[llvm-dev] Default compression level for -compress-debug-info=zlib?

[via llvm-dev]

More data on different compression levels will be good.  In this case we're compressing fairly consistent looking input data (a DWARF section) so I think we stand a good chance of being able to pick a very reasonable level.
I cringe at the thought of yet another user-facing knob, though.
--paulr

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of James Henderson via llvm-dev
Sent: Thursday, August 02, 2018 6:32 AM
To: Pavel Labath
Cc: LLVM Dev
Subject: Re: [llvm-dev] Default compression level for -compress-debug-info=zlib?

Also not an expert, but would it make sense for this to be configurable at a fine-grained level, perhaps with another option, or an extension to the compress-debug-sections switch interface? That way users who care about the finer details can configure it themselves. And we should pick sensible options for the default.

James

On 2 August 2018 at 11:08, Pavel Labath via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
I don't claim to be an expert, but I did some zlib compression
benchmarks in the past. IIRC, my conclusion from that was that the
"DEFAULT" zlib level (6) is indeed a very good default for a lot of
cases -- it does not generate much larger outputs, while being
significantly faster than the max level. This all depends on the data
set and what you intend to do with the resulting data, of course, but
I guess my point is you don't have to choose only between 1 and 9. I
think it would be interesting to at least get the data for the default
level before making choice.

cheers,
pl
On Thu, 2 Aug 2018 at 01:57, Rui Ueyama via llvm-dev
<llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
>
> Folks,
>
> I'd like to get expert's opinion on which compression level is suitable for lld's -compress-debug-section=zlib option, which let the linker compress .debug_* sections using zlib.
>
> Currently, lld uses compression level 9 which produces the smallest output in exchange for a longer link time. My question is, is this what people actually want? We didn't consciously choose compression level 9. That was just the default compression level for zlib::compress function.
>
> For an experiment, I created a patch to use compression level 1 instead of 9 and linked clang using that modified lld. By default, lld takes 1m4s to link clang with --compress-debug-sections=zlib. With that patch, it took only 31s.
>
> Here is a comparison of clang executable size with various configurations:
>
> no debug sections:    275 MB
> level 9 compression:  855 MB
> level 1 compression:  922 MB
> no compression:      2044 MB
>
> Given that the best compression takes significantly longer time than the fastest compression, we probably should change the default to level 1. Any objections?
>
> I wonder what is the best compression level when -O2 is passed to lld. We could use level 9 when -O2 is passed, but is there any reason to compress debug sections that hard in the first place?
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