llvm dev - Sep 2018 - [lldb-dev] [LLD] How to get rid of debug info of sections deleted by garbage collector

Hi,

After compiling an example.cpp file with "-c -ffunction-sections" and
linking with "--gc-sections" (used ld.lld), I am still seeing debug
info
for the sections deleted by garbage collector in the generated executable.

Are there any compiler/linker options and/or other tools in LLVM to get rid
of the above mentioned unneeded debug info?

If such options does not exist, what needs to be changed in the linker
(lld)?

Thanks,
Ramana
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On Wed, Sep 19, 2018 at 8:35 PM Venkata Ramanaiah Nalamothu via
lldb-dev <lldb-dev at lists.llvm.org> wrote:
>
> Hi,
>
> After compiling an example.cpp file with "-c -ffunction-sections"
and linking with "--gc-sections" (used ld.lld), I am still seeing
debug info for the sections deleted by garbage collector in the generated
executable.
>
> Are there any compiler/linker options and/or other tools in LLVM to get rid
of the above mentioned unneeded debug info?
>
> If such options does not exist, what needs to be changed in the linker
(lld)?
>
It's not easy. It's also format dependent. I assume you're talking
about ELF here. In first approximation, the linker does not GC section
marked without SHF_ALLOC. At some point we did an analysis and in
practice it turns out most of them are debug info.
I seem to recall that Cary Coutant had a proposal for ld.gold on how
to reclaim them without breaking, but I can't find it easily (cc:ing
him directly).

Thanks,

--
Davide

> -----Original Message-----
> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of
> Davide Italiano via llvm-dev
> Sent: Thursday, September 20, 2018 10:55 AM
> To: ramana.venkat83 at gmail.com; Cary Coutant
> Cc: llvm-dev; LLDB
> Subject: Re: [llvm-dev] [lldb-dev] [LLD] How to get rid of debug info of
> sections deleted by garbage collector
> 
> On Wed, Sep 19, 2018 at 8:35 PM Venkata Ramanaiah Nalamothu via
> lldb-dev <lldb-dev at lists.llvm.org> wrote:
> >
> > Hi,
> >
> > After compiling an example.cpp file with "-c
-ffunction-sections" and
> linking with "--gc-sections" (used ld.lld), I am still seeing
debug info
> for the sections deleted by garbage collector in the generated executable.
> >
> > Are there any compiler/linker options and/or other tools in LLVM to
get
> rid of the above mentioned unneeded debug info?
> >
> > If such options does not exist, what needs to be changed in the linker
> (lld)?
> >
> 
> It's not easy. It's also format dependent. I assume you're
talking
> about ELF here. In first approximation, the linker does not GC section
> marked without SHF_ALLOC. At some point we did an analysis and in
> practice it turns out most of them are debug info.
> I seem to recall that Cary Coutant had a proposal for ld.gold on how
> to reclaim them without breaking, but I can't find it easily (cc:ing
> him directly).
The short answer is: Nothing you can do currently.

I had a chat with some of the Sony linker guys last week about this.
Currently .debug_info is monolithic; we'd have to break it up in some
fashion that would correspond with the way .text is broken up with
-ffunction-sections, in such a way that the linker would automatically
paste the right pieces back together to form a syntactically correct 
.debug_info section in the final executable.  There are some gotchas
that would need to be designed correctly (e.g. reference from an
inlined-subprogram to its abstract instance) but it didn't seem like
the problems were insurmountable.

The ultimate design almost certainly requires agreement about what the
ELF pieces should look like, and a description in the DWARF spec so
that consumers (e.g. dumpers) of the .o files would understand about
the fragmented sections.  And then the linkers and dumpers have to
be modified to implement it all. :-)

Even without gc-sections, there is duplicate info to get rid of:
everything that ends up in a COMDAT, like template instantiations
and inline functions.  This is actually a much bigger win than
anything you'd see left behind by GC.
--paulr
> 
> Thanks,
> 
> --
> Davide
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