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

[Iain Sandoe <idsandoe@googlemail.com>]

> On 24 Jun 2019, at 14:31, 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)
> 
>> 
>>>> 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.

I already implemented a scheme (using three sections: INDEX, NAMES, PAYLOAD)  for Mach-O -
 since it doesn’t have  unlimited section count - it works - and hardly rocket science ;) 

- if one were to import the tabular portion of that at the start of a section and then the variable portion
 as a trailer … it could all be a single section.

iain

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