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

[Tom de Vries]

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
...

Thanks,
- Tom