So are we going to discuss dwarves? Or what? On Wed, May 3, 2017 at 11:05 AM, <dwarf-discuss-requ...@lists.dwarfstd.org> wrote:
> Send Dwarf-Discuss mailing list submissions to > dwarf-discuss@lists.dwarfstd.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://lists.dwarfstd.org/listinfo.cgi/dwarf-discuss-dwarfstd.org > or, via email, send a message with subject or body 'help' to > dwarf-discuss-requ...@lists.dwarfstd.org > > You can reach the person managing the list at > dwarf-discuss-ow...@lists.dwarfstd.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Dwarf-Discuss digest..." > > > Today's Topics: > > 1. Fission + cross-CU references (ref_addr) (David Blaikie) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Tue, 02 May 2017 19:09:40 +0000 > From: David Blaikie <dblai...@gmail.com> > To: "dwarf-discuss@lists.dwarfstd.org" > <dwarf-discuss@lists.dwarfstd.org> > Subject: [Dwarf-Discuss] Fission + cross-CU references (ref_addr) > Message-ID: > <CAENS6EuSZ+8__YTXdPgbTfKgs819sVPW972nRFcMF=p > b_xl...@mail.gmail.com> > Content-Type: text/plain; charset="utf-8" > > I've recently been trying to resolve the use of Fission in LLVM's ThinLTO > mode (though this would apply to plain LTO too). > > One of the things that happens here is that cross-CU DIE references > (DW_FORM_ref_addr) are used to describe inlining a function in one CU into > another CU. > > This format has been implemented in LLVM and GCC for ~years and seems to > work well outside of Fission. > > So the question is: what to do with Fission? > > It seemed to me that a good representation would be to produce multiple CUs > into a single DWO file, which GDB can't yet consume, but I'm working on > patches to help there. DW_FORM_ref_addr would not use any ELF relocation, > but be assumed to be "relative to the chunk of debug_info it was in" > (within the .dwo file) > > But what about DWP files? Currently binutils dwp produces records like > this: > > (this dwp contains 3 CUs, two from one LTO compile, and one from a > standalone compile linked in for comparison): > > Index Signature INFO ABBR LINE STR_OFF > ----- ------------------ -------- -------- -------- -------- > 2 0x7bd765349b7e7631 [2d, 65) [38, ae) [11, 22) [14, 3c) > 8 0x66f4e160661d2687 [00, 2d) [00, 38) [00, 11) [00, 14) > 11 0x32dd6d7121dd1d9a [65, 98) [38, ae) [11, 22) [14, 3c) > > So the ABBR/LINE/STR_OFF sections are kept as-is (no analysis is done to > find which portions of the dwo file are used by which CUs, etc), but the > INFO section is fragmented on the CU boundaries. Fragmenting the TYPES > section on the TU boundaries is necessary/useful for deduplication of > types, but this fragmenting of the CU makes it impossible (I think) to use > ref_addr in a dwp file. > > If this fragmenting were not done - consumers (GDB, etc) would need to > change to account for this - searching through the INFO range to find the > CU matching the signature, rather than knowing it starts at the start of > the INFO range. This could have a noticeable performance impact especially > in a full LTO build (where /all/ the CUs were in the same .dwo - so the > index would be entirely unhelpful, I think). > > Does all this sound right/sane - anyone have ideas/perspectives/thoughts on > how this should work? > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: <http://lists.dwarfstd.org/private.cgi/dwarf-discuss- > dwarfstd.org/attachments/20170502/155f11be/attachment.html> > > ------------------------------ > > Subject: Digest Footer > > _______________________________________________ > Dwarf-Discuss mailing list > Dwarf-Discuss@lists.dwarfstd.org > http://lists.dwarfstd.org/listinfo.cgi/dwarf-discuss-dwarfstd.org > > > ------------------------------ > > End of Dwarf-Discuss Digest, Vol 99, Issue 1 > ******************************************** >
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