[Bug gdb/28426] New: Step reverse over line with multiple

[Bug gdb/28426] New: Step reverse over line with multiple

[cel at us dot ibm.com]

https://sourceware.org/bugzilla/show_bug.cgi?id=28426

            Bug ID: 28426
           Summary: Step reverse over line with multiple
           Product: gdb
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: gdb
          Assignee: unassigned at sourceware dot org
          Reporter: cel at us dot ibm.com
  Target Milestone: ---

The gdb command step is supposed to step over the next line in the program.

When executing in reverse, the step command for a line that contains two
executable statements requires two step commands to get to the previous line
not one step command as expected.

The following is a test program to demonstrate the behavior.  It is based on
the regression test gdb.reverese/solib-reverse.c.

int main ()                                                                     
{                                                                               
  int b[2] = {5,8};                                                             

  b[0] = 1;
  b[1] = 2;

  b[0] = 6;   b[1] = 9;         /* generic statement, end part two */           

  b[1] = 10;
  b[0] = 20;

  return 0;
} /* end of main */

Compile the program with -g and run on gdb

Start by enabling recording and stepping forward.

(gdb) break main
Breakpoint 1 at 0x838: file solib-reverse-bug.c, line 2.
(gdb) run
Starting program: /home/carll/solib-reverse-bug 

Breakpoint 1, main () at solib-reverse-bug.c:2
2       {
(gdb) list
1       int main ()
2       {
3         int b[2] = {5,8};
4       
5         b[0] = 1;
6         b[1] = 2;
7       
8         b[0] = 6;   b[1] = 9;         /* generic statement, end part two */
9       
10        b[1] = 10;
(gdb) 
11        b[0] = 20;
12      
13        return 0;
14      }
(gdb) record
(gdb) step
3         int b[2] = {5,8};
(gdb) step
5         b[0] = 1;
(gdb) step
6         b[1] = 2;
(gdb) step
8         b[0] = 6;   b[1] = 9;         /* generic statement, end part two */
(gdb) step
10        b[1] = 10;
(gdb) step
11        b[0] = 20;
(gdb) step
13        return 0;

As expected step advances to the next line each time, specifically line 8.

Set the execution direction to reverse and step

(gdb) set exec-dir reverse
(gdb) step
11        b[0] = 20;
(gdb) step
10        b[1] = 10;
(gdb) step
8         b[0] = 6;   b[1] = 9;         /* generic statement, end part two */
(gdb) step
8         b[0] = 6;   b[1] = 9;         /* generic statement, end part two */
(gdb) step
6         b[1] = 2;
(gdb) step
5         b[0] = 1;
(gdb) step
3         int b[2] = {5,8};
(gdb) step

You will note, that line 8 required two steps to get to the previous line. 
Step over the other lines worked as expected.

This issue has been observed on Power PC and Intel.

Note the above bug results in gdb.reverse/solib-presave.exp and
gdb.reverse/solib-reverse.exp to fail.



./

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[Bug gdb/28426] Step reverse over line with multiple executable statements

[cel at us dot ibm.com]

https://sourceware.org/bugzilla/show_bug.cgi?id=28426

Carl E Love <cel at us dot ibm.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Step reverse over line with |Step reverse over line with
                   |multiple                    |multiple executable
                   |                            |statements

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[Bug gdb/28426] Step reverse over line with multiple executable statements

[vries at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28426

Tom de Vries <vries at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |vries at gcc dot gnu.org

--- Comment #1 from Tom de Vries <vries at gcc dot gnu.org> ---
Created attachment 14415
  --> https://sourceware.org/bugzilla/attachment.cgi?id=14415&action=edit
reproduce.sh

I've written a reproducer script reproducer.sh (generating test.c and gdb.in)
based on comment #0.

I first tried with gcc 7.5.0, but that one does not emit column info by
default, so I added -gcolumn-info:
...
$ gcc -g test.c ; readelf -wl a.out | grep -i column
$
$ gcc -g test.c -gcolumn-info ; readelf -wl a.out | grep -i column
  [0x0000010f]  Set column to 1
  [0x0000011d]  Set column to 7
  [0x00000120]  Set column to 8
  [0x00000125]  Set column to 20
  [0x00000128]  Set column to 8
  [0x0000012c]  Set column to 10
  [0x0000012f]  Set column to 1
...

Then, I used "b *main" to start at line 2 (otherwise, on x86_64-linux, it
starts at line 3).

Still, it doesn't reproduce for me with current trunk and x86_64-linux:
...
$ ./reproduce.sh
  ...
+b *main
Breakpoint 1 at 0x400497: file test.c, line 2.
+run

Breakpoint 1, main () at test.c:2
2       {
+record
+target record-full
+step
3         int b[2] = {5,8};
+step
5         b[0] = 1;
+step
6         b[1] = 2;
+step
8         b[0] = 6;   b[1] = 9;         /* generic statement, end part two */
+step
10        b[1] = 10;
+step
11        b[0] = 20;
+step
13        return 0;
+set exec-dir reverse
+step
11        b[0] = 20;
+step
10        b[1] = 10;
+step
8         b[0] = 6;   b[1] = 9;         /* generic statement, end part two */
+step
6         b[1] = 2;
+step
5         b[0] = 1;
+step
3         int b[2] = {5,8};
+step

No more reverse-execution history.
main () at test.c:2
2       {
...
in other words, after 7 steps forward and 7 steps back I end up at the same
point where I started: line 2.

That's both with gcc 7.5.0 and gcc 12.2.1.

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[Bug gdb/28426] Step reverse over line with multiple executable statements

[vries at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=28426

--- Comment #2 from Tom de Vries <vries at gcc dot gnu.org> ---
(In reply to Tom de Vries from comment #1)
> That's both with gcc 7.5.0 and gcc 12.2.1.

And I've tried both compiler also with gdb-12-branch, and all looks good there
as well.  So, I can't reproduce this on x86_64-linux.

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[Bug gdb/28426] Step reverse over line with multiple executable statements

[blarsen at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=28426

B. Larsen <blarsen at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |blarsen at redhat dot com

--- Comment #3 from B. Larsen <blarsen at redhat dot com> ---
As explained by Pedro Alves in this email thread
(https://sourceware.org/pipermail/gdb-patches/2023-January/196110.html), it is
possible to reproduce this bug in x86_64, with a modified reproducer. Consider
the following code:

void func1 ()
{
}

void func 2 ()
{
}

int main ()
{
  int v = 0;
  func 1 (); func 2 ();
  return v;
}

and the debug session:

(gdb) start
Temporary breakpoint 1 at 0x40111c: file t.c, line 10.
Starting program: /home/blarsen/Documents/downstream_build/gdb/reverse 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".

Temporary breakpoint 1, main () at t.c:10
10          int v = 0;
(gdb) record
(gdb) n
11          func1 (); func2 ();
(gdb) 
12          return v;
(gdb) rn
11          func1 (); func2 ();
(gdb) 
11          func1 (); func2 ();
(gdb) rn

No more reverse-execution history.
main () at t.c:10
10          int v = 0;
(gdb) 


Without a line before the `func1 (); func2 ()` GDB mistakenly reports that
we've reached the end of the recorded history, but that is an unrelated bug.
Compiling the example program with no column information, we get instead the
following session:

(gdb) start
Temporary breakpoint 1 at 0x40111c: file t.c, line 10.
Starting program: /home/blarsen/Documents/downstream_build/gdb/reverse 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".

Temporary breakpoint 1, main () at t.c:10
10          int v = 0;
(gdb) record
(gdb) n
11          func1 (); func2 ();
(gdb) 
12          return v;
(gdb) rn
11          func1 (); func2 ();
(gdb) 

No more reverse-execution history.
main () at t.c:10
10          int v = 0;
(gdb)

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[Bug gdb/28426] Step reverse over line with multiple executable statements

[cel at linux dot ibm.com]

https://sourceware.org/bugzilla/show_bug.cgi?id=28426

Carl E Love <cel at linux dot ibm.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |FIXED

--- Comment #4 from Carl E Love <cel at linux dot ibm.com> ---
Marking this as resolved as it is not failing any more.

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