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) > >>> 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 >>