[Bug gdb/25528] step will stopped at any "asm volatile" statement

[Bug gdb/25528] step will stopped at any "asm volatile" statement

[yangyibiao at outlook dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=25528

Yibiao Yang <yangyibiao at outlook dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |vries at gcc dot gnu.org,
                   |                            |yangyibiao at outlook dot com
            Summary|incorrect trace for "asm    |step will stopped at any
                   |volatile" statement         |"asm volatile" statement
         Resolution|---                         |FIXED
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #1 from Yibiao Yang <yangyibiao at outlook dot com> ---
In current version of GDB, statement with asm volatile will be skipped.

I was wondering that this is the default behavior of GDB.

Therefore, I think this bug in previous version is a bug of GDB.

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[Bug gdb/25528] step will stopped at any "asm volatile" statement

[vries at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=25528

--- Comment #2 from Tom de Vries <vries at gcc dot gnu.org> ---
(In reply to Yibiao Yang from comment #1)
> In current version of GDB, statement with asm volatile will be skipped.
> 
> I was wondering that this is the default behavior of GDB.
> 

Compiling with gcc 9.3.1, and looking at the line info using readline -wL, I
get:
...
CU: small.c:

Line number    Starting address    View    Stmt
4            0x400492               x
5            0x40049c               x
6            0x4004a2               x
6            0x4004a8               x
8            0x4004aa               x
9            0x4004b8               x
9            0x4004c5               x
10           0x4004c9               x
6            0x4004c9       1       x
6            0x4004d4               x
12           0x4004db               x
14           0x4004df               x
14           0x4004e3               x
14           0x4004ed               x
14           0x4004f0               x
...

while we have:
...
$ cat -n small.c
     1  const signed char NM[] = {-1, 0, 1};
     2
     3  __attribute__((noinline)) void f(short delta)
     4  {
     5    short p = 2, s;
     6    for (s = 0; s < 2; s++)
     7    {
     8      p += delta;
     9      if (NM[p] == 0)
    10        asm volatile("");
    11    }
    12  }
    13
    14  void main(void) { f(-1); }
    15
...

So, the line with the asm stmt does have line info, but shares the address with
another entry with another line number.

Using this debugging session, we can see that we do visit the address, but show
line 6 instead of line 10:
...
(gdb) 
9           if (NM[p] == 0)
(gdb) p /x $pc
$1 = 0x4004b8
(gdb) s
6         for (s = 0; s < 2; s++)
(gdb) p /x $pc
$2 = 0x4004c9
(gdb) 
...
which I'd say is a valid interpretation of the line number info.

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[Bug gdb/25528] step will stopped at any "asm volatile" statement

[vries at gcc dot gnu.org]

https://sourceware.org/bugzilla/show_bug.cgi?id=25528

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|FIXED                       |NOTABUG

--- Comment #3 from Tom de Vries <vries at gcc dot gnu.org> ---
(In reply to Yibiao Yang from comment #0)
> $ gdb -q ./a.out
> Reading symbols from ./a.out...
> (gdb) break small.c:9
> Breakpoint 1 at 0x113f: file small.c, line 9.
> (gdb) run
> Starting program: /home/yibiao/cv-gcov/a.out 
> 
> Breakpoint 1, f (delta=-1) at small.c:9
> 9	    if (NM[p] == 0)
> (gdb) step
> 10	      asm volatile("");
> (gdb) c
> Continuing.
> 
> Breakpoint 1, f (delta=-1) at small.c:9
> 9	    if (NM[p] == 0)
> (gdb) step
> 10	      asm volatile("");
> (gdb) q
> A debugging session is active.
> 
> 	Inferior 1 [process 4947] will be killed.
> 
> Quit anyway? (y or n) y
> 
> 
> 
> #############################################################################
> ##
> ### We can find that Line #10 @asm volatile("");@ is executed twice. 
> ### However, it is obvious that Line #9 "if (NM[p] == 0)" is True in the
> second for loop. 
> #############################################################################
> ##

I think this is a different variant of the same problem: the target of the true
_and_ false path end up at the same insn, which has two entries in the line
table.  So either we show the "true" line twice, or the "false" line twice.

Confusing, but not a gdb bug.

On the gcc side, it could be argued that this is exactly what you asked for by
providing an _empty_ asm stmt.

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