[LLVMdev] [lldb-dev] How is variable info retrieved in debugging for executables generated by llvm backend?

杨勇勇 triple.yang at gmail.com
Tue Feb 18 23:08:12 PST 2014


Thank you.

Here is an example and the attchment contains extra files including object
file and executable file.
I want to print for example the value of "a", but lldb command "frame
variable a" displays "0" and so does "b", and "c".
Meanwhile, the value is correct if I directly check registers or memory.

Following content is composed of three part:
1. C program.
2. assembly file including directives of dwarf debug info.
3. dwarf debug info dumpped from executable file with llvm-dwarfdump.

///////////////////////////////////////////////////////////////////////////////////////////
// C
int
main(int argc, char *argv[])
{
  int a = 11;
  int b = 22;
  int c = a+b;
  return c;
}

######################################
# asm including dwarf debugging info
.file "test.c"
.section .debug_info,"", at progbits
Lsection_info:
.section .debug_abbrev,"", at progbits
Lsection_abbrev:
.section .debug_aranges,"", at progbits
.section .debug_macinfo,"", at progbits
.section .debug_line,"", at progbits
Lsection_line:
.section .debug_loc,"", at progbits
.section .debug_pubtypes,"", at progbits
.section .debug_str,"MS", at progbits,1
Linfo_string:
.section .debug_ranges,"", at progbits
Ldebug_range:
.section .debug_loc,"", at progbits
Lsection_debug_loc:
.text
Ltext_begin:
.data
.file 1 "test.c"
.text
.global main
.type main, at function
main:                                   # @main
Lfunc_begin0:
.loc 1 3 0                   # test.c:3:0
# BB#0:                                 # %entry
.loc 1 2 0 prologue_end      # test.c:2:0
[SP + -4] = FP;;
[SP + -8] = J30;;
FP = SP;;
J31 = -40;;
SP = SP + J31;;
#DEBUG_VALUE: main:argc <- undef
#DEBUG_VALUE: main:argv <- undef
R2 = 0;;
R3 = 11;;
[FP + -12] = R2;;
[FP + -16] = R0;;
[FP + -20] = R1;;
.loc 1 4 0                   # test.c:4:0
[FP + -24] = R3;;
R2 = 22;;
.loc 1 5 0                   # test.c:5:0
[FP + -28] = R2;;
.loc 1 6 0                   # test.c:6:0
R3 = [FP + -24];;
R2 = R3 + R2 (T);;
[FP + -32] = R2;;
.loc 1 7 0                   # test.c:7:0
[FP + -36] = R0;;
R0 = R2;;
[FP + -40] = R1;;
SP = FP;;
J30 = [SP + -8];;
FP = [SP + -4];;
jump J30;;
Ltmp0:
Ltmp1:
.size main, Ltmp1-main
Lfunc_end0:

Ltext_end:
.data
Ldata_end:
.text
Lsection_end1:
.section .debug_info,"", at progbits
L.debug_info_begin0:
.int 148                     # Length of Compilation Unit Info
.short 2                       # DWARF version number
.int L.debug_abbrev_begin    # Offset Into Abbrev. Section
.byte 4                       # Address Size (in bytes)
.byte 1                       # Abbrev [1] 0xb:0x8d DW_TAG_compile_unit
.int Linfo_string0           # DW_AT_producer
.short 12                      # DW_AT_language
.int Linfo_string1           # DW_AT_name
.int 0                       # DW_AT_low_pc
.int Lsection_line           # DW_AT_stmt_list
.int Linfo_string2           # DW_AT_comp_dir
.byte 2                       # Abbrev [2] 0x22:0x5d DW_TAG_subprogram
.int Linfo_string3           # DW_AT_name
.byte 1                       # DW_AT_decl_file
.byte 2                       # DW_AT_decl_line
                                        # DW_AT_prototyped
.int 127                     # DW_AT_type
                                        # DW_AT_external
.int Lfunc_begin0            # DW_AT_low_pc
.int Lfunc_end0              # DW_AT_high_pc
.byte 2                       # DW_AT_frame_base
.byte 144
.byte 60
.byte 3                       # Abbrev [3] 0x38:0xe DW_TAG_formal_parameter
.int Linfo_string5           # DW_AT_name
.byte 1                       # DW_AT_decl_file
.byte 2                       # DW_AT_decl_line
.int 127                     # DW_AT_type
.byte 2                       # DW_AT_location
.byte 145
.byte 24
.byte 3                       # Abbrev [3] 0x46:0xe DW_TAG_formal_parameter
.int Linfo_string6           # DW_AT_name
.byte 1                       # DW_AT_decl_file
.byte 2                       # DW_AT_decl_line
.int 146                     # DW_AT_type
.byte 2                       # DW_AT_location
.byte 145
.byte 20
.byte 4                       # Abbrev [4] 0x54:0xe DW_TAG_variable
.int Linfo_string8           # DW_AT_name
.byte 1                       # DW_AT_decl_file
.byte 4                       # DW_AT_decl_line
.int 127                     # DW_AT_type
.byte 2                       # DW_AT_location
.byte 145
.byte 16
.byte 4                       # Abbrev [4] 0x62:0xe DW_TAG_variable
.int Linfo_string9           # DW_AT_name
.byte 1                       # DW_AT_decl_file
.byte 5                       # DW_AT_decl_line
.int 127                     # DW_AT_type
.byte 2                       # DW_AT_location
.byte 145
.byte 12
.byte 4                       # Abbrev [4] 0x70:0xe DW_TAG_variable
.int Linfo_string10          # DW_AT_name
.byte 1                       # DW_AT_decl_file
.byte 6                       # DW_AT_decl_line
.int 127                     # DW_AT_type
.byte 2                       # DW_AT_location
.byte 145
.byte 8
.byte 0                       # End Of Children Mark
.byte 5                       # Abbrev [5] 0x7f:0x7 DW_TAG_base_type
.int Linfo_string4           # DW_AT_name
.byte 5                       # DW_AT_encoding
.byte 4                       # DW_AT_byte_size
.byte 5                       # Abbrev [5] 0x86:0x7 DW_TAG_base_type
.int Linfo_string7           # DW_AT_name
.byte 6                       # DW_AT_encoding
.byte 1                       # DW_AT_byte_size
.byte 6                       # Abbrev [6] 0x8d:0x5 DW_TAG_pointer_type
.int 134                     # DW_AT_type
.byte 6                       # Abbrev [6] 0x92:0x5 DW_TAG_pointer_type
.int 141                     # DW_AT_type
.byte 0                       # End Of Children Mark
L.debug_info_end0:
.section .debug_abbrev,"", at progbits
L.debug_abbrev_begin:
.byte 1                       # Abbreviation Code
.byte 17                      # DW_TAG_compile_unit
.byte 1                       # DW_CHILDREN_yes
.byte 37                      # DW_AT_producer
.byte 14                      # DW_FORM_strp
.byte 19                      # DW_AT_language
.byte 5                       # DW_FORM_data2
.byte 3                       # DW_AT_name
.byte 14                      # DW_FORM_strp
.byte 17                      # DW_AT_low_pc
.byte 1                       # DW_FORM_addr
.byte 16                      # DW_AT_stmt_list
.byte 6                       # DW_FORM_data4
.byte 27                      # DW_AT_comp_dir
.byte 14                      # DW_FORM_strp
.byte 0                       # EOM(1)
.byte 0                       # EOM(2)
.byte 2                       # Abbreviation Code
.byte 46                      # DW_TAG_subprogram
.byte 1                       # DW_CHILDREN_yes
.byte 3                       # DW_AT_name
.byte 14                      # DW_FORM_strp
.byte 58                      # DW_AT_decl_file
.byte 11                      # DW_FORM_data1
.byte 59                      # DW_AT_decl_line
.byte 11                      # DW_FORM_data1
.byte 39                      # DW_AT_prototyped
.byte 25                      # DW_FORM_flag_present
.byte 73                      # DW_AT_type
.byte 19                      # DW_FORM_ref4
.byte 63                      # DW_AT_external
.byte 25                      # DW_FORM_flag_present
.byte 17                      # DW_AT_low_pc
.byte 1                       # DW_FORM_addr
.byte 18                      # DW_AT_high_pc
.byte 1                       # DW_FORM_addr
.byte 64                      # DW_AT_frame_base
.byte 10                      # DW_FORM_block1
.byte 0                       # EOM(1)
.byte 0                       # EOM(2)
.byte 3                       # Abbreviation Code
.byte 5                       # DW_TAG_formal_parameter
.byte 0                       # DW_CHILDREN_no
.byte 3                       # DW_AT_name
.byte 14                      # DW_FORM_strp
.byte 58                      # DW_AT_decl_file
.byte 11                      # DW_FORM_data1
.byte 59                      # DW_AT_decl_line
.byte 11                      # DW_FORM_data1
.byte 73                      # DW_AT_type
.byte 19                      # DW_FORM_ref4
.byte 2                       # DW_AT_location
.byte 10                      # DW_FORM_block1
.byte 0                       # EOM(1)
.byte 0                       # EOM(2)
.byte 4                       # Abbreviation Code
.byte 52                      # DW_TAG_variable
.byte 0                       # DW_CHILDREN_no
.byte 3                       # DW_AT_name
.byte 14                      # DW_FORM_strp
.byte 58                      # DW_AT_decl_file
.byte 11                      # DW_FORM_data1
.byte 59                      # DW_AT_decl_line
.byte 11                      # DW_FORM_data1
.byte 73                      # DW_AT_type
.byte 19                      # DW_FORM_ref4
.byte 2                       # DW_AT_location
.byte 10                      # DW_FORM_block1
.byte 0                       # EOM(1)
.byte 0                       # EOM(2)
.byte 5                       # Abbreviation Code
.byte 36                      # DW_TAG_base_type
.byte 0                       # DW_CHILDREN_no
.byte 3                       # DW_AT_name
.byte 14                      # DW_FORM_strp
.byte 62                      # DW_AT_encoding
.byte 11                      # DW_FORM_data1
.byte 11                      # DW_AT_byte_size
.byte 11                      # DW_FORM_data1
.byte 0                       # EOM(1)
.byte 0                       # EOM(2)
.byte 6                       # Abbreviation Code
.byte 15                      # DW_TAG_pointer_type
.byte 0                       # DW_CHILDREN_no
.byte 73                      # DW_AT_type
.byte 19                      # DW_FORM_ref4
.byte 0                       # EOM(1)
.byte 0                       # EOM(2)
.byte 0                       # EOM(3)
L.debug_abbrev_end:
.section .debug_aranges,"", at progbits
.section .debug_ranges,"", at progbits
.section .debug_macinfo,"", at progbits
.section .debug_str,"MS", at progbits,1
Linfo_string0:
.asciz "clang version 3.3 (/opt/git.repo/clang.git/
b422d20530588813b09057b45d5b383f0b175ced) (/opt/git.repo/llvm.git/
57b428f0a6be7b81bc364b0088992b1f820b516e)"
Linfo_string1:
.asciz "test.c"
Linfo_string2:
.asciz "/home/yangyy/workspace/newlib"
Linfo_string3:
.asciz "main"
Linfo_string4:
.asciz "int"
Linfo_string5:
.asciz "argc"
Linfo_string6:
.asciz "argv"
Linfo_string7:
.asciz "char"
Linfo_string8:
.asciz "a"
Linfo_string9:
.asciz "b"
Linfo_string10:
.asciz "c"

#########################################
# dwarf debug info dumpped from its executable file with llvm-dwarfdump
mspu.out: file format ELF32-mspu

.debug_abbrev contents:
Abbrev table for offset: 0x00000000
[1] DW_TAG_compile_unit DW_CHILDREN_yes
DW_AT_producer DW_FORM_strp
DW_AT_language DW_FORM_data2
DW_AT_name DW_FORM_strp
DW_AT_low_pc DW_FORM_addr
DW_AT_stmt_list DW_FORM_data4
DW_AT_comp_dir DW_FORM_strp

[2] DW_TAG_subprogram DW_CHILDREN_yes
DW_AT_name DW_FORM_strp
DW_AT_decl_file DW_FORM_data1
DW_AT_decl_line DW_FORM_data1
DW_AT_prototyped DW_FORM_flag_present
DW_AT_type DW_FORM_ref4
DW_AT_external DW_FORM_flag_present
DW_AT_low_pc DW_FORM_addr
DW_AT_high_pc DW_FORM_addr
DW_AT_frame_base DW_FORM_block1

[3] DW_TAG_formal_parameter DW_CHILDREN_no
DW_AT_name DW_FORM_strp
DW_AT_decl_file DW_FORM_data1
DW_AT_decl_line DW_FORM_data1
DW_AT_type DW_FORM_ref4
DW_AT_location DW_FORM_block1

[4] DW_TAG_variable DW_CHILDREN_no
DW_AT_name DW_FORM_strp
DW_AT_decl_file DW_FORM_data1
DW_AT_decl_line DW_FORM_data1
DW_AT_type DW_FORM_ref4
DW_AT_location DW_FORM_block1

[5] DW_TAG_base_type DW_CHILDREN_no
DW_AT_name DW_FORM_strp
DW_AT_encoding DW_FORM_data1
DW_AT_byte_size DW_FORM_data1

[6] DW_TAG_pointer_type DW_CHILDREN_no
DW_AT_type DW_FORM_ref4


.debug_info contents:
0x00000000: Compile Unit: length = 0x00000094 version = 0x0002 abbr_offset
= 0x0000 addr_size = 0x04 (next CU at 0x00000098)

0x0000000b: DW_TAG_compile_unit [1] *
0x0000000c:   DW_AT_producer [DW_FORM_strp] ( .debug_str[0x00000000] =
"clang version 3.3 (/opt/git.repo/clang.git/
b422d20530588813b09057b45d5b383f0b175ced) (/opt/git.repo/llvm.git/
57b428f0a6be7b81bc364b0088992b1f820b516e)")
0x00000010:   DW_AT_language [DW_FORM_data2] (0x000c)
0x00000012:   DW_AT_name [DW_FORM_strp] ( .debug_str[0x00000099] = "test.c")
0x00000016:   DW_AT_low_pc [DW_FORM_addr] (0x0000000000000000)
0x0000001a:   DW_AT_stmt_list [DW_FORM_data4] (0x00000000)
0x0000001e:   DW_AT_comp_dir [DW_FORM_strp] ( .debug_str[0x000000a0] =
"/home/yangyy/workspace/newlib")

0x00000022:   DW_TAG_subprogram [2] *
0x00000023:     DW_AT_name [DW_FORM_strp] ( .debug_str[0x000000be] = "main")
0x00000027:     DW_AT_decl_file [DW_FORM_data1] (0x01)
0x00000028:     DW_AT_decl_line [DW_FORM_data1] (0x02)
0x00000029:     DW_AT_prototyped [DW_FORM_flag_present] (true)
0x00000029:     DW_AT_type [DW_FORM_ref4] (cu + 0x007f => {0x0000007f})
0x0000002d:     DW_AT_external [DW_FORM_flag_present] (true)
0x0000002d:     DW_AT_low_pc [DW_FORM_addr] (0x0000000000000028)
0x00000031:     DW_AT_high_pc [DW_FORM_addr] (0x0000000000000084)
0x00000035:     DW_AT_frame_base [DW_FORM_block1] (<0x02> 90 3c )

0x00000038:     DW_TAG_formal_parameter [3]
0x00000039:       DW_AT_name [DW_FORM_strp] ( .debug_str[0x000000c7] =
"argc")
0x0000003d:       DW_AT_decl_file [DW_FORM_data1] (0x01)
0x0000003e:       DW_AT_decl_line [DW_FORM_data1] (0x02)
0x0000003f:       DW_AT_type [DW_FORM_ref4] (cu + 0x007f => {0x0000007f})
0x00000043:       DW_AT_location [DW_FORM_block1] (<0x02> 91 18 )

0x00000046:     DW_TAG_formal_parameter [3]
0x00000047:       DW_AT_name [DW_FORM_strp] ( .debug_str[0x000000cc] =
"argv")
0x0000004b:       DW_AT_decl_file [DW_FORM_data1] (0x01)
0x0000004c:       DW_AT_decl_line [DW_FORM_data1] (0x02)
0x0000004d:       DW_AT_type [DW_FORM_ref4] (cu + 0x0092 => {0x00000092})
0x00000051:       DW_AT_location [DW_FORM_block1] (<0x02> 91 14 )

0x00000054:     DW_TAG_variable [4]
0x00000055:       DW_AT_name [DW_FORM_strp] ( .debug_str[0x000000d6] = "a")
0x00000059:       DW_AT_decl_file [DW_FORM_data1] (0x01)
0x0000005a:       DW_AT_decl_line [DW_FORM_data1] (0x04)
0x0000005b:       DW_AT_type [DW_FORM_ref4] (cu + 0x007f => {0x0000007f})
0x0000005f:       DW_AT_location [DW_FORM_block1] (<0x02> 91 10 )

0x00000062:     DW_TAG_variable [4]
0x00000063:       DW_AT_name [DW_FORM_strp] ( .debug_str[0x000000d8] = "b")
0x00000067:       DW_AT_decl_file [DW_FORM_data1] (0x01)
0x00000068:       DW_AT_decl_line [DW_FORM_data1] (0x05)
0x00000069:       DW_AT_type [DW_FORM_ref4] (cu + 0x007f => {0x0000007f})
0x0000006d:       DW_AT_location [DW_FORM_block1] (<0x02> 91 0c )

0x00000070:     DW_TAG_variable [4]
0x00000071:       DW_AT_name [DW_FORM_strp] ( .debug_str[0x000000da] = "c")
0x00000075:       DW_AT_decl_file [DW_FORM_data1] (0x01)
0x00000076:       DW_AT_decl_line [DW_FORM_data1] (0x06)
0x00000077:       DW_AT_type [DW_FORM_ref4] (cu + 0x007f => {0x0000007f})
0x0000007b:       DW_AT_location [DW_FORM_block1] (<0x02> 91 08 )

0x0000007e:     NULL

0x0000007f:   DW_TAG_base_type [5]
0x00000080:     DW_AT_name [DW_FORM_strp] ( .debug_str[0x000000c3] = "int")
0x00000084:     DW_AT_encoding [DW_FORM_data1] (0x05)
0x00000085:     DW_AT_byte_size [DW_FORM_data1] (0x04)

0x00000086:   DW_TAG_base_type [5]
0x00000087:     DW_AT_name [DW_FORM_strp] ( .debug_str[0x000000d1] = "char")
0x0000008b:     DW_AT_encoding [DW_FORM_data1] (0x06)
0x0000008c:     DW_AT_byte_size [DW_FORM_data1] (0x01)

0x0000008d:   DW_TAG_pointer_type [6]
0x0000008e:     DW_AT_type [DW_FORM_ref4] (cu + 0x0086 => {0x00000086})

0x00000092:   DW_TAG_pointer_type [6]
0x00000093:     DW_AT_type [DW_FORM_ref4] (cu + 0x008d => {0x0000008d})

0x00000097:   NULL

.debug_frame contents:


.debug_aranges contents:

.debug_line contents:
Line table prologue:
   total_length: 0x00000037
        version: 2
prologue_length: 0x0000001d
min_inst_length: 1
default_is_stmt: 1
      line_base: -5
     line_range: 14
    opcode_base: 13
standard_opcode_lengths[DW_LNS_copy] = 0
standard_opcode_lengths[DW_LNS_advance_pc] = 1
standard_opcode_lengths[DW_LNS_advance_line] = 1
standard_opcode_lengths[DW_LNS_set_file] = 1
standard_opcode_lengths[DW_LNS_set_column] = 1
standard_opcode_lengths[DW_LNS_negate_stmt] = 0
standard_opcode_lengths[DW_LNS_set_basic_block] = 0
standard_opcode_lengths[DW_LNS_const_add_pc] = 0
standard_opcode_lengths[DW_LNS_fixed_advance_pc] = 1
standard_opcode_lengths[DW_LNS_set_prologue_end] = 0
standard_opcode_lengths[DW_LNS_set_epilogue_begin] = 0
standard_opcode_lengths[DW_LNS_set_isa] = 1
                Dir  Mod Time   File Len   File Name
                ---- ---------- ---------- ---------------------------
file_names[  1]    0 0x00000000 0x00000000 test.c

Address            Line   Column File   ISA Flags
------------------ ------ ------ ------ --- -------------
0x0000000000000028      2      0      1   0  is_stmt prologue_end
0x0000000000000050      4      0      1   0  is_stmt
0x0000000000000058      5      0      1   0  is_stmt
0x000000000000005c      6      0      1   0  is_stmt
0x0000000000000068      7      0      1   0  is_stmt
0x0000000000000084      7      0      1   0  is_stmt end_sequence

.debug_str contents:
0x00000000: "clang version 3.3 (/opt/git.repo/clang.git/
b422d20530588813b09057b45d5b383f0b175ced) (/opt/git.repo/llvm.git/
57b428f0a6be7b81bc364b0088992b1f820b516e)"
0x00000099: "test.c"
0x000000a0: "/home/yangyy/workspace/newlib"
0x000000be: "main"
0x000000c3: "int"
0x000000c7: "argc"
0x000000cc: "argv"
0x000000d1: "char"
0x000000d6: "a"
0x000000d8: "b"
0x000000da: "c"

.debug_ranges contents:

.debug_pubnames contents:
Length:                0
Version:               0
Offset in .debug_info: 0
Size:                  0

  Offset    Name

.debug_abbrev.dwo contents:
< EMPTY >



2014-02-19 2:28 GMT+08:00 Greg Clayton <gclayton at apple.com>:

> All of this information is contained in the DWARF debug info that you must
> generate. Are you generating DWARF? If not, you will need to. If so, please
> attach an example program that contains DWARF and specify which function
> you are having trouble getting variable information for.
>
> Greg Clayton
>
> On Feb 18, 2014, at 12:44 AM, 杨勇勇 <triple.yang at gmail.com> wrote:
>
> > Hi, all
> >
> > I ported llvm backend and lldb recently. Both tools can basically work.
> > lldb is able to debug programs in asm style and frame unwinding is OK.
> >
> > But "frame variable XX" does not work because lldb is not able to
> determine the address of
> > XX from debug info.
> >
> > Can someone give any clue?
> > Thanks in advance.
> >
> > --
> > 杨勇勇 (Yang Yong-Yong)
> > _______________________________________________
> > lldb-dev mailing list
> > lldb-dev at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/lldb-dev
>
>


-- 
杨勇勇 (Yang Yong-Yong)
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