dwblaikie
commented
4 years ago another case of jumping into the middle of a line:
.LBB0_1: .loc 1 6 7 is_stmt 1 # ./example.c:6:7 mov ecx, dword ptr [rip + .La$local] .LBB0_3: .loc 1 6 8 is_stmt 0 # ./example.c:6:8 xor eax, eax cmp ecx, 1 setne al .loc 1 9 1 is_stmt 1 # ./example.c:9:1 ret
The first reference to line 6 is the load from 'a', but that's conditional - if the code came from the 'if (d)' block, then 'a's value is known without the need to load it, so the code skips over the load (missing the "start" of line 6 in the process).
Extended Description
$ clang --version clang version 11.0.0 (/home/yibiao/.cache/yay/llvm-git/llvm-project 871beba234a83a2a02da9dedbd59b91a1bfbd7af) Target: x86_64-pc-linux-gnu Thread model: posix InstalledDir: /usr/bin
$ lldb --version lldb version 11.0.0 clang revision 871beba234a83a2a02da9dedbd59b91a1bfbd7af llvm revision 871beba234a83a2a02da9dedbd59b91a1bfbd7af
$ clang -g -O2 small.c
$ lldb a.out (lldb) target create "a.out" Current executable set to '/home/yibiao/Debugger/a.out' (x86_64). (lldb) b 6 Breakpoint 1: where = a.out`main + 29 at small.c:6:7, address = 0x000000000040112d (lldb) r Process 578366 launched: '/home/yibiao/Debugger/a.out' (x86_64) Process 578366 exited with status = 0 (0x00000000) (lldb)
/**** As showed above, Line 6 is not hit by lldb when setting breakpoint. However, it was hit by lldb when using step-i *****/
$ lldb a.out (lldb) target create "a.out" Current executable set to '/home/yibiao/Debugger/a.out' (x86_64). (lldb) b main Breakpoint 1: where = a.out`main at small.c:4:7, address = 0x0000000000401110 (lldb) r Process 578410 launched: '/home/yibiao/Debugger/a.out' (x86_64) Process 578410 stopped
3 int main() { -> 4 if (d) 5 a = b==0; 6 if (a!=1) 7 return 1; (lldb) si -c 7 Process 578410 stopped
$ cat small.c int a, b, d=1;
int main() { if (d) a = b==0; if (a!=1) return 1; return 0; }