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

[Tom de Vries <tdevries@suse.de>]

On 8/28/20 3:32 PM, Pedro Alves wrote:
> On 8/27/20 2:49 PM, Tom de Vries wrote:
>> On 8/27/20 2:41 PM, Pedro Alves wrote:
>>> On 8/27/20 12:52 PM, Tom de Vries wrote:
>>>> Hi,
>>>>
>>>> Consider test-case test.c:
>>>> ...
>>>> $ cat test.c
>>>> int main (void) {
>>>>   return 0;
>>>>  L1:
>>>>   (void)0;
>>>> }
>>>> ...
>>>>
>>>> Compiled with debug info:
>>>> ...
>>>> $ gcc test.c -g
>>>> ...
>>>>
>>>> When attempting to set a breakpoint at L1, which is a label without address:
>>>> ...
>>>>  <1><f4>: Abbrev Number: 2 (DW_TAG_subprogram)
>>>>     <f5>   DW_AT_name        : main
>>>>  <2><115>: Abbrev Number: 3 (DW_TAG_label)
>>>>     <116>   DW_AT_name        : L1
>>>>     <119>   DW_AT_decl_file   : 1
>>>>     <11a>   DW_AT_decl_line   : 5
>>>>  <2><11b>: Abbrev Number: 0
>>>
>>> Is this a debug info bug,
>>
>> Strictly speaking, this is a debug info bug.  The standard says that:
>> ...
>> The label entry has a DW_AT_low_pc attribute whose value is the address
>> of the first executable instruction for the location identified by the
>> label in the source program.
>> ...
>>
>> But I interpret the missing DW_AT_low_pc attribute as: there is a label
>> in the source, but the corresponding code has been optimized out.
>>
>>> or is the debug info telling us that the
>>> address of the label is the same as the line number's address?
>>>
>>> How about looking up the line number address instead of throwing
>>> an error?
>>>
>>
>> Well, in this particular case, that wouldn't help.
>>
>> With L1 at line 3:
>> ...
>> $ cat -n test.c
>>      1  int main (void) {
>>      2    return 0;
>>      3   L1:
>>      4    (void)0;
>>      5  }
>>      6
>> ...
>> there's no corresponding address:
>> ...
>> $ readelf -wL a.out
>> CU: test.c:
>> File name                            Line number    Starting address
>> View    Stmt
>> test.c                                         1            0x400497
>>            x
>> test.c                                         2            0x40049b
>>            x
>> test.c                                         5            0x4004a0
>>            x
>> test.c                                         -            0x4004a2
>> ...
>>
>> My suspicion is that this won't be useful in general.
> 
> I don't understand the "not useful" remark.  If a user does gets
> the error, they'll probably do:
> 
>   "b 3",
> 
> and they'll get a breakpoint at line 5, no?
> 
> That's very likely what a user would do after the error.
> 
> IMO GDB should do that for the user.
> 
> So far I don't agree with your patch.
> 

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.

Thanks,
- Tom