Re: [reverse/record] adjust_pc_after_break in reverse execution mode?

teawater
2008-10-18T03:07:08Z

Great! Please check it in maintree.

On Sat, Oct 18, 2008 at 09:10, Pedro Alves <pedro@codesourcery.com> wrote:
> Just noticed this, while looking at the code, so I tried it out against
> the record target (x86) on the reverse-20080930-branch branch.
>
> 4 int main ()
> 5 {
> 6 asm ("nop");
> 7 asm ("nop");
> 8 asm ("nop");
> 9 asm ("nop");
> 10 }
>
> (gdb) disassemble
> Dump of assembler code for function main:
> 0x08048344 <main+0>: lea 0x4(%esp),%ecx
> 0x08048348 <main+4>: and $0xfffffff0,%esp
> 0x0804834b <main+7>: pushl -0x4(%ecx)
> 0x0804834e <main+10>: push %ebp
> 0x0804834f <main+11>: mov %esp,%ebp
> 0x08048351 <main+13>: push %ecx
> 0x08048352 <main+14>: nop
> 0x08048353 <main+15>: nop
> 0x08048354 <main+16>: nop
> 0x08048355 <main+17>: nop
> 0x08048356 <main+18>: pop %ecx
>
> Now let's try reverse continuing until hitting a breakpoint at 0x8048353 (line 7):
>
> (gdb) b 7
> Breakpoint 1 at 0x8048353: file nop.c, line 7.
> (gdb) start
> Temporary breakpoint 2 at 0x8048352: file nop.c, line 6.
> Starting program: /home/pedro/gdb/reverse-20080930-branch/build32/gdb/nop
>
> Temporary breakpoint 2, main () at nop.c:6
> 6 asm ("nop");
> (gdb) record
> (gdb) n
>
> Breakpoint 1, main () at nop.c:7
> 7 asm ("nop");
> (gdb) n
> 8 asm ("nop");
> (gdb) n
> 9 asm ("nop");
> (gdb) p $pc
> $1 = (void (*)()) 0x8048355 <main+17>
> (gdb) reverse-continue
> Continuing.
>
> Breakpoint 1, main () at nop.c:7
> 7 asm ("nop");
> (gdb) p $pc
> $1 = (void (*)()) 0x8048353 <main+15>
> (gdb)
>
> Now, let's try reverse continuing to a breakpoint at 0x8048353 (line 6),
> but this time, let's also sneak a breakpoint at 0x8048352 (line 6):
>
> (gdb) start
> Temporary breakpoint 1 at 0x8048352: file nop.c, line 6.
> Starting program: /home/pedro/gdb/reverse-20080930-branch/build32/gdb/nop
>
> Temporary breakpoint 1, main () at nop.c:6
> 6 asm ("nop");
> (gdb) b 6
> Breakpoint 2 at 0x8048352: file nop.c, line 6.
> (gdb) b 7
> Breakpoint 3 at 0x8048353: file nop.c, line 7.
> (gdb) record
> (gdb) n
>
> Breakpoint 3, main () at nop.c:7
> 7 asm ("nop");
> (gdb) n
> 8 asm ("nop");
> (gdb) n
> 9 asm ("nop");
> (gdb) p $pc
> $1 = (void (*)()) 0x8048355 <main+17>
> (gdb) reverse-continue
> Continuing.
>
> Breakpoint 2, main () at nop.c:6
> 6 asm ("nop");
> (gdb) p $pc
> $1 = (void (*)()) 0x8048352 <main+14>
>
> Oh-oh. Not good.
>
> So, in the second example, reverse execution should continue until
> breakpoint 3, but, adjust_pc_after_break finds a breakpoint
> at `PC - decr_pc_after_break' (1 on x86), adjusts the PC, and then we
> report breakpoint 2 being hit. The first example didn't trip on the
> problem, because there was no breakpoint at `PC - 1' when GDB went to
> look if adjustment was needed.
>
> I'm guessing the attached patch should be correct for all
> targets/archs, or could it be your targets are behaving differently?
>
> --
> Pedro Alves
>