lldb wrongly stopped at a statement within a nested for statement by si (step instruction)

[Quuxplusone]

Bugzilla Link	PR45920
Status	CONFIRMED
Importance	P normal
Reported by	Yibiao Yang (yangyibiao@nju.edu.cn)
Reported on	2020-05-13 20:13:04 -0700
Last modified on	2020-05-14 10:23:18 -0700
Version	trunk
Hardware	PC Linux
CC	dblaikie@gmail.com, jdevlieghere@apple.com, jeremy.morse.llvm@gmail.com, josh@joshmatthews.net, llvm-bugs@lists.llvm.org, neeilans@live.com, paul.robinson@am.sony.com, richard-llvm@metafoo.co.uk, vsk@apple.com
Fixed by commit(s)
Attachments
Blocks	PR38768
Blocked by
See also

$ lldb --version
lldb version 11.0.0
  clang revision 871beba234a83a2a02da9dedbd59b91a1bfbd7af
  llvm revision 871beba234a83a2a02da9dedbd59b91a1bfbd7af

$ 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 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 + 11 at small.c:4:10, address =
0x000000000040111b
(lldb) r
Process 13529 launched: '/home/yibiao/Debugger/a.out' (x86_64)
Process 13529 stopped
* thread #1, name = 'a.out', stop reason = breakpoint 1.1
    frame #0: 0x000000000040111b a.out`main at small.c:4:10
   1    int main ()
   2    {
   3      int x, y;
-> 4      for (x = __INT_MAX__ - 1; x < __INT_MAX__; x++)
   5        for (y = -1; y <= 0; y++)
   6          if ((x + 1 - y) != (int) (x + 1U - y))
   7            return 1;
(lldb) si -c 35
Process 13529 stopped
* thread #1, name = 'a.out', stop reason = instruction step into
    frame #0: 0x000000000040113a a.out`main at small.c:5:5
   2    {
   3      int x, y;
   4      for (x = __INT_MAX__ - 1; x < __INT_MAX__; x++)
-> 5        for (y = -1; y <= 0; y++)
   6          if ((x + 1 - y) != (int) (x + 1U - y))
   7            return 1;
   8      return 0;
(lldb) var
(int) x = 2147483646
(int) y = 1
(lldb) si
Process 13529 stopped
* thread #1, name = 'a.out', stop reason = instruction step into
    frame #0: 0x0000000000401179 a.out`main at small.c:7:16
   4      for (x = __INT_MAX__ - 1; x < __INT_MAX__; x++)
   5        for (y = -1; y <= 0; y++)
   6          if ((x + 1 - y) != (int) (x + 1U - y))
-> 7            return 1;
   8      return 0;
   9    }
(lldb)

/******************************
lldb is wrongly stopped at Line 7.
However, when setting breakpoint at Line 7. The program is directly exit.
*******************************/

$ lldb a.out
(lldb) target create "a.out"
Current executable set to '/home/yibiao/Debugger/a.out' (x86_64).
(lldb) b 7
Breakpoint 1: where = a.out`main + 74 at small.c:7:9, address =
0x000000000040115a
(lldb) r
Process 13589 launched: '/home/yibiao/Debugger/a.out' (x86_64)
Process 13589 exited with status = 0 (0x00000000)

$ cat small.c
int main ()
{
  int x, y;
  for (x = __INT_MAX__ - 1; x < __INT_MAX__; x++)
    for (y = -1; y <= 0; y++)
      if ((x + 1 - y) != (int) (x + 1U - y))
        return 1;
  return 0;
}

[Quuxplusone]

Thanks for the bug report -- could you include the command line used to compile
the source too please. Assuming -O0, this looks very similar to bug 45676 and
its related bug 19864 -- clang is labelling the exit of the inner loop with
what we would consider the wrong location:

> for.end:                                          ; preds = %for.cond1
>   br label %for.inc7, !dbg !51

> 51 = !DILocation(line: 8, column: 16, scope: !29)

Where line 8 is the line with "return 1;". I can replicate this with gdb, so
switching component to clang.

[Quuxplusone]

I am sorry! I forgot to include the commands, here is the command I used to compile this code:

$ clang -g small.c