[llvm-dev] DWARF Generator

[Eric Christopher via llvm-dev]

On Fri, Nov 18, 2016 at 5:51 PM Robinson, Paul <paul.robinson at sony.com>
wrote:

> > Regarding DWARF parsing speed where strings are concerned:  DWARF 5 will
> > ruin this because DW_FORM_strx is a ULEB; so, every DIE that has a
> > DW_AT_name or DW_AT_linkage_name will become variable-size (just as bad
> > as DW_FORM_string).  In a RelWithDebInfo build of Clang, I got just over
> > 100 million DIEs and 40.2% had DW_AT_name.  I didn't try to count DIEs
> > that had DW_AT_linkage_name without DW_AT_name; it can happen, though, so
> > the number of variable-size DIEs in DWARF 5 will actually be higher than
> > that.
>
> More data from my RelWithDebInfo build of Clang:
>
> 102,654,608 total DIEs
>  95,864,960 fixed-size in DWARF 4
>  58,189,225 fixed-size in DWARF 5, assuming
>             all FORM_strp attributes become FORM_strx
>
> That is, fixed-size DIEs go from 93% to 57% of all DIEs.
> If it's really faster to load fixed-size DIEs, this is likely to have
> a noticeable effect.
>
> >
> > Maybe we should propose a fixed-size variant of FORM_strx, or that strx
> > just be fixed size?  The public-comment review period for DWARF 5 is not
> > over yet, and this seems like a valid concern.
>
> I have a little more data to look at but I am going to propose this to
> the DWARF committee.
>
>
Agreed. Let's talk offline about a proposal here if you'd like.

-eric


> > --paulr
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> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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