Re: [PATCH 1/3] Check function is GC'ed

[Doug Evans <dje at google dot com>]

Yao Qi writes:
 > I see the following fail on arm-none-eabi target,
 > 
 > (gdb) b 24^M
 > Breakpoint 1 at 0x4: file
 > ../../../../git/gdb/testsuite/gdb.base/break-on-linker-gcd-function.cc,
 > line 24.^M
 > (gdb) FAIL: gdb.base/break-on-linker-gcd-function.exp: b 24
 > 
 > Currently, we are using flag has_section_at_zero to determine whether
 > address zero in debug info means the corresponding code has been
 > GC'ed, like this:
 > 
 > 	case DW_LNE_set_address:
 > 	  address = read_address (abfd, line_ptr, cu, &bytes_read);
 > 
 > 	  if (address == 0 && !dwarf2_per_objfile->has_section_at_zero)
 > 	    {
 > 	      /* This line table is for a function which has been
 > 		 GCd by the linker.  Ignore it.  PR gdb/12528 */
 > 
 > However, this is incorrect on some bare metal targets, as .text
 > section is located at 0x0, so dwarf2_per_objfile->has_section_at_zero
 > is true.  If a function is GC'ed by linker, the address is zero.  GDB
 > thinks address zero is a function's address rather than this function
 > is GC'ed.
 > 
 > In this patch, we choose 'textlow' field of partial symtabl to check
 > whether 'textlow' is zero.  If it isn't, address zero really means the
 > function is GC'ed.  This patch fixes the fail above.  Note that this
 > patch only fixes the problem on the path that partial symtab is used.
 > On other paths partial symtab isn't used (start gdb with --readnow for
 > example), I don't find a good way to fix it.
 > 
 > It is regression tested on x86_64-linux, arm-none-eabi,
 > arm-none-linux-gnueabi.  OK to apply?
 > 
 > gdb:
 > 
 > 2014-08-20  Yao Qi  <yao@codesourcery.com>
 > 
 > 	* dwarf2read.c (dwarf_decode_lines_1): Skip the line table if
 > 	PST->textlow is greater than zero.
 > ---
 >  gdb/dwarf2read.c | 9 ++++++++-
 >  1 file changed, 8 insertions(+), 1 deletion(-)
 > 
 > diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
 > index b4d53c8..cf2ce76 100644
 > --- a/gdb/dwarf2read.c
 > +++ b/gdb/dwarf2read.c
 > @@ -17229,6 +17229,8 @@ dwarf_decode_lines_1 (struct line_header *lh, const char *comp_dir,
 >        /* Decode the table.  */
 >        while (!end_sequence)
 >  	{
 > +	  struct partial_symtab *pst = NULL;
 > +
 >  	  op_code = read_1_byte (abfd, line_ptr);
 >  	  line_ptr += 1;
 >  	  if (line_ptr > line_end)
 > @@ -17291,7 +17293,12 @@ dwarf_decode_lines_1 (struct line_header *lh, const char *comp_dir,
 >  		case DW_LNE_set_address:
 >  		  address = read_address (abfd, line_ptr, cu, &bytes_read);
 >  
 > -		  if (address == 0 && !dwarf2_per_objfile->has_section_at_zero)
 > +		  if (!decode_for_pst_p && !dwarf2_per_objfile->using_index)
 > +		    pst = cu->per_cu->v.psymtab;
 > +
 > +		  if (address == 0
 > +		      && (!dwarf2_per_objfile->has_section_at_zero
 > +			  || (pst != NULL && pst->textlow > address)))
 >  		    {
 >  		      /* This line table is for a function which has been
 >  			 GCd by the linker.  Ignore it.  PR gdb/12528 */

Hi.

I'd like to solve this for both partial syms and .gdb_index.

We want to, essentially, discard the entry if address is outside the
range of the cu (the range needn't be contiguous, but for the task
at hand I don't think it matters).  I haven't dug into this too deeply,
but if we have lowpc (which for example is that read_file_scope has
before it calls handle_DW_AT_stmt_list (which calls dwarf_decode_lines),
can we use that in the test?  Can we arrange for all callers of
dwarf_decode_lines_1 pass lowpc down to it? [Or both lowpc,highpc
for a more complete test, but for the task at hand we've only been
checking, effectively, lowpc all along so I don't mind leaving it at that
for now.]