On Mon, Jun 24, 2019 at 3:31 PM Martin Liška <mli...@suse.cz> wrote: > > On 6/24/19 2:44 PM, Richard Biener wrote: > > On Mon, Jun 24, 2019 at 2:12 PM Martin Liška <mli...@suse.cz> wrote: > >> > >> On 6/24/19 2:02 PM, Richard Biener wrote: > >>> On Fri, Jun 21, 2019 at 4:01 PM Martin Liška <mli...@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... > > > >>> 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 > >> >