[ was: Re: [PATCH 2/3] gdb: Don't reorder line table entries too much
when sorting. ]

On 05-06-2020 18:00, Tom de Vries wrote:
> On 05-06-2020 16:49, Tom de Vries wrote:
>> On 23-12-2019 02:51, Andrew Burgess wrote:
>>> I had to make a small adjustment in find_pc_sect_line in order to
>>> correctly find the previous line in the line table.  In some line
>>> tables I was seeing an actual line entry and an end of sequence marker
>>> at the same address, before this commit these would reorder to move
>>> the end of sequence marker before the line entry (end of sequence has
>>> line number 0).  Now the end of sequence marker remains in its correct
>>> location, and in order to find a previous line we should step backward
>>> over any end of sequence markers.
>>>
>>> As an example, the binary:
>>>   gdb/testsuite/outputs/gdb.dwarf2/dw2-ranges-func/dw2-ranges-func-lo-cold
>>>
>>> Has this line table before the patch:
>>>
>>>   INDEX    LINE ADDRESS
>>>   0          48 0x0000000000400487
>>>   1         END 0x000000000040048e
>>>   2          52 0x000000000040048e
>>>   3          54 0x0000000000400492
>>>   4          56 0x0000000000400497
>>>   5         END 0x000000000040049a
>>>   6          62 0x000000000040049a
>>>   7         END 0x00000000004004a1
>>>   8          66 0x00000000004004a1
>>>   9          68 0x00000000004004a5
>>>   10         70 0x00000000004004aa
>>>   11         72 0x00000000004004b9
>>>   12        END 0x00000000004004bc
>>>   13         76 0x00000000004004bc
>>>   14         78 0x00000000004004c0
>>>   15         80 0x00000000004004c5
>>>   16        END 0x00000000004004cc
>>>
>>> And after this patch:
>>>
>>>   INDEX    LINE ADDRESS
>>>   0          48 0x0000000000400487
>>>   1          52 0x000000000040048e
>>>   2         END 0x000000000040048e
>>>   3          54 0x0000000000400492
>>>   4          56 0x0000000000400497
>>>   5         END 0x000000000040049a
>>>   6          62 0x000000000040049a
>>>   7          66 0x00000000004004a1
>>>   8         END 0x00000000004004a1
>>>   9          68 0x00000000004004a5
>>>   10         70 0x00000000004004aa
>>>   11         72 0x00000000004004b9
>>>   12        END 0x00000000004004bc
>>>   13         76 0x00000000004004bc
>>>   14         78 0x00000000004004c0
>>>   15         80 0x00000000004004c5
>>>   16        END 0x00000000004004cc
>>>
>>> When calling find_pc_sect_line with the address 0x000000000040048e, in
>>> both cases we find entry #3, we then try to find the previous entry,
>>> which originally found this entry '2         52 0x000000000040048e',
>>> after the patch it finds '2         END 0x000000000040048e', which
>>> cases the lookup to fail.
>>>
>>> By skipping the END marker after this patch we get back to the correct
>>> entry, which is now #1: '1          52 0x000000000040048e', and
>>> everything works again.
>>
>> I start to suspect that you have been working around an incorrect line
>> table.
>>
>> Consider this bit:
>> ...
>>    0          48 0x0000000000400487
>>    1          52 0x000000000040048e
>>    2         END 0x000000000040048e
>> ...
>>
>> The end marker marks the address one past the end of the sequence.
>> Therefore, it makes no sense to have an entry in the sequence with the
>> same address as the end marker.
>>
>> [ dwarf doc:
>>
>> end_sequence:
>>
>> A boolean indicating that the current address is that of the first byte
>> after the end of a sequence of target machine instructions. end_sequence
>> terminates a sequence of lines; therefore other information in the same
>> row is not meaningful.
>>
>> DW_LNE_end_sequence:
>>
>> The DW_LNE_end_sequence opcode takes no operands. It sets the
>> end_sequence register of the state machine to “true” and appends a row
>> to the matrix using the current values of the state-machine registers.
>> Then it resets the registers to the initial values specified above (see
>> Section 6.2.2). Every line number program sequence must end with a
>> DW_LNE_end_sequence instruction which creates a row whose address is
>> that of the byte after the last target machine instruction of the sequence.
>>
>> ]
>>
>> The incorrect entry is generated by this dwarf assembler sequence:
>> ...
>>                 {DW_LNS_copy}
>>                 {DW_LNE_end_sequence}
>> ...
>>
>> I think we should probably fix the dwarf assembly test-cases.
>>
>> If we want to handle this in gdb, the thing that seems most logical to
>> me is to ignore this kind of entries.
> 
> Hmm, that seems to be done already, in buildsym_compunit::record_line.
> 
> Anyway, I was looking at the line table for
> gdb.dwarf2/dw2-ranges-base.exp, and got a line table with subsequent end
> markers:
> ...
> INDEX  LINE   ADDRESS            IS-STMT
> 0      31     0x00000000004004a7 Y
> 1      21     0x00000000004004ae Y
> 2      END    0x00000000004004ae Y
> 3      11     0x00000000004004ba Y
> 4      END    0x00000000004004ba Y
> 5      END    0x00000000004004c6 Y
> ...
> 
> By using this patch:
> ...
> diff --git a/gdb/buildsym.c b/gdb/buildsym.c
> index 33bf6523e9..76f0b54ff6 100644
> --- a/gdb/buildsym.c
> +++ b/gdb/buildsym.c
> @@ -943,6 +943,10 @@ buildsym_compunit::end_symtab_with_blockvector
> (struct block *static_block,
>             = [] (const linetable_entry &ln1,
>                   const linetable_entry &ln2) -> bool
>               {
> +               if (ln1.pc == ln2.pc
> +                   && ((ln1.line == 0) != (ln2.line == 0)))
> +                 return ln1.line == 0 ? true : false;
> +
>                 return (ln1.pc < ln2.pc);
>               };
> 
> ...
> I get the expected:
> ...
> INDEX  LINE   ADDRESS            IS-STMT
> 0      31     0x00000000004004a7 Y
> 1      END    0x00000000004004ae Y
> 2      21     0x00000000004004ae Y
> 3      END    0x00000000004004ba Y
> 4      11     0x00000000004004ba Y
> 5      END    0x00000000004004c6 Y
> ...

Any comments on patch below?

Thanks,
- Tom