The document discusses issues with line number information in the .debug_line section when relocation is not applied properly. It shows a C program with line number information extracted using llvm-dwarfdump and llvm-objdump. Setting a breakpoint at line 6 fails as the address is incorrect without relocation applied to .debug_line. Precise line number information relies on correct address information in .debug_line.

1. Issues in .debug_line
// test.c
1 #include <stdio.h>
2
3 int main()
4 {
5 printf("a");
6 printf("b");
7 printf("c");
8 return 0;
9 }
lui a1, 0
addi a1, a1, 0
sw a0, -16(s0)
c.mv a0, a1
auipc ra, 0
jalr ra, ra, 0
lui a1, 0
addi a1, a1, 0
sw a0, -20(s0)
compile

2. Issues in .debug_line
// llvm-dwarfdump
file_names[ 1] 0 0x00000000 0x00000000 test.c
Address Line Column File ISA Discriminator
------------------ ------ ------ ------ --- -------------
0x00000000000101c0 4 0 1 0 0
0x00000000000101d0 5 3 1 0 0
0x00000000000101e6 6 3 1 0 0
0x00000000000101fc 7 3 1 0 0
0x0000000000010212 8 3 1 0 0
0x000000000001022a 8 3 1 0 0
// llvm-objdump -disassemble
101d0: b7 85 01 00 lui a1, 24
101d4: 93 85 05 27 addi a1, a1, 624
101d8: 23 28 a4 fe sw a0, -16(s0)
101dc: 2e 85 c.mv a0, a1
101de: c9 2a c.jal 466
101e0: b7 85 01 00 lui a1, 24
101e4: 93 85 45 27 addi a1, a1, 628
101e8: 23 26 a4 fe sw a0, -20(s0)
lui a1, 0
addi a1, a1, 0
sw a0, -16(s0)
c.mv a0, a1
auipc ra, 0
jalr ra, ra, 0
lui a1, 0
addi a1, a1, 0
sw a0, -20(s0)
If there is no relocation for .debug_line, address will be wrong.
In the case, the address of line 6 is not existed.
link

3. (gdb) b test.c:6
Breakpoint 1 at 0x101e6: file test.c, line 6.
(gdb) r
Starting program: /home/users/kai/sandbox/llvm-test/debug-line/test
ac[Inferior 1 (process 42000) exited normally]
Issues in .debug_line
Insert a breakpoint at line 6, the program will not hit the breakpoint.

4. • “we design a byte-coded language for a state machine
and store a stream of bytes in the object file instead of the
matrix. This language can be much more compact than
the matrix.”
How to Encode Line Number
Information in DWARF

5. How to Encode Address
Delta
• DW_LNS_advance_pc
• Takes a single unsigned LEB128 operand as the operation advance
and modifies the address.
• unsigned LEB128 is a variable length operand.
• DW_LNS_fixed_advance_pc (No supported in LLVM yet. D46850)
• Takes a single uhalf (unencoded) operand and adds it to the address
register of the state machine and sets the op_index register to 0.
• Relaxation may change the address delta after linking. We use the
fixed length operand to make it easier to generate fixups
in .debug_line.