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

Tom de Vries
2020-08-27T13:50:00Z

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