Re: [PATCH] Fix skip.exp test failure observed with gcc-9.2.0

[Simon Marchi <simark@simark.ca>]

On 2019-12-15 1:12 p.m., Bernd Edlinger wrote:
> On 12/15/19 2:05 PM, Simon Marchi wrote:
>> On 2019-12-15 6:30 a.m., Bernd Edlinger wrote:
>>> Hi,
>>>
>>> this is the split out patch on skip.exp which fixes a pre-existing
>>> compatibilty issue with that test case and gcc-9.2.0 (and gcc-10 from
>>> trunk of a few weeks ago at least, likely other versions too).
>>>
>>>
>>> Is it OK for trunk?
>>>
>>>
>>> Thanks
>>> Bernd.
>>>
>>>
>>
>> Hi Bernd,
>>
>> Just wondering, were you able to figure out which change in debug info lead
>> to this behavior change?  The behavior with gcc 9.2.0 seems better to be me,
>> I think your patch is ok.
>>
> 
> Yes indeed.  The change started with gcc-8.1.0 when -gcolumn-info was enabled
> by default.  -gcolumn-info was first implemented in gcc-7.1.0 but default-disabled,
> so you can get the altered behavior already with gcc-7 if you manually enable
> -gcolumn-info.
> 
> So previously there was just one point where line line 30 (of skip.c) started:
> 
>   [0x00000032]  Advance Line by 27 to 28
>   [0x00000034]  Copy
>   [0x00000035]  Special opcode 63: advance Address by 4 to 0x4004cb and Line by 2 to 30
>   [0x00000036]  Advance PC by constant 17 to 0x4004dc
>   [0x00000037]  Special opcode 7: advance Address by 0 to 0x4004dc and Line by 2 to 32
> 
> with column-info we have line 30 three times with different column:
> 
>   [0x00000034]  Advance Line by 27 to 28
>   [0x00000036]  Copy
>   [0x00000037]  Set column to 9
>   [0x00000039]  Special opcode 63: advance Address by 4 to 0x4004c6 and Line by 2 to 30
>   [0x0000003a]  Set column to 17
>   [0x0000003c]  Special opcode 75: advance Address by 5 to 0x4004cb and Line by 0 to 30
>   [0x0000003d]  Set column to 3
>   [0x0000003f]  Special opcode 75: advance Address by 5 to 0x4004d0 and Line by 0 to 30
>   [0x00000040]  Special opcode 105: advance Address by 7 to 0x4004d7 and Line by 2 to 32
> 
> 
> That could probably be filtered in dwarf2read.c to keep the old behavior, but I agree
> that the new behavior makes still sense, even if we cannot really use the column info
> in the line number info.

That is actually some very good info for anyone wondering why this change was introduced!

Could you please put it in the commit message for posterity?

>> I would just remove the unrelated whitespace fix before merging.
>>
> 
> Okay, I just replicated you advice regarding 8-space tab columns on
> skip-inline.exp in the other patch.  Exactly those 2 lines were copied
> where the tabs were not used correctly.

Great thanks.  Note that we can also fix skip.exp, just in another, obvious patch.

Simon