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

[Martin Liška]

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)

> 
>>> 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. 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.

Thoughts?
Martin

> 
> Richard.
> 
>> Martin
>>
>>>
>>> Richard.
>>>
>>>>
>>>> Martin
>>