Re: [PATCH] Add .gnu.lto_.meta section.

["Martin Liška" <mliska@suse.cz>]

On 6/24/19 8:05 PM, Richard Biener wrote:
> On Mon, Jun 24, 2019 at 3:31 PM Martin Liška <mliska@suse.cz> wrote:
>>
>> On 6/24/19 2:44 PM, Richard Biener wrote:
>>> On Mon, Jun 24, 2019 at 2:12 PM Martin Liška <mliska@suse.cz> wrote:
>>>>
>>>> On 6/24/19 2:02 PM, Richard Biener wrote:
>>>>> On Fri, Jun 21, 2019 at 4:01 PM Martin Liška <mliska@suse.cz> wrote:
>>>>>>
>>>>>> On 6/21/19 2:57 PM, Jan Hubicka wrote:
>>>>>>> This looks like good step (and please stream it in host independent
>>>>>>> way). I suppose all these issues can be done one-by-one.
>>>>>>
>>>>>> So there's a working patch for that. However one will see following errors
>>>>>> when using an older compiler or older LTO bytecode:
>>>>>>
>>>>>> $ gcc main9.o -flto
>>>>>> lto1: fatal error: bytecode stream in file ‘main9.o’ generated with LTO version -25480.4493 instead of the expected 9.0
>>>>>>
>>>>>> $ gcc main.o
>>>>>> lto1: internal compiler error: compressed stream: data error
>>>>>
>>>>> This is because of your change to bitfields or because with the old
>>>>> scheme the header with the
>>>>> version is compressed (is it?).
>>>>
>>>> Because currently also the header is compressed.
>>>
>>> That was it, yeah :/  Stupid decisions in the past.
>>>
>>> I guess we have to bite the bullet and do this kind of incompatible
>>> change, accepting
>>> the odd error message above.
>>>
>>>>> I'd simply avoid any layout changes
>>>>> in the version check range.
>>>>
>>>> Well, then we have to find out how to distinguish between compression algorithms.
>>>>
>>>>>
>>>>>> To be honest, I would prefer the new .gnu.lto_.meta section.
>>>>>> Richi why is that so ugly?
>>>>>
>>>>> Because it's a change in the wrong direction and doesn't solve the
>>>>> issue we already
>>>>> have (cannot determine if a section is compressed or not).
>>>>
>>>> That's not true, the .gnu.lto_.meta section will be always uncompressed and we can
>>>> also backport changes to older compiler that can read it and print a proper error
>>>> message about LTO bytecode version mismatch.
>>>
>>> We can always backport changes, yes, but I don't see why we have to.
>>
>> I'm fine with the backward compatibility break. But we should also consider lto-plugin.c
>> that is parsing following 2 sections:
>>
>>     91  #define LTO_SECTION_PREFIX      ".gnu.lto_.symtab"
>>     92  #define LTO_SECTION_PREFIX_LEN  (sizeof (LTO_SECTION_PREFIX) - 1)
>>     93  #define OFFLOAD_SECTION         ".gnu.offload_lto_.opts"
>>     94  #define OFFLOAD_SECTION_LEN     (sizeof (OFFLOAD_SECTION) - 1)
> 
> Yeah, I know.  And BFD and gold hard-coded those __gnu_lto_{v1,slim} symbols...

Yep, they do, 'nm' is also using that.

> 
>>>
>>>>> ELF section overhead
>>>>> is quite big if you have lots of small functions.
>>>>
>>>> My patch is actually shrinking space as I'm suggesting to add _one_ extra ELF section
>>>> and remove the section header from all other LTO sections. That will save space
>>>> for all function sections.
>>>
>>> But we want the header there to at least say if the section is
>>> compressed or not.
>>> The fact that we have so many ELF section means we have the redundant version
>>> info everywhere.
>>>
>>> We should have a single .gnu.lto_ section (and also get rid of those
>>> __gnu_lto_v1 and __gnu_lto_slim COMMON symbols - checking for
>>> existence of a symbol is more expensive compared to existence
>>> of a section).
>>
>> I like removal of the 2 aforementioned sections. To be honest I would recommend to
>> add a new .gnu.lto_.meta section.
> 
> Why .meta?  Why not just .gnu.lto_?

Works for me.

> 
>> We can use it instead of __gnu_lto_v1 and we can
>> have a flag there instead of __gnu_lto_slim. As a second step, I'm willing to concatenate all
>>
>>   LTO_section_function_body,
>>   LTO_section_static_initializer
>>
>> sections into a single one. That will require an index that will have to be created. I can discuss
>> that with Honza as he suggested using something smarter than function names.
> 
> I think the index belongs to symtab?
> 
> Let's properly do it if we want to change it.  Removing of
> __gnu_lto_v1/slim is going to be
> the most intrusive change btw. and orthogonal to the section changes.

I'm fine with a proper change. So do I understand that correctly that:
- we'll come up with .gnu.lto_ section that will be used by bfd, gold and nm
  to detect LTO objects
- for some time, we'll keep __gnu_lto_v1 and __gnu_lto_slim for backward
  compatibility with older binutils tool
- in couple of year, the legacy support will be removed

?

Martin

> 
> Richard.
> 
>>
>> Thoughts?
>> Martin
>>
>>>
>>> Richard.
>>>
>>>> Martin
>>>>
>>>>>
>>>>> Richard.
>>>>>
>>>>>>
>>>>>> Martin
>>>>
>>