800,12 +1908,12 @@ dwarf2_get_pc_bounds (struct die_info *d
 > >  >       labels are not in the output, so the relocs get a value of 0.
 > >  >       If this is a discarded function, mark the pc bounds as invalid,
 > >  >       so that GDB will ignore it.  */
 > >  > -  if (low == 0 && (bfd_get_file_flags (objfile->obfd) & HAS_RELOC) == 0)
 > >  > +  if (low == 0 && (bfd_get_file_flags (obfd) & HAS_RELOC) == 0)
 > >  >      return 0;
 > >  >  
 > >  >    *lowpc = low;
 > >  >    *highpc = high;
 > >  > -  return 1;
 > >  > +  return ret;
 > >  >  }
 > >  >  
 > >  >  /* Add an aggregate field to the field list.  */
 > > 
 > 
 > -- 
 > Daniel Jacobowitz
 > MontaVista Software                         Debian GNU/Linux Developer