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

Pedro Alves
2020-08-28T15:14:08Z

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?