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

[Andreas Arnez <arnez@linux.vnet.ibm.com>]

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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


[-- Attachment #2: implvalue.c --]
[-- Type: text/plain, Size: 603 bytes --]

/* 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;
}

[-- Attachment #3: readelf -wio output --]
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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>


[-- Attachment #4: GDB session --]
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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}