[LLVMdev] LTO question

Eric Christopher echristo at gmail.com
Fri Dec 26 16:45:18 PST 2014


>
>
> Comparison with LLVM is described in the second article. It took about
> 40min to finish building Firefox with llvm using lto and -g. The
> following is a quote:
>
> "This graph shows issues with debug info memory use. LLVM goes up to
> 35GB. LLVM developers are also working on debug info merging
> improvements (equivalent to what GCC's type merging is) and the
> situation has improved in last two releases until the current shape.
> Older LLVM checkouts happily run out of 60GB memory & 60GB swap on my
> machine.".
>

As an aside:

This has been largely alleviated on programs the size of firefox. The
current work that Duncan is doing is designed to reduce memory usage even
more - quite a bit.

That said, debug info quality for types and locations with LTO is quite
good under LLVM if you're willing to spend the memory versus the current
state of gcc. GCC has far better optimized location information than gcc,
but both are, of course, improving rapidly.

-eric


>
>
> >
> > I'd be interested to hear more about the LTO design you folks are
> working on, whenever you're ready to share the details.
>
> We will share the details as soon as we can -- possibly some time in Jan
> 2015.
>
>
> > I read the GCC design docs on LTO, and I'm curious how similar or
> different your approach will be.  For example, the 3-phase approach of
> WHOPR is fairly sophisticated (it actually follows closely some research
> done at Rice U. and IBM on scalable interprocedural analysis, in the same
> group where Preston did his Ph.D.).
>
> In Google, we care mostly about peak optimization performance. Peak
> Optimization is basically PGO + CMO. For cross-module optimization
> (CMO) to be usable for large applications, small memory footprint is
> just one aspect of it,  and fast build time is equally important. Peak
> optimization is not only used in release build but  in developer
> workflow too. This means build time with CMO should be close to O2
> time as much as possible.  It is important to compiler engineers too
> -- you don't want to wait for more than 20min to hit a breakpoint in
> debugging a compiler problem :)
>
> For this reason, GCC LTO is not used in Google. Instead, the much more
> scalable solution called LIPO is widely used for CMO:
> https://gcc.gnu.org/wiki/LightweightIpo. LIPO by design requires PGO.
>
> While LIPO is scalable, it has its own limitation that prevents the
> compiler from maximizing the benefit of CMO. The new design is
> intended to solve the problem with more very aggressive objectives.
> The new design is pretty simple and shares the basic principles of
> LIPO without requiring PGO (though it still works best with PGO). It
> still fits in LTO framework, so that toolchain support change is
> minimized. For now, without giving details, I can share  some of the
> objectives of the new design:
>
>     * The build should be almost fully parallelizable (at both process
>       level and build machine node level)
>     * The build should scale to programs with *any/unlimited* size
>       (measured in number of TUs). It should handle programs 10x, or 100x
>        the size of Firefox.
>     * The build time should be very close to non-LTO build, and can be
>       considered to be turned on *by default* for O2 or at least O3
>       compilations.
>     * When turned on the by default, it can eliminate the need for
>       users to put inline functions in header files (thus greatly help
>       improving parsing time)
>     * Most of the benefit of CMO comes from cross module inlining and
>       cross module indirect call promotions.  By default, the design
>       only enables these two, but it is still compatible with any whole
>       program analysis which can be turned on with additional options.
>
> thanks,
>
> David
>
> On Thu, Dec 25, 2014 at 11:55 PM, Adve, Vikram Sadanand <
> vadve at illinois.edu> wrote:
>>
>> Diego, Teresa, David,
>>
>> Sorry for my delayed reply; I left for vacation right after sending my
>> message about this.
>>
>> Diego, it wasn't explicit from your message whether LLVM LTO can handle
>> Firefox-scale programs, which you said GCC can handle.  I assumed that's
>> what you meant, but could you confirm that?  I understand that neither can
>> handle the very large Google applications, but that's probably not a
>> near-term concern for a project like the one Charles is embarking on.
>>
>> I'd be interested to hear more about the LTO design you folks are working
>> on, whenever you're ready to share the details.  I read the GCC design docs
>> on LTO, and I'm curious how similar or different your approach will be.
>> For example, the 3-phase approach of WHOPR is fairly sophisticated (it
>> actually follows closely some research done at Rice U. and IBM on scalable
>> interprocedural analysis, in the same group where Preston did his Ph.D.).
>>
>> For now, I would like to introduce you all to Charles, so that he has
>> access to people working on this issue, which will probably continue to be
>> a concern for his project.  I have copied you on my reply to him.
>>
>> Thanks for the information.
>>
>> --Vikram S. Adve
>> Visiting Professor, Computer Science, EPFL
>> Professor, Department of Computer Science
>> University of Illinois at Urbana-Champaign
>> vadve at illinois.edu
>> http://llvm.org
>>
>>
>>
>>
>> On Dec 16, 2014, at 3:48 AM, Teresa Johnson <tejohnson at google.com> wrote:
>>
>> > On Fri, Dec 12, 2014 at 1:59 PM, Diego Novillo <dnovillo at google.com>
>> wrote:
>> >> On 12/12/14 15:56, Adve, Vikram Sadanand wrote:
>> >>>
>> >>> I've been asked how LTO in LLVM compares to equivalent capabilities
>> >>> in GCC.  How do the two compare in terms of scalability?  And
>> >>> robustness for large applications?
>> >>
>> >>
>> >> Neither GCC nor LLVM can handle our (Google) large applications.
>> They're
>> >> just too massive for the kind of linking done by LTO.
>> >>
>> >> When we built GCC's LTO, we were trying to address this by creating a
>> >> partitioned model, where the analysis phase and the codegen phase are
>> split
>> >> to allow working on partial callgraphs
>> >> (http://gcc.gnu.org/wiki/LinkTimeOptimization for details).
>> >>
>> >> This allows to split and parallelize the initial bytecode generation
>> and the
>> >> final optimization/codegen. However, the analysis is still implemented
>> as a
>> >> single process. We found that we cannot even load summaries, types and
>> >> symbols in an efficient way.
>> >>
>> >> It does allow for programs like Firefox to be handled. So, if by "big"
>> you
>> >> need to handle something of that size, this model can doit.
>> >>
>> >> With LLVM, I can't even load the IR for one of our large programs on a
>> box
>> >> with 64Gb of RAM.
>> >>
>> >>> Also, are there any ongoing efforts or plans to improve LTO in LLVM
>> >>> in the near future?
>> >>
>> >>
>> >> Yes. We are going to be investing in this area very soon. David and
>> Teresa
>> >> (CC'd) will have details.
>> >
>> > Still working out the details, but we are investigating a solution
>> > that is scalable to very large programs. We'll share the design in the
>> > near future when we have more details worked out so that we can get
>> > feedback.
>> >
>> > Thanks!
>> > Teresa
>> >
>> >>
>> >>
>> >> Diego.
>> >
>> >
>> >
>> > --
>> > Teresa Johnson | Software Engineer | tejohnson at google.com |
>> 408-460-2413
>>
>>
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