Re: Should a DW_OP_implicit_value be taken from the left end?

Andreas Arnez <[email protected]>
Newsgroups gmane.comp.gdb.devel
Message-ID <[email protected]> 
Thanks for looking into this.

On Wed, Dec 21 2016, Jan Kratochvil wrote:

> When looking at the patch I think the patch may be wrong, I think the
> endianity should affect at least also the lines:
>
>         case DWARF_VALUE_LITERAL:
> ...
>             ldata = ctx.data + byte_offset;
>             n -= byte_offset;

Right, that's another issue.  If we revert the change for
DW_OP_implicit_value, it doesn't matter anymore, but the same bug occurs
with DW_OP_stack_value as well.  I'll handle that separately.

> I really do not mind reverting the patch if you think so, that is up to the
> maintainers.

Note that I don't intend to revert the whole patch, only the part
affecting DW_OP_implicit_value (DWARF_VALUE_LITERAL).

> But when you ask me I miss here stating what the current GCC version does
> produce.  Is GDB behavior fixed with current GCC by your proposed patch
> revert?  If it breaks do you plan to change/fix also GCC?  Does the GDB
> testsuite (particulerly the entryval testcases) have no regressions on s390*?

Right, the test suite has no regressions on s390*.  And GCC can indeed
emit DWARF code that breaks with current GDB on s390x; and the proposed
patch would fix that.  See attachment for an example.

> From my mail you reference I understand it as that my patch did fix some
> entryval testcases with GCC that time.  Unfortunately the entryval testcases
> are provided as .S files prebuilt by GCC that time and they are difficult to
> reproduce with newer GCC as -O2 -g code changes too much with different GCC
> version, breaking various compiled code assumptions of the .exp file.

Your patch certainly fixed a real problem with DW_OP_stack_value, but I
doubt that the changes to the handling of DW_OP_implicit_value did.  In
the patch description you stated:

  "I am not completely sure with the DWARF standard understanding for
 DW_OP_implicit_value (DWARF_VALUE_LITERAL) but I think it also should
 be fixed (if such DWARF is valid at all)."

And Tom Tromey answered:

  "I think I asked about this when implementing this DWARF addition in
  gdb, but I can't find the messages right now."

Is there any any other information about the rationale of that change?

--
Andreas
implvalue.c (text/plain, 603 B) 
/* DWARF test case that gets the compiler to emit DW_OP_implicit_value as
   well as multiple instances of DW_OP_implicit_pointer, each pointing
   into the implicit value with varying offsets.  */

volatile int v;

static void
bar (char *a, char *b, char *c, char *d)
{
  v = *a + *b + *c + *d;
}

int
main ()
{
  __int128 u = (((__int128) 0x11223344556677 << 64) | 0x8899aabbccddeeff);
  char *a = (char *) &u;

  bar (a, a + 4, a + 8, a + 12);

  /* Ensure that a location list is needed for u, so DW_AT_const_value can
     not be used.  */
  u = ~u;
  bar (a, a + 4, a + 8, a + 12);

  return 0;
}
implvalue.debug (text/plain, 6.2 KB) 
Contents of the .debug_info section:

  Compilation Unit @ offset 0x0:
   Length:        0x159 (32-bit)
   Version:       4
   Abbrev Offset: 0x0
   Pointer Size:  8
 <0><b>: Abbrev Number: 1 (DW_TAG_compile_unit)
    <c>   DW_AT_producer    : (indirect string, offset: 0x21): GNU C11 7.0.0 20161222 (experimental) -march=zEC12 -mno-vx -mzarch -m64 -g -O3
    <10>   DW_AT_language    : 12	(ANSI C99)
    <11>   DW_AT_name        : (indirect string, offset: 0x10): implvalue.c
    <15>   DW_AT_comp_dir    : (indirect string, offset: 0x0): /home/arnez/tmp
    <19>   DW_AT_ranges      : 0x0
    <1d>   DW_AT_low_pc      : 0x0
    <25>   DW_AT_stmt_list   : 0x0
 <1><29>: Abbrev Number: 2 (DW_TAG_variable)
    <2a>   DW_AT_name        : v
    <2c>   DW_AT_decl_file   : 1
    <2d>   DW_AT_decl_line   : 5
    <2e>   DW_AT_type        : <0x43>
    <32>   DW_AT_external    : 1
    <32>   DW_AT_location    : 9 byte block: 3 0 0 0 0 10 0 20 3c 	(DW_OP_addr: 1000203c)
 <1><3c>: Abbrev Number: 3 (DW_TAG_base_type)
    <3d>   DW_AT_byte_size   : 4
    <3e>   DW_AT_encoding    : 5	(signed)
    <3f>   DW_AT_name        : int
 <1><43>: Abbrev Number: 4 (DW_TAG_volatile_type)
    <44>   DW_AT_type        : <0x3c>
 <1><48>: Abbrev Number: 5 (DW_TAG_subprogram)
    <49>   DW_AT_external    : 1
    <49>   DW_AT_name        : (indirect string, offset: 0x1c): main
    <4d>   DW_AT_decl_file   : 1
    <4e>   DW_AT_decl_line   : 14
    <4f>   DW_AT_type        : <0x3c>
    <53>   DW_AT_low_pc      : 0x10000418
    <5b>   DW_AT_high_pc     : 0x18
    <63>   DW_AT_frame_base  : 1 byte block: 9c 	(DW_OP_call_frame_cfa)
    <65>   DW_AT_GNU_all_call_sites: 1
    <65>   DW_AT_sibling     : <0x11b>
 <2><69>: Abbrev Number: 6 (DW_TAG_variable)
    <6a>   DW_AT_name        : u
    <6c>   DW_AT_decl_file   : 1
    <6d>   DW_AT_decl_line   : 16
    <6e>   DW_AT_type        : <0x11b>
    <72>   DW_AT_location    : 0x0 (location list)
 <2><76>: Abbrev Number: 7 (DW_TAG_variable)
    <77>   DW_AT_name        : a
    <79>   DW_AT_decl_file   : 1
    <7a>   DW_AT_decl_line   : 17
    <7b>   DW_AT_type        : <0x122>
    <7f>   DW_AT_location    : 6 byte block: f2 0 0 0 69 0 	(DW_OP_GNU_implicit_pointer: <0x69> 0)
 <2><86>: Abbrev Number: 8 (DW_TAG_inlined_subroutine)
    <87>   DW_AT_abstract_origin: <0x12f>
    <8b>   DW_AT_low_pc      : 0x10000418
    <93>   DW_AT_high_pc     : 0xc
    <9b>   DW_AT_call_file   : 1
    <9c>   DW_AT_call_line   : 19
    <9d>   DW_AT_sibling     : <0xd2>
 <3><a1>: Abbrev Number: 9 (DW_TAG_formal_parameter)
    <a2>   DW_AT_abstract_origin: <0x137>
    <a6>   DW_AT_location    : 6 byte block: f2 0 0 0 69 0 	(DW_OP_GNU_implicit_pointer: <0x69> 0)
 <3><ad>: Abbrev Number: 9 (DW_TAG_formal_parameter)
    <ae>   DW_AT_abstract_origin: <0x140>
    <b2>   DW_AT_location    : 6 byte block: f2 0 0 0 69 4 	(DW_OP_GNU_implicit_pointer: <0x69> 4)
 <3><b9>: Abbrev Number: 9 (DW_TAG_formal_parameter)
    <ba>   DW_AT_abstract_origin: <0x149>
    <be>   DW_AT_location    : 6 byte block: f2 0 0 0 69 8 	(DW_OP_GNU_implicit_pointer: <0x69> 8)
 <3><c5>: Abbrev Number: 9 (DW_TAG_formal_parameter)
    <c6>   DW_AT_abstract_origin: <0x152>
    <ca>   DW_AT_location    : 6 byte block: f2 0 0 0 69 c 	(DW_OP_GNU_implicit_pointer: <0x69> 12)
 <3><d1>: Abbrev Number: 0
 <2><d2>: Abbrev Number: 10 (DW_TAG_inlined_subroutine)
    <d3>   DW_AT_abstract_origin: <0x12f>
    <d7>   DW_AT_low_pc      : 0x10000424
    <df>   DW_AT_high_pc     : 0x6
    <e7>   DW_AT_call_file   : 1
    <e8>   DW_AT_call_line   : 24
 <3><e9>: Abbrev Number: 9 (DW_TAG_formal_parameter)
    <ea>   DW_AT_abstract_origin: <0x137>
    <ee>   DW_AT_location    : 6 byte block: f2 0 0 0 69 0 	(DW_OP_GNU_implicit_pointer: <0x69> 0)
 <3><f5>: Abbrev Number: 9 (DW_TAG_formal_parameter)
    <f6>   DW_AT_abstract_origin: <0x140>
    <fa>   DW_AT_location    : 6 byte block: f2 0 0 0 69 4 	(DW_OP_GNU_implicit_pointer: <0x69> 4)
 <3><101>: Abbrev Number: 9 (DW_TAG_formal_parameter)
    <102>   DW_AT_abstract_origin: <0x149>
    <106>   DW_AT_location    : 6 byte block: f2 0 0 0 69 8 	(DW_OP_GNU_implicit_pointer: <0x69> 8)
 <3><10d>: Abbrev Number: 9 (DW_TAG_formal_parameter)
    <10e>   DW_AT_abstract_origin: <0x152>
    <112>   DW_AT_location    : 6 byte block: f2 0 0 0 69 c 	(DW_OP_GNU_implicit_pointer: <0x69> 12)
 <3><119>: Abbrev Number: 0
 <2><11a>: Abbrev Number: 0
 <1><11b>: Abbrev Number: 11 (DW_TAG_base_type)
    <11c>   DW_AT_byte_size   : 16
    <11d>   DW_AT_encoding    : 5	(signed)
    <11e>   DW_AT_name        : (indirect string, offset: 0x70): __int128
 <1><122>: Abbrev Number: 12 (DW_TAG_pointer_type)
    <123>   DW_AT_byte_size   : 8
    <124>   DW_AT_type        : <0x128>
 <1><128>: Abbrev Number: 11 (DW_TAG_base_type)
    <129>   DW_AT_byte_size   : 1
    <12a>   DW_AT_encoding    : 8	(unsigned char)
    <12b>   DW_AT_name        : (indirect string, offset: 0x79): char
 <1><12f>: Abbrev Number: 13 (DW_TAG_subprogram)
    <130>   DW_AT_name        : bar
    <134>   DW_AT_decl_file   : 1
    <135>   DW_AT_decl_line   : 8
    <136>   DW_AT_prototyped  : 1
    <136>   DW_AT_inline      : 1	(inlined)
 <2><137>: Abbrev Number: 14 (DW_TAG_formal_parameter)
    <138>   DW_AT_name        : a
    <13a>   DW_AT_decl_file   : 1
    <13b>   DW_AT_decl_line   : 8
    <13c>   DW_AT_type        : <0x122>
 <2><140>: Abbrev Number: 14 (DW_TAG_formal_parameter)
    <141>   DW_AT_name        : b
    <143>   DW_AT_decl_file   : 1
    <144>   DW_AT_decl_line   : 8
    <145>   DW_AT_type        : <0x122>
 <2><149>: Abbrev Number: 14 (DW_TAG_formal_parameter)
    <14a>   DW_AT_name        : c
    <14c>   DW_AT_decl_file   : 1
    <14d>   DW_AT_decl_line   : 8
    <14e>   DW_AT_type        : <0x122>
 <2><152>: Abbrev Number: 14 (DW_TAG_formal_parameter)
    <153>   DW_AT_name        : d
    <155>   DW_AT_decl_file   : 1
    <156>   DW_AT_decl_line   : 8
    <157>   DW_AT_type        : <0x122>
 <2><15b>: Abbrev Number: 0
 <1><15c>: Abbrev Number: 0

Contents of the .debug_loc section:

    Offset   Begin            End              Expression
    00000000 0000000010000418 0000000010000424 (DW_OP_implicit_value 16 byte block: 0 11 22 33 44 55 66 77 88 99 aa bb cc dd ee ff )
    00000024 0000000010000424 000000001000042a (DW_OP_implicit_value 16 byte block: ff ee dd cc bb aa 99 88 77 66 55 44 33 22 11 0 )
    00000048 <End of list>
implvalue.gdblog (text/plain, 676 B) 
Reading symbols from implvalue...done.
(gdb) b main
Breakpoint 1 at 0x10000418: file implvalue.c, line 15.
(gdb) r
Starting program: /home/arnez/tmp/implvalue 

Breakpoint 1, main () at implvalue.c:19
(gdb) si
0x000000001000041e in bar (a=<synthetic pointer>, b=<synthetic pointer>, 
    c=<synthetic pointer>, d=<synthetic pointer>) at implvalue.c:15
(gdb) p/x (char [4]) {*a, *b, *c, *d}
$1 = {0xff, 0xff, 0xff, 0xff}
(gdb) p/x *(char (*)[16]) a
$2 = {0x0, 0x11, 0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88, 0x99, 0xaa, 0xbb, 0xcc, 0xdd, 
  0xee, 0xff}
(gdb) p/x (char [8]) {a[0], a[1], a[2], a[3], a[4], a[5], a[6], a[7]}
$3 = {0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff}
lmpx.com only provides a reader for public news (NNTP) servers. It is not affiliated with the servers or forums shown here and is not responsible for the content of articles, which is written by their respective authors.