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

[George Rimar via llvm-dev]

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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