[llvm-dev] [RFC][Dwarf Library] Relocations for DWO sections

[David Blaikie via llvm-dev]

On Fri, Jul 23, 2021 at 1:18 PM Alexander Yermolovich <ayermolo at fb.com>
wrote:

> Thanks for replying in the patch. Left my reply.
> We are using it to deal with dwarf relocation overflows.
>

Ah, that's good to know. FWIW we've started to hit some overflows even in
Split DWARF on larger binaries (and/or those making especially heavy use of
expression templates - creating an exceptional amount of DWARF/long symbol
names

A couple of ideas to address this particular overflow (which section(s) did
you manage to overflow? We're dealing with .debug_str[.dwo] overflow in
particular) that I'm looking into are:
Simplified template names (
https://lists.llvm.org/pipermail/llvm-dev/2021-June/150903.html ) - emit
only the base name ("foo") of a template rather than all the template
parameters ("foo<int>") - and then reconstruct the full name by using the
DW_TAG_template_type_parameters, etc.
Reconstituted Mangled names (
https://groups.google.com/g/llvm-dev/c/2jMqDjdChuQ/m/HpOpWy8pAwAJ ) - skip
mangled names when they can be reconstituted from the DWARF structural
representation (eg: "void f1(int) { }" -> "_Z2f1i" but we could build the
latter from DWARF's representation that says f1 has one "int" parameter).


> We considered DWARF64, but split dwarf seems like a more traveled path. As
> for single vs split my understanding is that single plays nicer with our
> build system ATM.
>

Ah, fair enough.


> ------------------------------
> *From:* David Blaikie <dblaikie at gmail.com>
> *Sent:* Friday, July 23, 2021 7:41 AM
> *To:* Alexander Yermolovich <ayermolo at fb.com>
> *Cc:* llvm-dev at lists.llvm.org <llvm-dev at lists.llvm.org>
> *Subject:* Re: [llvm-dev] [RFC][Dwarf Library] Relocations for DWO
> sections
>
> General premise sounds correct to me (that we shouldn't be processing
> those sections, etc). I've replied to the patch - thanks for taking a look
> at this!
>
> (out of curiosity: What are you using Split DWARF single mode for (if you
> can speak to the application)?)
>
> On Thu, Jul 22, 2021 at 9:10 PM Alexander Yermolovich via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Hello
>
> I observed when DWARF Context is created for DWO object (split dwarf
> single mode), that relocations for .debug_info are processed and are stored
> in a map. This adds quite a bit of memory overhead. This doesn't seem like
> it is needed for DWO Context. Context created through
> API DWARFContext::getDWOContext. Am I missing something?
>
> Illustrative patch to fix this:
> https://reviews.llvm.org/D106624
>
> Thank you,
> Alex
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