[Dwarf-Discuss] Dwarf-Discuss Digest, Vol 99, Issue 1

[Aaron Robinson]

So are we going to discuss dwarves? Or what?

On Wed, May 3, 2017 at 11:05 AM, <dwarf-discuss-request at lists.dwarfstd.org>
wrote:

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>    1. Fission + cross-CU references (ref_addr) (David Blaikie)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Tue, 02 May 2017 19:09:40 +0000
> From: David Blaikie <dblaikie at gmail.com>
> To: "dwarf-discuss at lists.dwarfstd.org"
>         <dwarf-discuss at lists.dwarfstd.org>
> Subject: [Dwarf-Discuss] Fission + cross-CU references (ref_addr)
> Message-ID:
>         <CAENS6EuSZ+8__YTXdPgbTfKgs819sVPW972nRFcMF=p
> B_XLuSw at 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?
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