[llvm-dev] [RFC] - Deduplication of debug information in linkers (LLD).

[David Blaikie via llvm-dev]

On Sat, Dec 16, 2017 at 11:40 AM George Rimar <grimar at accesssoftek.com>
wrote:

> Or following workflow:
>
> Split dwarf is used to make linker to proccess less, like relocations,
> right ?
>
Partly, though the main motivation as far as I know, was to have to provide
fewer bytes to the linker at all. That's why something like Apple's scheme
(leave the debug info in the object files, but have the linker ignore them
- then merge the debug info separately in dsymutil) wasn't applicable -
because that still leaves large object files. For a distributed build
system like Google's, with very large binaries, the presence of the bytes,
even if they're ignored/not processed by the linker, was problematic.

> What about (I think I heard this somewhere, not sure idea is mine, it was
> a year ago I think).
>
> What about something that combines output to a linker output, so it could
> do optimizations
>
Not sure I understand what you're suggesting here, sorry :/

> of DWARF data (probably no need to proccess relocations at all, so it
> would be fast probably).
>
> And combine everything to a single file.
>
> Some kind of mix. Not sure it makes sence. Just wondering.
>
>
> Best regards,
> George | Developer | Access Softek, Inc
> ------------------------------
> *От:* George Rimar
> *Отправлено:* 16 декабря 2017 г. 22:27
> *Кому:* David Blaikie
>
> *Копия:* Sean Silva; llvm-dev at lists.llvm.org; Rui Ueyama; Rafael Espindola
> *Тема:* Re: [llvm-dev] [RFC] - Deduplication of debug information in
> linkers (LLD).
>
> ​But could not we for example do split dwarf, but for example do dedup of
> types ?
>
> I do not mean right now, but in a theory ?
>
>
> Best regards,
> George | Developer | Access Softek, Inc
> ------------------------------
> *От:* David Blaikie <dblaikie at gmail.com>
> *Отправлено:* 16 декабря 2017 г. 22:25
> *Кому:* George Rimar
> *Копия:* Sean Silva; llvm-dev at lists.llvm.org; Rui Ueyama; Rafael Espindola
> *Тема:* Re: [llvm-dev] [RFC] - Deduplication of debug information in
> linkers (LLD).
>
>
>
> On Sat, Dec 16, 2017 at 4:08 AM George Rimar <grimar at accesssoftek.com>
> wrote:
>
>> >Wasn't our (lld/ELF's) position on debug info size that we should focus
>> on providing a great split-dwarf workflow and not try go too far out of our
>> way to deduplicate >or otherwise reduce debug info size inside LLD? I
>> recall there being some patches that made linking of large debug binaries
>> like 1.5GB+ clang faster, but we decided to >reject those changes because
>> split-dwarf was the "right" solution.
>> >
>> >Rafael, Rui?
>> >
>> >(I even recall Rafael saying at one point that a great split-dwarf
>> workflow was one of the key things he considered as necessary for him to
>> consider LLD "done")
>> >
>> >-- Sean Silva
>>
>> I probably not the right person to suggest (still mostly learning here
>> for now, so would like to be on a fence in general),
>> but it looks for me that splitting DWARF and deduplicating DWARF is a
>> bit othogonal things.
>> It feels for me that there is a room to do both things and have a benefit
>> from combinatiion ?
>>
>
> The two features/directions don't really compose - if the DWARF is split,
> then the linker never sees the DWARF (it's not in the object files), so has
> no deduplication to do. (llvm-dwp might see it, so the deduplication can
> happen there)
>
>
>>
>> George.
>>
>
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