<table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Issue</th>
<td>
<a href=https://github.com/llvm/llvm-project/issues/108165>108165</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>
[LLDB] lldb prints different values at different optimization levels
</td>
</tr>
<tr>
<th>Labels</th>
<td>
new issue
</td>
</tr>
<tr>
<th>Assignees</th>
<td>
</td>
</tr>
<tr>
<th>Reporter</th>
<td>
IOVOVHS
</td>
</tr>
</table>
<pre>
When printing an array variable, the value of one of the array elements is incorrect. This is somewhat similar to the previous question(https://github.com/llvm/llvm-project/issues/107977). There is an array element that is not used, so it is printed as 0, and the program does not update the variable.
compile:clang,-O2.
```
(lldb) b func_1
Breakpoint 1: where = random_833`main + 17 [inlined] func_1 at random_833.c:1332, address = 0x0000000000007771
(lldb) r
Process 449091 launched: '/home/hzw/experiment/csmith_prog/clang_O2/random_833' (x86_64)
Process 449091 stopped
* thread #1, name = 'random_833', stop reason = breakpoint 1.1
frame #0: 0x000055555555b771 random_833`main [inlined] func_1 at random_833.c:227:414
224 for (i = 0; i < 3; i++)
225 l_2080[i] = 0xB102D357L;
226 }
-> 227 (**g_666) = (safe_div_func_uint32_t_u_u(0x2E9D030DL, .......
(lldb) p l_10
(uint32_t) $0 = 9
(lldb) p l_2150
(int16_t [9]) $2 = ([0] = 11894, [1] = 11894, [2] = 11894, [3] = 11894, [4] = 11894, [5] = 0, [6] = 11894, [7] = 11894, [8] = 11894)
```
compile:clang,-O0,line 227
```
.....
(lldb) p l_10
(uint32_t) $0 = 10
(lldb) p l_2150
(int16_t [9]) $1 = ([0] = 11894, [1] = 11894, [2] = 11894, [3] = 11894, [4] = 11894, [5] = 11894, [6] = 11894, [7] = 11894, [8] = 11894)
```
As you can see, the value of element [5] is different between -O0 and -O2. This is because element [5] is never used, so there seems to be something wrong with the debug information written during optimization.
The value of l_10 is also problematic. In the same line, the value is different under different optimization levels. Under -O2, the debug information is the constant 9, which is the initialization value of the variable l_10, but later on line 227, the value of l_10 should be 10, just as the value printed under -O0, because the value of l_10 has been updated in this line. Or is this acceptable?
source file
``` c
//about l_2150
178 int16_t l_2150[9] = {0x2E76L,0x2E76L,0x2E76L,0x2E76L,0x2E76L,0x2E76L,0x2E76L,0x2E76L,0x2E76L};
179 uint64_t **l_2151 = &g_574;
...
227 ...g_2149) >= (-1L)) >= l_2150[2])...
228 int16_t c_1 = l_2150[4] - l_2150[0];
229 int16_t c_2 = l_2150[2] * l_2150[0];
230 int16_t c_3 = l_2150[8] - l_2150[1];
231 int16_t c_4 = l_2150[1] - l_2150[2];
232 int16_t c_5 = l_2150[3] + l_2150[2];
233 int16_t c_6 = l_2150[8] + l_2150[8];
//about l_10
175 uint32_t l_10 = 9UL;
...
214 l_10++;
....
```
debug info:
```
0x00000a9e: DW_TAG_variable
DW_AT_location (indexed (0x2) loclist = 0x00000092:
[0x000000000000114b, 0x0000000000001591): DW_OP_constu 0x2e76, DW_OP_stack_value, DW_OP_piece 0x2, DW_OP_constu 0x2e76, DW_OP_stack_value, DW_OP_piece 0x2, DW_OP_constu 0x2e76, DW_OP_stack_value, DW_OP_piece 0x2, DW_OP_constu 0x2e76, DW_OP_stack_value, DW_OP_piece 0x2, DW_OP_constu 0x2e76, DW_OP_stack_value, DW_OP_piece 0x2, DW_OP_piece 0x2, DW_OP_constu 0x2e76, DW_OP_stack_value, DW_OP_piece 0x2, DW_OP_constu 0x2e76, DW_OP_stack_value, DW_OP_piece 0x2, DW_OP_constu 0x2e76, DW_OP_stack_value, DW_OP_piece 0x2)
DW_AT_abstract_origin (0x0000060d "l_2150")
0x00000a98: DW_TAG_variable
DW_AT_const_value (9)
DW_AT_abstract_origin (0x00000605 "l_10")
```
Similar to the previous question, because only one array element was used, the debug information only had a constant for the value of an array variable, and the values of the remaining elements were 0. But here, the length of the array element is 9, and in the debug information, eight values are written for the variable, so the remaining one is naturally printed as 0.
For the variable l_10, the value is always the initialization value 9, even if there is an update statement later, but because l_10 has never been used anywhere else, maybe this is acceptable?
This is all I understand about debuggers. If there are any mistakes, please point them out.
clang --version：
clang version 19.0.0git (https://github.com/llvm/llvm-project.git b4ab52c8e71e819c13606de3500043eaa701e1ea)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /home/hzw/Downloads/llvm-project/build/bin
Build config: +unoptimized, +assertions
</pre>
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