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

Tue Dec 5 12:13:28 PST 2017

If you're interested in things you can do in the linker for this - you
might consider something more aggressive: Fully DWARF aware deduplication.

This could be done hopefully by reusing some of the code in the dsymutil
implementation in LLVM.

This would be much more effective (and without the possible
context-sensitive tradeoffs) than using type units. Though it'd possibly
have a big tradeoff in link time and/or linker memory usage (I'm not sure
how much dsymutil needs/uses of either).

It doesn't seem especially important to implement the DWARF5 types ->
debug_info thing for this situation, the type units as they are (in
debug_types) offer the same size benefits here. But sure, if anyone wanted
to implement it at some point, that'd be fine.

I think Paul covered some of the reasons type units might not be a
reasonable default.

One additional reason is that if you use Split DWARF (another great way to
massively reduce the amount of debug info going to the linker) type units
are mostly /just/ overhead in the .dwo files: since the debug info is not
linked, there's no opportunity to remove the duplication anyway (unless
you're making a DWP - like a dsym file)

On Mon, Dec 4, 2017 at 7:11 AM George Rimar via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi all !
>
> We have an issue with LLD, it is  "relocation R_X86_64_32 out of range"
> (PR31109)
> which occurs during resolving relocations in debug sections. It looks
> happens
> because .debug_info section can be too large sometimes and 32x relocation
> is not enough
> to represent the value. One of possible solutions looks to be to
> deduplicate information
> to reduce .debug_info size.
> The rest of mail contains information about experiments I did, the
> obtained results and
> some questions and suggestions as well.
>
> I was investigating idea to deduplicate debug types information. Idea is
> described at
> p276 of DWARF4 specification (http://www.dwarfstd.org/doc/DWARF4.pdf). It
> suggests
> to split types information out of .debug_info and emit multiple
> .debug_types sections
> with use of COMDATs. Both clang and gcc I tested implements
> -fdebug-types-section flag for that:
>
> -fdebug-types-section, -fno-debug-types-section
> Place debug types in their own section (ELF Only)
> gcc's description is here:
> https://gcc.gnu.org/onlinedocs/gcc-6.4.0/gcc/Debugging-Options.html#Debugging-Options
> .
>
> This flag is disabled by default. I compared clang binaries to see the
> difference
> with and without the linker side optimisation.
> 1) Clang built with -g has size of 1.7 GB, .debug_info section size is
> 894.5 Mb.
> 2) Clang built with -g -fdebug-types-section has size of 1.0 GB.
>    .debug_types size is 26.267 MB, .debug_info size is 227.7 MB.
>
> Difference is huge and I believe shows (though probably for most of
> readers here it was
> already obvious) that optimization can be useful. Though
> -fdebug-types-section is disabled by default.
> Looks it was initially disabled because not all of DWARF consumers were
> aware of .debug_types section.
>
> Now in 2017 situation is different. I think most of DWARF consumers knows
> about .debug_types, but:
> 1) DWARF5 specification explicitly eliminates the .debug_types section
> introduced in DWARF4:
>    p8, "1.4 Changes from Version 4 to Version 5"
> http://dwarfstd.org/doc/DWARF5.pdf
> 2) Instead of emiting multiple .debug_types it suggests to emit multiple
> .debug_info COMDAT
>    sections. (p375, p376).
>
> And it seems currently there is no way to make clang to emit multiple
> .debug_info with type information
> like DWARF5 suggests. I tried command line below:
> -g -fdebug-types-section -gdwarf-5
> It still emits .debug_types and does not look there is a flag for emiting
> multiple .debug_info.
> Looking at whole LLVM code (lib/mc, lib/CodeGen) actually it seems it is
> just always assumed .debug_info is
> a unique section in object.
> (also not sure why clang emits .debug_types when -gdwarf-5 flag is set, as
> this section is incompatible with v5,
> probably it is a bug).
>
> So my questions are following:
> 1) Do we want to try to implement multiple .debug_info approach ? As it
> seems can be very useful sometimes.
> 2) For now in LLD may be we may want to extend our error message from
> "relocation X out of range" to something
>    suggesting to use -fdebug-types-section (only for relocations in debug
> sections) ?
> 3) Why -fdebug-types-section is disabled by default ?
>
> 
> Best regards,
> George | Developer | Access Softek, Inc
> _______________________________________________
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> llvm-dev at lists.llvm.org
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>
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