[llvm-dev] Reducing DWARF emitter memory consumption

Eric Christopher via llvm-dev
Sat, 06 Feb 2016 01:53:01 -0000

On Fri, Feb 5, 2016 at 5:51 PM Mehdi Amini via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
> > On Feb 5, 2016, at 5:40 PM, Peter Collingbourne <peter at
pcc.me.uk> wrote:
> >
> > On Fri, Feb 05, 2016 at 04:58:45PM -0800, Mehdi Amini wrote:
> >>
> >>> On Feb 5, 2016, at 3:17 PM, Peter Collingbourne via llvm-dev
<
> llvm-dev at lists.llvm.org> wrote:
> >>>
> >>> Hi all,
> >>>
> >>> We have profiled [1] the memory usage in LLVM when LTO'ing
Chromium,
> and
> >>> we've found that one of the top consumers of memory is the
DWARF
> emitter in
> >>> lib/CodeGen/AsmPrinter/Dwarf*.
> >>
> >> I'm staring at the profile attached to the post #15 on the
link you
> posted, can you confirm that the Dwarf emitter accounts for
> 6.7%+15.6%=22.3% of the the total allocated memory?
> >> If I understand correctly the numbers, this does not tell anything
> about how much the Dwarf emitter accounts on the *peak memory* usage (could
> be more, could be nothing...).
> >
> > I think these nodes represent allocations from the DWARF emitter:
> >
> > DwarfDebug::DwarfDebug 9.5%
> > DwarfDebug::endFunction 15.6%
> > DIEValueList::addValue 9.1%
> > total 34.2%
> >
> > I believe they are totals, but my reading of the code is that the
DWARF
> > emitter does not deallocate its memory until the end of code
generation,
>
> That's sad :(
>
> > so total ~= peak in this case.
>
> Assuming the peak occurs during CodeGen (which is what I on my profile),
> that sounds pretty reasonable!
>
> Thanks for the information (and the work!).
>
> Another question I have, is how worse the split codegen make the
> situation? Naively there will be a lot of redundancy in the split modules,
> for ThinLTO Teresa has to proceed with care to limit the amount of
> duplication.
>
>
Hmm? Can you reword this slightly? I'm not sure what you're asking here.

-eric


> Mehdi
>
>
> >
> > I am not surprised by these figures -- see e.g. DIEValueList::Node
which
> in
> > the worst case can use up to 24 bytes on a 1-byte DWARF attribute
record.
> >
> > Ivan was the person who collected the numbers, he may be able to
comment
> more.
> >
> > Thanks,
> > --
> > Peter
>
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