Weird reverse problems

["Marc Khouzam" <marc dot khouzam at ericsson dot com>]

Hi,

so I'm trying Reverse Debugging by having applied teawater's patches
on top of HEAD which contains Michael S reverse infrastructure.

I ran into some bizarre behavior when reverse stepping.
I found that with (I'm guessing) library functions, a reverse next
will skip a line.  If you look at the simple session I ran below
(a bit long, but very simple) you can see that on the reverse, 
every two printf gets skipped.
Also, on a reverse step, multiple sleep() can be skipped.

Weird no?

I tried to look at the code, but performing steps gets pretty
low level, so I gave up.

Thanks

Marc

GNU gdb (GDB) 6.8.50.20090113-cvs
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This GDB was configured as "i686-pc-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
(gdb) l
1       #include <stdio.h>
2       #include <unistd.h>
3
4       int foo(int i) {
5           return i;
6       }
7       int main()
8       {
9           sleep(0);//1
10          sleep(0);//2
(gdb) 
11          sleep(0);//3
12          sleep(0);//4
13          printf("A\n");
14          printf("B\n");
15          printf("C\n");
16          printf("D\n");
17          int a = 0;
18          a++;
19          printf("E\n");
20          printf("F\n");
(gdb) 
21          printf("G\n");
22          printf("H\n");
23          return a;
24      }
(gdb) b main
Breakpoint 1 at 0x80484bd: file a.cc, line 9.
(gdb) r
Starting program: /local/home/lmckhou/testing/a.out 

Breakpoint 1, main () at a.cc:9
9           sleep(0);//1
(gdb) record
(gdb) b 24
Breakpoint 2 at 0x804855b: file a.cc, line 24.
(gdb) c
Continuing.
A
B
C
D
E
F
G
H

Breakpoint 2, main () at a.cc:24
24      }
(gdb) rn
23          return a;
(gdb) rn
21          printf("G\n");    <--- Missed a printf here!
(gdb) rn
19          printf("E\n");    <--- Missed a printf here!
(gdb) rn
18          a++;
(gdb) rn
17          int a = 0;
(gdb) rn
15          printf("C\n");    <--- Missed a printf here!
(gdb) rn
13          printf("A\n");    <--- Missed a printf here!
(gdb) rn
11          sleep(0);//3    <--- Missed a sleep here!
(gdb) rn

No more reverse-execution history.
main () at a.cc:9
9           sleep(0);//1
(gdb) n
10          sleep(0);//2
(gdb) n
11          sleep(0);//3
(gdb) n
12          sleep(0);//4
(gdb) n
13          printf("A\n");
(gdb) rs
12          sleep(0);//4
(gdb) rs
11          sleep(0);//3
(gdb) rs
10          sleep(0);//2
(gdb) n
11          sleep(0);//3
(gdb) n
12          sleep(0);//4
(gdb) n
13          printf("A\n");
(gdb) n
14          printf("B\n");
(gdb) n
15          printf("C\n");
(gdb) rs
14          printf("B\n");
(gdb) 

No more reverse-execution history.
main () at a.cc:9
9           sleep(0);//1       <--- Woah, missed a bunch of stuff on the
reverse step!