[cfe-dev] [llvm-dev] Your help needed: List of LLVM Open Projects 2017

[Sean Silva via cfe-dev]

On Mon, Jan 16, 2017 at 3:32 PM, Sean Silva <chisophugis at gmail.com> wrote:

>
>
> On Mon, Jan 16, 2017 at 2:31 PM, Davide Italiano <davide at freebsd.org>
> wrote:
>
>> On Mon, Jan 16, 2017 at 2:07 PM, Mehdi Amini <mehdi.amini at apple.com>
>> wrote:
>> >
>> > On Jan 16, 2017, at 1:47 PM, Sean Silva <chisophugis at gmail.com> wrote:
>> >
>> >
>> >
>> > On Mon, Jan 16, 2017 at 1:25 PM, Davide Italiano <davide at freebsd.org>
>> wrote:
>> >>
>> >> On Mon, Jan 16, 2017 at 12:31 PM, Sean Silva via llvm-dev
>> >> <llvm-dev at lists.llvm.org> wrote:
>> >> > Do we have any open projects on LLD?
>> >> >
>> >> > I know we usually try to avoid any big "projects" and mainly add/fix
>> >> > things
>> >> > in response to user needs, but just wondering if somebody has any
>> ideas.
>> >> >
>> >>
>> >> I'm not particularly active in lld anymore, but the last big item I'd
>> >> like to see implemented is Pettis-Hansen layout.
>> >> http://perso.ensta-paristech.fr/~bmonsuez/Cours/B6-4/Article
>> s/papers15.pdf
>> >> (mainly because it improves performances of the final executable).
>> >> GCC/gold have an implementation of the algorithm that can be used as
>> >> base. I'll expand if anybody is interested.
>> >> Side note: I'd like to propose a couple of llvm projects as well, I'll
>> >> sit down later today and write them.
>> >
>> >
>> >
>> > I’m not sure, can you confirm that such layout optimization on ELF
>> requires
>> > -ffunction-sections?
>> >
>>
>> For the non-LTO case, I think so.
>>
>> > Also, for clang on OSX the best layout we could get is to order
>> functions in
>> > the order in which they get executed at runtime.
>> >
>>
>> That's what we already do for lld. We collect and order file (run a
>> profiler) and pass that to the linker that lays out functions
>> accordingly.
>> This is to improve startup time for a class of startup-time-sensitive
>> operations. The algorithm proposed by Pettis (allegedly) aims to
>> reduce the TLB misses as it tries to lay out hot functions (or
>> functions that are likely to  be called together near in the final
>> binary).
>>
>
> IIRC from when I looked at the paper a while ago, it is mostly just a
> "huffman tree construction" type algorithm (agglomerating based on highest
> probability) and assumes that if two functions are hot then they are likely
> to be needed together. This is not always the case.
>
> E.g. consider a server that accepts RPC requests and based on those
> requests either does Foo or Bar which are largely disjoint. It's entirely
> possible for the top two functions of the profile to be one in Foo and one
> in Bar, but laying them out near each other doesn't make sense since there
> is never locality (for a given RPC, either Foo or Bar gets run). A static
> call graph analysis can provide the needed signals to handle this case
> better.
>
>
Hence you said "allegedly" :) I know we've talked about this before. Just
wanted to put the backstory of the "allegedly" on the list.

-- Sean Silva


> -- Sean Silva
>
>
>>
>> >
>> > For FullLTO it is conceptually pretty easy to get profile data we need
>> for
>> > this, but I'm not sure about the ThinLTO case.
>> >
>> > Teresa, Mehdi,
>> >
>> > Are there any plans (or things already working!) for getting profile
>> data
>> > from ThinLTO in a format that the linker can use for code layout? I
>> assume
>> > that profile data is being used already to guide importing, so it may
>> just
>> > be a matter of siphoning that off.
>> >
>> >
>> > I’m not sure what kind of “profile information” is needed, and what
>> makes it
>> > easier for MonolithicLTO compared to ThinLTO?
>> >
>> > Or maybe that layout code should be inside LLVM; maybe part of the
>> general
>> > LTO interface? It looks like the current gcc plugin calls back into gcc
>> for
>> > the actual layout algorithm itself (function call
>> > find_pettis_hansen_function_layout) rather than the reordering logic
>> living
>> > in the linker:
>> > https://android.googlesource.com/toolchain/gcc/+/3f73d6ef904
>> 58b45bbbb33ef4c2b174d4662a22d/gcc-4.6/function_reordering_
>> plugin/function_reordering_plugin.c
>> >
>> >
>> > I was thinking about this: could this be done by reorganizing the module
>> > itself for LTO?
>> >
>> > That wouldn’t help non-LTO and ThinLTO though.
>>
>> This is a dimension that I think can be explored. The fact that it
>> wouldn't help with other modes of operation is completely orthogonal,
>> in particular until it's proven that this kind of optimization makes
>> sense with ThinLTO (and if it doesn't, it can be an optimization ran
>> only during full LTO).
>>
>> --
>> Davide
>>
>> "There are no solved problems; there are only problems that are more
>> or less solved" -- Henri Poincare
>>
>
>
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