Re: [PATCH][gdb/breakpoint] Handle setting breakpoint on label without address

[Pedro Alves <pedro@palves.net>]

On 8/28/20 2:53 PM, Tom de Vries wrote:

> I see what you mean, but let's try this counter-example:
> ...
>  cat -n test.c
>      1  int
>      2  main (void)
>      3  {
>      4    goto L2;
>      5
>      6   L3:
>      7    return 0;
>      8
>      9   L1:
>     10    (void)0;
>     11    return 1;
>     12
>     13   L2:
>     14    goto L3;
>     15  }
>     16
> ...
> compiled like this:
> ...
> $ gcc test.c -g
> ...
> 
> With the patch, we're not able to set a breakpoint at L1, and setting
> the breakpoint at the corresponding line, line 9:
> ...
> $ gdb a.out
> Reading symbols from a.out...
> (gdb) b main:L1
> Location main:L1 not available
> (gdb) b 9
> Breakpoint 1 at 0x40049c: file test.c, line 14.
> (gdb)
> ...
> yields a breakpoint at line 14, a piece of code that's not reachable
> from L1.
> 
> To me, label L1 and line 14 are unrelated enough to convince me to not
> do this automatically.

Well, OK.  All the code after line 7 is unreacheable.

Then using that rationale, why don't we reject a breakpoint at line 9 too?
Line 14 is not reacheable from line 9 either.  Can you justify the
difference in a small sentence/paragraph?

It would be perhaps nice if "break" accepted an option to prevent the
move-to-following-insn -- for all kinds of breakpoints -- like lldb's
"break set -m" option:

       -m <boolean> ( --move-to-nearest-code <boolean> )
            Move breakpoints to nearest code. If not set the target.move-to-nearest-codesetting is used.

BTW, I noticed that, setting the breakpoint at the label _after_ running
to main "works":

 Reading symbols from testsuite/outputs/gdb.base/label-without-address/label-without-address...
 (gdb) b main:L1
 Location main:L1 not available
 (gdb) start
 Temporary breakpoint 1 at 0x1131: file testsuite/gdb.base/label-without-address.c, line 21.
 Starting program: testsuite/outputs/gdb.base/label-without-address/label-without-address 

 Temporary breakpoint 1, main () at testsuite/gdb.base/label-without-address.c:21
 21        return 0;
 (gdb) b main:L1
 Breakpoint 2 at 0x555555554000: file testsuite/gdb.base/label-without-address.c, line 22.
 (gdb) info breakpoints 
 Num     Type           Disp Enb Address            What
 2       breakpoint     keep y   0x0000555555554000 main:L1

I see the same with your more complicated example.  What's going on here?