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

Doug Evans
2014-08-22T17:40:17Z

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.]