On Mon, Jun 24, 2019 at 3:31 PM Martin Liška <[email protected]> wrote:
>
> On 6/24/19 2:44 PM, Richard Biener wrote:
> > On Mon, Jun 24, 2019 at 2:12 PM Martin Liška <[email protected]> wrote:
> >>
> >> On 6/24/19 2:02 PM, Richard Biener wrote:
> >>> On Fri, Jun 21, 2019 at 4:01 PM Martin Liška <[email protected]> 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...
> >
> >>> 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_?
> 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.
Richard.
>
> Thoughts?
> Martin
>
> >
> > Richard.
> >
> >> Martin
> >>
> >>>
> >>> Richard.
> >>>
> >>>>
> >>>> Martin
> >>
>
