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

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

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.

Thanks,
- Tom