Re: Symbol can't be found unless type 'tab'

[Jan Kratochvil <jan.kratochvil@redhat.com>]

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On Wed, 12 Sep 2012 09:44:44 +0200, Yao Qi wrote:
> On 09/10/2012 09:18 PM, Jan Kratochvil wrote:
> >On Mon, 10 Sep 2012 14:51:29 +0200, Yao Qi wrote:
> >>>  <1><2c>: Abbrev Number: 3 (DW_TAG_variable)
> >>>     <2d>   DW_AT_name        : i
> >>>     <2f>   DW_AT_type        : <0x23>
> >>>     <33>   DW_AT_location    : 0x0      (location list)
> >I guess because GDB does not like such complicated location expression for
> >glboal variables.
> 
> Jan, thanks for the pointer.
> 
> The "symptom" is symbol is not found in psymtab, but in symtab when
> I press 'tab' which triggers converting psymtab to symtab.  I wonder
> why symbol 'i' is *not* included in psymtab.

Because it has the complex location I suggested above.


> Examine the source of
> dwarf2read.c:add_partial_symbol, especially the case block for
> 'DW_TAG_variable', gives me some clue, and I add attribute
> DW_AT_external to symbol 'i' like this,
> 
>  <1><33>: Abbrev Number: 3 (DW_TAG_variable)
>     <34>   DW_AT_name        : i
>     <36>   DW_AT_type        : <0x2a>
>     <3a>   DW_AT_external    : 1  <--- here
>     <3b>   DW_AT_location    : 0x0      (location list)
> 
> and then symbol 'i' can be found by GDB without typing extra 'tab'.

It is difficult to talk about the problem when you have not provided the
testcase, I have provided mine now.

But DW_AT_external is not significant to GDB as GDB can display even static
variables, even when they are not in current CU.

It is good to verify 'maintenance info symtabs' is empty during these
experiments, when one tries to print the variable.

I am very sure that this complex DW_AT_location is not properly supported by
psymtabs in GDB, this is one of the reasons I was doing the fix and its later
undo as I referenced in the previous mail.

Why didn't use just normal DWARF block, instead of the location list?
This part does not matter for your testcase and it will work with GDB.


Jan

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int i;

[-- Attachment #3: i.s --]
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	.file	"i.c"
	.text
.Ltext0:
	.comm	i,4,4
.Letext0:
	.file 1 "i.c"
	.section	.debug_info,"",@progbits
.Ldebug_info0:
	.long	1f - 2f	# Length of Compilation Unit Info
2:
	.value	0x4	# DWARF version number
	.long	.Ldebug_abbrev0	# Offset Into Abbrev. Section
	.byte	0x8	# Pointer Size (in bytes)
	.uleb128 0x1	# (DIE (0xb) DW_TAG_compile_unit)
	.long	.LASF0	# DW_AT_producer: "GNU C 4.6.3 20120306 (Red Hat 4.6.3-2) -mtune=generic -march=x86-64 -g"
	.byte	0x2	# DW_AT_language
	.ascii "i.c\0"	# DW_AT_name
#	.long	.LASF1	# DW_AT_comp_dir: "/home/jkratoch/t"
#	.long	.Ldebug_line0	# DW_AT_stmt_list
	.uleb128 0x2	# (DIE (0x1d) DW_TAG_variable)
	.ascii "i\0"	# DW_AT_name
#	.byte	0x1	# DW_AT_decl_file (i.c)
#	.byte	0x1	# DW_AT_decl_line
	.long	die30-.Ldebug_info0	# DW_AT_type
			# DW_AT_external
	.uleb128 0x9	# DW_AT_location
	.byte	0x3	# DW_OP_addr
	.quad	i
die30:
	.uleb128 0x3	# (DIE (0x30) DW_TAG_base_type)
	.byte	0x8	# DW_AT_byte_size
	.byte	0x5	# DW_AT_encoding
	.ascii "int\0"	# DW_AT_name
	.byte	0	# end of children of DIE 0xb
1:
	.section	.debug_abbrev,"",@progbits
.Ldebug_abbrev0:
	.uleb128 0x1	# (abbrev code)
	.uleb128 0x11	# (TAG: DW_TAG_compile_unit)
	.byte	0x1	# DW_children_yes
	.uleb128 0x25	# (DW_AT_producer)
	.uleb128 0xe	# (DW_FORM_strp)
	.uleb128 0x13	# (DW_AT_language)
	.uleb128 0xb	# (DW_FORM_data1)
	.uleb128 0x3	# (DW_AT_name)
	.uleb128 0x8	# (DW_FORM_string)
#	.uleb128 0x1b	# (DW_AT_comp_dir)
#	.uleb128 0xe	# (DW_FORM_strp)
#	.uleb128 0x10	# (DW_AT_stmt_list)
#	.uleb128 0x17	# (DW_FORM_sec_offset)
	.byte	0
	.byte	0
	.uleb128 0x2	# (abbrev code)
	.uleb128 0x34	# (TAG: DW_TAG_variable)
	.byte	0	# DW_children_no
	.uleb128 0x3	# (DW_AT_name)
	.uleb128 0x8	# (DW_FORM_string)
#	.uleb128 0x3a	# (DW_AT_decl_file)
#	.uleb128 0xb	# (DW_FORM_data1)
#	.uleb128 0x3b	# (DW_AT_decl_line)
#	.uleb128 0xb	# (DW_FORM_data1)
	.uleb128 0x49	# (DW_AT_type)
	.uleb128 0x13	# (DW_FORM_ref4)
#	.uleb128 0x3f	# (DW_AT_external)
#	.uleb128 0x19	# (DW_FORM_flag_present)
	.uleb128 0x2	# (DW_AT_location)
	.uleb128 0x18	# (DW_FORM_exprloc)
	.byte	0
	.byte	0
	.uleb128 0x3	# (abbrev code)
	.uleb128 0x24	# (TAG: DW_TAG_base_type)
	.byte	0	# DW_children_no
	.uleb128 0xb	# (DW_AT_byte_size)
	.uleb128 0xb	# (DW_FORM_data1)
	.uleb128 0x3e	# (DW_AT_encoding)
	.uleb128 0xb	# (DW_FORM_data1)
	.uleb128 0x3	# (DW_AT_name)
	.uleb128 0x8	# (DW_FORM_string)
	.byte	0
	.byte	0
	.byte	0
	.section	.debug_line,"",@progbits
.Ldebug_line0:
	.section	.debug_str,"MS",@progbits,1
.LASF0:
	.string	"GNU C 4.6.3 20120306 (Red Hat 4.6.3-2) -mtune=generic -march=x86-64 -g"
.LASF1:
	.string	"/home/jkratoch/t"
	.ident	"GCC: (GNU) 4.6.3 20120306 (Red Hat 4.6.3-2)"
	.section	.note.GNU-stack,"",@progbits