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

[James Henderson via llvm-dev]

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> 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> 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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