[llvm-dev] unified debug information despite function/data sections flags

[Alexey Lapshin via llvm-dev]

 >As for reducing debug info size by omitting debug info descriptions of 
dead code - Apple/MachO's dsymutil does this, and I believe Alexey 
Lapshin is working on trying to get similar behavior into lld, possibly 
(or as a post-link tool).

yep. I am working on the tool similar to dsymutil but for non-darwin 
platform. dsymutil is able to remove all unreferenced debug info. That 
work is currently in progress. It has two main parts:

1) speedup DWARFLinker library and reduce final debug info size - 
https://reviews.llvm.org/D96035
2) create a post link tool using DWARFLinker library to do similar 
functionality as dsymutil does - https://reviews.llvm.org/D86539 
https://reviews.llvm.org/D88827

Alexey.

On 30.09.2021 09:51, David Blaikie wrote:
> You can differentiate dead function descriptions from others on most 
> platforms by checking if the low_pc == 0. If 0 is a valid instruction 
> address on your architecture, you can use an lld feature to set a more 
> authoritative/unambiguous tombstone value for dead code addresses, 
> passing something like:
> /-z 'dead-reloc-in-nonalloc=.debug_ranges=0xfffffffffffffffe' //-z 'dead-reloc-in-nonalloc=.debug_loc=0xfffffffffffffffe' //-z 'dead-reloc-in-nonalloc=.debug_*=0xffffffffffffffff' /
> to the linker.
>
> As for reducing debug info size by omitting debug info descriptions of 
> dead code - Apple/MachO's dsymutil does this, and I believe Alexey 
> Lapshin is working on trying to get similar behavior into lld, 
> possibly (or as a post-link tool).
>
> There's also the possibility of using comdats to make the linker's job 
> easier - I think there might be ways to structure the DWARF into 
> chunks that could be deduplicated and dropped naturally by a linker's 
> existing comdat support, but I haven't fully prototyped it. I think 
> there was a thread a while back with JHenderson and others discussing 
> this possibility further.
>
> - Dave
>
> On Wed, Sep 29, 2021 at 12:50 PM Youssefi, Anna via llvm-dev 
> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>
>     Hi,
>
>     I was wondering if there are any plans to separate debug
>     information into distinct sections accordingly when the compiler
>     flags -ffunction-sections and/or -fdata-sections are used. If an
>     unreferenced function is removed from the link, it makes no sense
>     for its associated debug information to still be included.  As we
>     rely on the debug information for stack usage analysis, we wind up
>     displaying stack usage statistics for unreferenced functions that
>     were eliminated from the link if debug information for any other
>     referenced functions is in the same debug section.  It seems that
>     others have run into this problem previously so I wanted to check
>     whether there are any plans to change the behavior.
>
>     Thanks,
>
>     Anna Youssefi
>
>     Texas Instruments, Codegen group
>
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