* [gdb]Line number zero in line table gives incorrect debugging experince with gdb
[not found] ` <CAMyYBzwV9dr5Aue-rrrbtLRc30F==OaVih91kpzF8KpBtaqMNQ@mail.gmail.com>
@ 2020-08-27 15:02 ` Jaydeep Chauhan
2020-08-28 16:59 ` Line " Tom Tromey
0 siblings, 1 reply; 3+ messages in thread
From: Jaydeep Chauhan @ 2020-08-27 15:02 UTC (permalink / raw)
To: gdb
[-- Attachment #1: Type: text/plain, Size: 3814 bytes --]
Hi Everyone,
We have a situation with GDB(latest trunk) and below testcase
referred for further discussion :
$ cat gdb.c
1 /* demo testcase */
2 int garr[2];
3 int main() {
4
5 if((garr[0] && garr[1])==0){
6 garr[0]=1;
7 garr[1]=1;
8 }
9 else {
10 garr[0]=2;
11 garr[1]=2;
12 }
13 printf("%d %d\n",garr[0], garr[1]);
14 return 0;
15 }
$clang -target arm64 -g -c gdb.c
$./llvm-dwarfdump --debug-line gdb.o
...
...
Address Line Column File ISA Discriminator Flags
------------------ ------ ------ ------ --- ------------- -------------
0x0000000000000000 3 0 1 0 0 is_stmt
0x000000000000000c 0 0 1 0 0 is_stmt prologue_end
0x000000000000001c 5 13 1 0 0 is_stmt
0x000000000000002c 5 21 1 0 0
0x0000000000000030 0 21 1 0 0
0x0000000000000034 5 24 1 0 0
0x0000000000000038 5 21 1 0 0
0x0000000000000044 0 21 1 0 0
0x0000000000000048 5 12 1 0 0
0x000000000000004c 0 12 1 0 0
0x0000000000000054 6 24 1 0 0 is_stmt
0x000000000000005c 7 24 1 0 0 is_stmt
0x0000000000000060 8 9 1 0 0 is_stmt
0x0000000000000064 0 9 1 0 0
0x000000000000006c 10 25 1 0 0 is_stmt
0x0000000000000074 11 25 1 0 0 is_stmt
0x0000000000000078 0 0 1 0 0
0x000000000000007c 13 27 1 0 0 is_stmt
0x0000000000000084 13 36 1 0 0
0x0000000000000088 0 0 1 0 0
0x0000000000000090 13 10 1 0 0
0x0000000000000098 14 10 1 0 0 is_stmt
0x00000000000000a8 14 10 1 0 0 is_stmt end_sequence
=================================================================================================
Let’s demonstrate two issues that we see w.r.t gdb that are
related to zero line number.
1)As per above line table prologue_end is set to line zero and we set
breakpoint on function then gdb info was stated like below .
(gdb) b main
Breakpoint 1 at 0x800005ec: file gdb.c, line 0.
It should be, something like
(gdb) b main
Breakpoint 1 at 0x800005ec: file gdb.c, line 3.
2)
Breakpoint 1, main () at gdb.c:0
1 /* demo testcase */
(gdb) n
5 if((garr[0] && garr[1])==0){
(gdb)
1 /* demo testcase */
(gdb)
5 if((garr[0] && garr[1])==0){
(gdb)
1 /* demo testcase */
(gdb)
6 garr[0]=1;
(gdb)
7 garr[1]=1;
(gdb)
8 }
(gdb)
1 /* demo testcase */
(gdb)
13 printf("%d %d\n",garr[0], garr[1]);
(gdb)
1 /* demo testcase */
(gdb)
13 printf("%d %d\n",garr[0], garr[1]);
(gdb)
1 1
14 return 0;
(gdb)
Here the sequence like “5->1->5->6” should be “5->5->6” and as per
DWARF standard ,ZERO line has a well-defined meaning and stands for
"no source line corresponds to this location".
From LLVM point of view the ZERO-LINE debug info added to the
compiler generated remat statement’s like loading global constant
/value ,which doesn’t have “.loc” info from the source.
Please share your valuable thoughts on this issue’s.
Assembly and object files are added in mail.
Thanks in Advance,
Jaydeep.
[-- Attachment #2: gdb.s --]
[-- Type: text/plain, Size: 12340 bytes --]
.text
.file "gdb.c"
.file 1 "/home/bft/Jaydeep/llvm_all/check/llvm-project/install/bin" "gdb.c"
.globl main // -- Begin function main
.p2align 2
.type main,@function
main: // @main
.Lfunc_begin0:
.loc 1 3 0 // gdb.c:3:0
.cfi_sections .debug_frame
.cfi_startproc
// %bb.0:
sub sp, sp, #48 // =48
stp x29, x30, [sp, #32] // 16-byte Folded Spill
add x29, sp, #32 // =32
.cfi_def_cfa w29, 16
.cfi_offset w30, -8
.cfi_offset w29, -16
.Ltmp0:
.loc 1 0 0 prologue_end // gdb.c:0:0
adrp x8, garr
adrp x9, garr
add x9, x9, :lo12:garr
stur wzr, [x29, #-4]
.loc 1 5 13 // gdb.c:5:13
ldr w8, [x8, :lo12:garr]
mov w10, #0
str x9, [sp, #16] // 8-byte Folded Spill
str w10, [sp, #12] // 4-byte Folded Spill
.loc 1 5 21 is_stmt 0 // gdb.c:5:21
cbz w8, .LBB0_2
// %bb.1:
.loc 1 0 21 // gdb.c:0:21
ldr x8, [sp, #16] // 8-byte Folded Reload
.loc 1 5 24 // gdb.c:5:24
ldr w9, [x8, #4]
.loc 1 5 21 // gdb.c:5:21
cmp w9, #0 // =0
cset w9, ne
str w9, [sp, #12] // 4-byte Folded Spill
.Ltmp1:
.LBB0_2:
.loc 1 0 21 // gdb.c:0:21
ldr w8, [sp, #12] // 4-byte Folded Reload
.loc 1 5 12 // gdb.c:5:12
tbnz w8, #0, .LBB0_4
// %bb.3:
.loc 1 0 12 // gdb.c:0:12
mov w8, #1
.Ltmp2:
adrp x9, garr
.Ltmp3:
.loc 1 6 24 is_stmt 1 // gdb.c:6:24
str w8, [x9, :lo12:garr]
ldr x9, [sp, #16] // 8-byte Folded Reload
.loc 1 7 24 // gdb.c:7:24
str w8, [x9, #4]
.loc 1 8 9 // gdb.c:8:9
b .LBB0_5
.Ltmp4:
.LBB0_4:
.loc 1 0 9 is_stmt 0 // gdb.c:0:9
mov w8, #2
adrp x9, garr
.Ltmp5:
.loc 1 10 25 is_stmt 1 // gdb.c:10:25
str w8, [x9, :lo12:garr]
ldr x9, [sp, #16] // 8-byte Folded Reload
.loc 1 11 25 // gdb.c:11:25
str w8, [x9, #4]
.Ltmp6:
.LBB0_5:
.loc 1 0 0 is_stmt 0 // gdb.c:0:0
adrp x8, garr
.Ltmp7:
.loc 1 13 27 is_stmt 1 // gdb.c:13:27
ldr w1, [x8, :lo12:garr]
ldr x8, [sp, #16] // 8-byte Folded Reload
.loc 1 13 36 is_stmt 0 // gdb.c:13:36
ldr w2, [x8, #4]
.loc 1 0 0 // gdb.c:0:0
adrp x0, .L.str
add x0, x0, :lo12:.L.str
.loc 1 13 10 // gdb.c:13:10
bl printf
mov w8, wzr
.loc 1 14 10 is_stmt 1 // gdb.c:14:10
mov w0, w8
ldp x29, x30, [sp, #32] // 16-byte Folded Reload
add sp, sp, #48 // =48
ret
.Ltmp8:
.Lfunc_end0:
.size main, .Lfunc_end0-main
.cfi_endproc
// -- End function
.type garr,@object // @garr
.bss
.globl garr
.p2align 2
garr:
.zero 8
.size garr, 8
.type .L.str,@object // @.str
.section .rodata.str1.1,"aMS",@progbits,1
.L.str:
.asciz "%d %d\n"
.size .L.str, 7
.section .debug_abbrev,"",@progbits
.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 16 // DW_AT_stmt_list
.byte 23 // DW_FORM_sec_offset
.byte 27 // DW_AT_comp_dir
.byte 14 // DW_FORM_strp
.byte 17 // DW_AT_low_pc
.byte 1 // DW_FORM_addr
.byte 18 // DW_AT_high_pc
.byte 6 // DW_FORM_data4
.byte 0 // EOM(1)
.byte 0 // EOM(2)
.byte 2 // Abbreviation Code
.byte 52 // DW_TAG_variable
.byte 0 // DW_CHILDREN_no
.byte 3 // DW_AT_name
.byte 14 // DW_FORM_strp
.byte 73 // DW_AT_type
.byte 19 // DW_FORM_ref4
.byte 63 // DW_AT_external
.byte 25 // DW_FORM_flag_present
.byte 58 // DW_AT_decl_file
.byte 11 // DW_FORM_data1
.byte 59 // DW_AT_decl_line
.byte 11 // DW_FORM_data1
.byte 2 // DW_AT_location
.byte 24 // DW_FORM_exprloc
.byte 0 // EOM(1)
.byte 0 // EOM(2)
.byte 3 // Abbreviation Code
.byte 1 // DW_TAG_array_type
.byte 1 // DW_CHILDREN_yes
.byte 73 // DW_AT_type
.byte 19 // DW_FORM_ref4
.byte 0 // EOM(1)
.byte 0 // EOM(2)
.byte 4 // Abbreviation Code
.byte 33 // DW_TAG_subrange_type
.byte 0 // DW_CHILDREN_no
.byte 73 // DW_AT_type
.byte 19 // DW_FORM_ref4
.byte 55 // DW_AT_count
.byte 11 // DW_FORM_data1
.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 36 // DW_TAG_base_type
.byte 0 // DW_CHILDREN_no
.byte 3 // DW_AT_name
.byte 14 // DW_FORM_strp
.byte 11 // DW_AT_byte_size
.byte 11 // DW_FORM_data1
.byte 62 // DW_AT_encoding
.byte 11 // DW_FORM_data1
.byte 0 // EOM(1)
.byte 0 // EOM(2)
.byte 7 // Abbreviation Code
.byte 46 // DW_TAG_subprogram
.byte 0 // DW_CHILDREN_no
.byte 17 // DW_AT_low_pc
.byte 1 // DW_FORM_addr
.byte 18 // DW_AT_high_pc
.byte 6 // DW_FORM_data4
.byte 64 // DW_AT_frame_base
.byte 24 // DW_FORM_exprloc
.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 63 // DW_AT_external
.byte 25 // DW_FORM_flag_present
.byte 0 // EOM(1)
.byte 0 // EOM(2)
.byte 0 // EOM(3)
.section .debug_info,"",@progbits
.Lcu_begin0:
.word .Ldebug_info_end0-.Ldebug_info_start0 // Length of Unit
.Ldebug_info_start0:
.hword 4 // DWARF version number
.word .debug_abbrev // Offset Into Abbrev. Section
.byte 8 // Address Size (in bytes)
.byte 1 // Abbrev [1] 0xb:0x68 DW_TAG_compile_unit
.word .Linfo_string0 // DW_AT_producer
.hword 12 // DW_AT_language
.word .Linfo_string1 // DW_AT_name
.word .Lline_table_start0 // DW_AT_stmt_list
.word .Linfo_string2 // DW_AT_comp_dir
.xword .Lfunc_begin0 // DW_AT_low_pc
.word .Lfunc_end0-.Lfunc_begin0 // DW_AT_high_pc
.byte 2 // Abbrev [2] 0x2a:0x15 DW_TAG_variable
.word .Linfo_string3 // DW_AT_name
.word 63 // DW_AT_type
// DW_AT_external
.byte 1 // DW_AT_decl_file
.byte 2 // DW_AT_decl_line
.byte 9 // DW_AT_location
.byte 3
.xword garr
.byte 3 // Abbrev [3] 0x3f:0xc DW_TAG_array_type
.word 75 // DW_AT_type
.byte 4 // Abbrev [4] 0x44:0x6 DW_TAG_subrange_type
.word 82 // DW_AT_type
.byte 2 // DW_AT_count
.byte 0 // End Of Children Mark
.byte 5 // Abbrev [5] 0x4b:0x7 DW_TAG_base_type
.word .Linfo_string4 // DW_AT_name
.byte 5 // DW_AT_encoding
.byte 4 // DW_AT_byte_size
.byte 6 // Abbrev [6] 0x52:0x7 DW_TAG_base_type
.word .Linfo_string5 // DW_AT_name
.byte 8 // DW_AT_byte_size
.byte 7 // DW_AT_encoding
.byte 7 // Abbrev [7] 0x59:0x19 DW_TAG_subprogram
.xword .Lfunc_begin0 // DW_AT_low_pc
.word .Lfunc_end0-.Lfunc_begin0 // DW_AT_high_pc
.byte 1 // DW_AT_frame_base
.byte 109
.word .Linfo_string6 // DW_AT_name
.byte 1 // DW_AT_decl_file
.byte 3 // DW_AT_decl_line
.word 75 // DW_AT_type
// DW_AT_external
.byte 0 // End Of Children Mark
.Ldebug_info_end0:
.section .debug_str,"MS",@progbits,1
.Linfo_string0:
.asciz "clang version 12.0.0 (https://github.com/llvm/llvm-project.git 6795f3fe9c4bf78df934de5ae448ab908948318d)" // string offset=0
.Linfo_string1:
.asciz "gdb.c" // string offset=105
.Linfo_string2:
.asciz "/home/bft/Jaydeep/llvm_all/check/llvm-project/install/bin" // string offset=111
.Linfo_string3:
.asciz "garr" // string offset=169
.Linfo_string4:
.asciz "int" // string offset=174
.Linfo_string5:
.asciz "__ARRAY_SIZE_TYPE__" // string offset=178
.Linfo_string6:
.asciz "main" // string offset=198
.ident "clang version 12.0.0 (https://github.com/llvm/llvm-project.git 6795f3fe9c4bf78df934de5ae448ab908948318d)"
.section ".note.GNU-stack","",@progbits
.addrsig
.addrsig_sym printf
.addrsig_sym garr
.section .debug_line,"",@progbits
.Lline_table_start0:
[-- Attachment #3: gdb.c --]
[-- Type: text/x-c-code, Size: 297 bytes --]
/* demo testcase */
int garr[2];
int main() {
if((garr[0] && garr[1])==0){
garr[0]=1;
garr[1]=1;
}
else {
garr[0]=2;
garr[1]=2;
}
printf("%d %d\n",garr[0], garr[1]);
return 0;
}
[-- Attachment #4: gdb.o --]
[-- Type: application/octet-stream, Size: 3488 bytes --]
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