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

[llvmbot]

Bugzilla Link	45920
Version	trunk
OS	Linux
Blocks	#38116
Reporter	LLVM Bugzilla Contributor
CC	@dwblaikie,@JDevlieghere,@jmorse,@jdm,@pogo59,@zygoloid,@vedantk

Extended Description

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

$ clang --version
clang version 11.0.0 (/home/yibiao/.cache/yay/llvm-git/llvm-project 871beba)
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;
}

[jmorse]

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.

[llvmbot]

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

$ clang -g small.c

[iamanonymouscs]

The latest reproducible code is as follow.
$ cat small.c

#include <limits.h>

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;
}

I updated clang and lldb to the latest trunk version and found that lldb can still incorrectly reaches to line 9 at instruction-level stepping.

$ clang --version
Ubuntu clang version 16.0.0-++20221107091524+a2620e00ffa2-1exp120221107091626.847

[llvmbot]

@llvm/issue-subscribers-debuginfo

[llvmbot]

@llvm/issue-subscribers-lldb

[dwblaikie]

yeah, I'll have to look back a long ways - this is a long-standing bug/issue, I think Chandler filed it years ago so there's something to dup this against eventually.

The issue is that we set the jump back to the start of the loop at the end of the loop (with {}, that'd be the } but without them it's the last statement in the loop) - GCC I think does something similar, but due to a code layout choice it doesn't come up in the same way/isn't as obvious if I recall correctly.

[dwblaikie]

ah, here. #44948 refers to #20238