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