[LLVMdev] RFC: ThinLTO Impementation Plan

[Teresa Johnson]

On Thu, May 14, 2015 at 2:09 PM, Eric Christopher <echristo at gmail.com> wrote:
>
>
> On Thu, May 14, 2015 at 1:35 PM Teresa Johnson <tejohnson at google.com> wrote:
>>
>> On Thu, May 14, 2015 at 1:18 PM, Eric Christopher <echristo at gmail.com>
>> wrote:
>> >
>> >
>> > On Thu, May 14, 2015 at 1:11 PM David Blaikie <dblaikie at gmail.com>
>> > wrote:
>> >>
>> >> On Thu, May 14, 2015 at 12:53 PM, Eric Christopher <echristo at gmail.com>
>> >> wrote:
>> >>>
>> >>>
>> >>>
>> >>> On Thu, May 14, 2015 at 11:34 AM Daniel Berlin <dberlin at dberlin.org>
>> >>> wrote:
>> >>>>
>> >>>> On Thu, May 14, 2015 at 11:14 AM, Eric Christopher
>> >>>> <echristo at gmail.com>
>> >>>> wrote:
>> >>>> > I'm not sure this is a particularly great assumption to make.
>> >>>>
>> >>>> Which part?
>> >>>
>> >>>
>> >>> The binutils part :)
>> >>>
>> >>>>
>> >>>>
>> >>>> >  We have to
>> >>>> > support a lot of different build systems and tools and
>> >>>> > concentrating
>> >>>> > on
>> >>>> > something that just binutils uses isn't particularly friendly here.
>> >>>> I think you may have misunderstood
>> >>>> His point was exactly that they want to be transparent to *all of*
>> >>>> these
>> >>>> tools.
>> >>>> You are saying "we should be friendly to everyone". He is saying the
>> >>>> same thing.
>> >>>> We should be friendly to everyone. The friendly way to do this is to
>> >>>> not require all of these tools build plugins to handle bitcode.
>> >>>>
>> >>>> Hence, elf-wrapped bitcode.
>> >>>
>> >>>
>> >>> Oh, I understood. I just don't know that I agree. To do anything with
>> >>> the
>> >>> tools will require some knowledge of bitcode anyhow or need the
>> >>> plugin. I'm
>> >>> saying that as a baseline start we should look at how to do this using
>> >>> the
>> >>> tools we've got rather than wrapping things for no real gain.
>> >>
>> >>
>> >> That doesn't seem strictly true - the ar situation (which I'm lead to
>> >> believe is in use in our build system & others, one would assume). With
>> >> the
>> >> symbol table included as proposed, ar can be used without any knowledge
>> >> of
>> >> the bitcode or need for a plugin.
>> >>
>> >
>> > For some bits, sure. Optimizing for ar seems a bit silly, why not 'ld
>> > -r'?
>>
>> But as mentioned, ld -r can work on native object wrapped bitcode
>> without a plugin as well.
>>
>
> How? It's not like any partial linking is going to go on inside the bitcode
> if the linker doesn't understand bitcode.

It allows us to delay the actual linking until the full link step,
thereby enabling ThinLTO on those modules.

As we discussed offline, the current ld -r behavior with the plugin is
to compile all the way down to machine code. The alternative if we use
straight bitcode is to tell the plugin to stop early after combining
the bitcode and emit bitcode back out, with the thinlto function info
also combined.

>
>>
>> > Agreed. The ar situation is interesting because one thing we discussed
>> > after
>> > you wandered off was just adding a ToC section to bitcode as it is and
>> > then
>> > having the tools handle that. Would seem to accomplish at least the
>> > goals as
>> > I've seen them up to this point without worrying too much.
>>
>> The ToC section is a way we can encode the function index/summary into
>> bitcode, but won't help integrate with existing tools. The main issue
>> we are trying to solve is integrating transparently with existing
>> binutils tools in use in our build system and probably elsewhere.
>>
>
> Right. I'm not entirely sure what use we're going to see in the existing
> tools that we want to encompass here. There's some of it for convenience
> (i.e. nm etc for developers), but they can use a tool that understands
> bitcode and we can make the existing llvm tools suffice for these needs.

My understanding from our discussion is that the llvm versions of
those tools do not accept native object files, so that is not
something that will work in the short term.

The best alternative to native wrapped bitcode seems to be relying on
the plugin (and changing its behavior for ld -r). Which means that the
ability to use some of the tools from the native toolchain out of the
box, and build systems such as ours have to be taught to use it.

>
> I think the way of looking at this is that we can:
>
> a) go with wrapping things in native object formats, this means
>  - some tools continue to work at the cost of additional I/O and space at
> compile/link time
>  - we still have to update some tools to work at all
>
> b) we extend those tools/our own tools and have them be drop in replacements
> to the existing tools. They'll understand the bitcode format natively,
> they'll be smaller, and we'll be able to push the state of the art in
> tooling/analysis a bit more in the future without having to rework thin lto.
>
> It's basically a set of trade-offs and for llvm we've historically gone the
> b direction.
>
>> >
>> > At any rate, I think this aspect of the proposal needs a bit of
>> > discussion
>> > and some mapping out of the pros and cons here.
>>
>> Sure, we can continue to discuss and I will try to lay out the pros/cons.
>
>
> Excellent.
>
> -eric
>
>>
>>
>> Teresa
>>
>> >
>> > -eric
>> >
>> >>>
>> >>> I've talked to Teresa a bit offline and we're going to talk more later
>> >>> (and discuss on the list), but there are some discussions about how to
>> >>> make
>> >>> this work either with just bitcode/llvm tools and so not requiring
>> >>> integration on all platforms. The latter is what I consider as
>> >>> particularly
>> >>> friendly :)
>> >>>
>> >>> -eric
>> >>>
>> >>>>
>> >>>>
>> >>>>
>> >>>> > I also
>> >>>> > can't imagine how it's necessary for any of the lto aspects as
>> >>>> > currently
>> >>>> > written in the proposal.
>> >>>> >
>> >>>> > -eric
>> >>>> >
>> >>>> > On Thu, May 14, 2015 at 9:26 AM Xinliang David Li
>> >>>> > <xinliangli at gmail.com>
>> >>>> > wrote:
>> >>>> >>
>> >>>> >> The design objective is to make thinLTO mostly transparent to
>> >>>> >> binutil
>> >>>> >> tools to enable easy integration with any build system in the
>> >>>> >> wild.
>> >>>> >> 'Pass-through' mode with 'ld -r' instead of the partial LTO mode
>> >>>> >> is
>> >>>> >> another
>> >>>> >> reason.
>> >>>> >>
>> >>>> >> David
>> >>>> >>
>> >>>> >> On Thu, May 14, 2015 at 7:30 AM, Teresa Johnson
>> >>>> >> <tejohnson at google.com>
>> >>>> >> wrote:
>> >>>> >>>
>> >>>> >>> On Thu, May 14, 2015 at 7:22 AM, Eric Christopher
>> >>>> >>> <echristo at gmail.com>
>> >>>> >>> wrote:
>> >>>> >>> > So, what Alex is saying is that we have these tools as well and
>> >>>> >>> > they
>> >>>> >>> > understand bitcode just fine, as well as every object format -
>> >>>> >>> > not
>> >>>> >>> > just
>> >>>> >>> > ELF.
>> >>>> >>> > :)
>> >>>> >>>
>> >>>> >>> Right, there are also LLVM specific versions (llvm-ar, llvm-nm)
>> >>>> >>> that
>> >>>> >>> handle bitcode similarly to the way the standard tool + plugin
>> >>>> >>> does.
>> >>>> >>> But the goal we are trying to achieve is to allow the standard
>> >>>> >>> system
>> >>>> >>> versions of the tools to handle these files without requiring a
>> >>>> >>> plugin. I know the LLVM tool handles other object formats, but
>> >>>> >>> I'm
>> >>>> >>> not
>> >>>> >>> sure how that helps here? We're not planning to replace those
>> >>>> >>> tools,
>> >>>> >>> just allow the standard system versions to handle the
>> >>>> >>> intermediate
>> >>>> >>> objects produced by ThinLTO.
>> >>>> >>>
>> >>>> >>> Thanks,
>> >>>> >>> Teresa
>> >>>> >>>
>> >>>> >>> >
>> >>>> >>> > -eric
>> >>>> >>> >
>> >>>> >>> >
>> >>>> >>> > On Thu, May 14, 2015, 6:55 AM Teresa Johnson
>> >>>> >>> > <tejohnson at google.com>
>> >>>> >>> > wrote:
>> >>>> >>> >>
>> >>>> >>> >> On Wed, May 13, 2015 at 11:23 PM, Xinliang David Li
>> >>>> >>> >> <xinliangli at gmail.com> wrote:
>> >>>> >>> >> >
>> >>>> >>> >> >
>> >>>> >>> >> > On Wed, May 13, 2015 at 10:46 PM, Alex Rosenberg
>> >>>> >>> >> > <alexr at leftfield.org>
>> >>>> >>> >> > wrote:
>> >>>> >>> >> >>
>> >>>> >>> >> >> "ELF-wrapped bitcode" seems potentially controversial to
>> >>>> >>> >> >> me.
>> >>>> >>> >> >>
>> >>>> >>> >> >> What about ar, nm, and various ld implementations adds this
>> >>>> >>> >> >> requirement?
>> >>>> >>> >> >> What about the LLVM implementations of these tools is
>> >>>> >>> >> >> lacking?
>> >>>> >>> >> >
>> >>>> >>> >> >
>> >>>> >>> >> > Sorry I can not parse your questions properly. Can you make
>> >>>> >>> >> > it
>> >>>> >>> >> > clearer?
>> >>>> >>> >>
>> >>>> >>> >> Alex is asking what the issue is with ar, nm, ld -r and
>> >>>> >>> >> regular
>> >>>> >>> >> bitcode that makes using elf-wrapped bitcode easier.
>> >>>> >>> >>
>> >>>> >>> >> The issue is that generally you need to provide a plugin to
>> >>>> >>> >> these
>> >>>> >>> >> tools in order for them to understand and handle bitcode
>> >>>> >>> >> files.
>> >>>> >>> >> We'd
>> >>>> >>> >> like standard tools to work without requiring a plugin as much
>> >>>> >>> >> as
>> >>>> >>> >> possible. And in some cases we want them to be handled
>> >>>> >>> >> different
>> >>>> >>> >> than
>> >>>> >>> >> the way bitcode files are handled with the plugin.
>> >>>> >>> >>
>> >>>> >>> >> nm: Without a plugin, normal bitcode files are inscrutable.
>> >>>> >>> >> When
>> >>>> >>> >> provided the gold plugin it can emit the symbols.
>> >>>> >>> >>
>> >>>> >>> >> ar: Without a plugin, it will create an archive of bitcode
>> >>>> >>> >> files,
>> >>>> >>> >> but
>> >>>> >>> >> without an index, so it can't be handled by the linker even
>> >>>> >>> >> with
>> >>>> >>> >> a
>> >>>> >>> >> plugin on an -flto link. When ar is provided the gold plugin
>> >>>> >>> >> it
>> >>>> >>> >> does
>> >>>> >>> >> create an index, so the linker + gold plugin handle it
>> >>>> >>> >> appropriately
>> >>>> >>> >> on an -flto link.
>> >>>> >>> >>
>> >>>> >>> >> ld -r: Without a plugin, fails when provided bitcode inputs.
>> >>>> >>> >> When
>> >>>> >>> >> provided the gold plugin, it handles them but compiles them
>> >>>> >>> >> all
>> >>>> >>> >> the
>> >>>> >>> >> way through to ELF executable instructions via a partial LTO
>> >>>> >>> >> link.
>> >>>> >>> >> This is where we would like to differ in behavior (while also
>> >>>> >>> >> not
>> >>>> >>> >> requiring a plugin) with ELF-wrapped bitcode: we would like
>> >>>> >>> >> the
>> >>>> >>> >> ld -r
>> >>>> >>> >> output file to still contain ELF-wrapped bitcode, delaying the
>> >>>> >>> >> LTO
>> >>>> >>> >> until the full link step.
>> >>>> >>> >>
>> >>>> >>> >> Let me know if that helps address your concerns.
>> >>>> >>> >>
>> >>>> >>> >> Thanks,
>> >>>> >>> >> Teresa
>> >>>> >>> >>
>> >>>> >>> >> >
>> >>>> >>> >> > David
>> >>>> >>> >> >
>> >>>> >>> >> >>
>> >>>> >>> >> >>
>> >>>> >>> >> >> Alex
>> >>>> >>> >> >>
>> >>>> >>> >> >> > On May 13, 2015, at 7:44 PM, Teresa Johnson
>> >>>> >>> >> >> > <tejohnson at google.com>
>> >>>> >>> >> >> > wrote:
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > I've included below an RFC for implementing ThinLTO in
>> >>>> >>> >> >> > LLVM,
>> >>>> >>> >> >> > looking
>> >>>> >>> >> >> > forward to feedback and questions.
>> >>>> >>> >> >> > Thanks!
>> >>>> >>> >> >> > Teresa
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > RFC to discuss plans for implementing ThinLTO upstream.
>> >>>> >>> >> >> > Background
>> >>>> >>> >> >> > can
>> >>>> >>> >> >> > be found in slides from EuroLLVM 2015:
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > https://drive.google.com/open?id=0B036uwnWM6RWWER1ZEl5SUNENjQ&authuser=0)
>> >>>> >>> >> >> > As described in the talk, we have a prototype
>> >>>> >>> >> >> > implementation, and
>> >>>> >>> >> >> > would like to start staging patches upstream. This RFC
>> >>>> >>> >> >> > describes
>> >>>> >>> >> >> > a
>> >>>> >>> >> >> > breakdown of the major pieces. We would like to commit
>> >>>> >>> >> >> > upstream
>> >>>> >>> >> >> > gradually in several stages, with all functionality off
>> >>>> >>> >> >> > by
>> >>>> >>> >> >> > default.
>> >>>> >>> >> >> > The core ThinLTO importing support and tuning will
>> >>>> >>> >> >> > require
>> >>>> >>> >> >> > frequent
>> >>>> >>> >> >> > change and iteration during testing and tuning, and for
>> >>>> >>> >> >> > that
>> >>>> >>> >> >> > part
>> >>>> >>> >> >> > we
>> >>>> >>> >> >> > would like to commit rapidly (off by default). See the
>> >>>> >>> >> >> > proposed
>> >>>> >>> >> >> > staged
>> >>>> >>> >> >> > implementation described in the Implementation Plan
>> >>>> >>> >> >> > section.
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > ThinLTO Overview
>> >>>> >>> >> >> > ==============
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > See the talk slides linked above for more details. The
>> >>>> >>> >> >> > following
>> >>>> >>> >> >> > is a
>> >>>> >>> >> >> > high-level overview of the motivation.
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > Cross Module Optimization (CMO) is an effective means for
>> >>>> >>> >> >> > improving
>> >>>> >>> >> >> > runtime performance, by extending the scope of
>> >>>> >>> >> >> > optimizations
>> >>>> >>> >> >> > across
>> >>>> >>> >> >> > source module boundaries. Without CMO, the compiler is
>> >>>> >>> >> >> > limited to
>> >>>> >>> >> >> > optimizing within the scope of single source modules. Two
>> >>>> >>> >> >> > solutions
>> >>>> >>> >> >> > for enabling CMO are Link-Time Optimization (LTO), which
>> >>>> >>> >> >> > is
>> >>>> >>> >> >> > currently
>> >>>> >>> >> >> > supported in LLVM and GCC, and
>> >>>> >>> >> >> > Lightweight-Interprocedural
>> >>>> >>> >> >> > Optimization (LIPO). However, each of these solutions has
>> >>>> >>> >> >> > limitations
>> >>>> >>> >> >> > that prevent it from being enabled by default. ThinLTO is
>> >>>> >>> >> >> > a
>> >>>> >>> >> >> > new
>> >>>> >>> >> >> > approach that attempts to address these limitations, with
>> >>>> >>> >> >> > a
>> >>>> >>> >> >> > goal
>> >>>> >>> >> >> > of
>> >>>> >>> >> >> > being enabled more broadly. ThinLTO is designed with many
>> >>>> >>> >> >> > of
>> >>>> >>> >> >> > the
>> >>>> >>> >> >> > same
>> >>>> >>> >> >> > principals as LIPO, and therefore its advantages, without
>> >>>> >>> >> >> > any of
>> >>>> >>> >> >> > its
>> >>>> >>> >> >> > inherent weakness. Unlike in LIPO where the module group
>> >>>> >>> >> >> > decision
>> >>>> >>> >> >> > is
>> >>>> >>> >> >> > made at profile training runtime, ThinLTO makes the
>> >>>> >>> >> >> > decision
>> >>>> >>> >> >> > at
>> >>>> >>> >> >> > compile time, but in a lazy mode that facilitates large
>> >>>> >>> >> >> > scale
>> >>>> >>> >> >> > parallelism. The serial linker plugin phase is designed
>> >>>> >>> >> >> > to
>> >>>> >>> >> >> > be
>> >>>> >>> >> >> > razor
>> >>>> >>> >> >> > thin and blazingly fast. By default this step only does
>> >>>> >>> >> >> > minimal
>> >>>> >>> >> >> > preparation work to enable the parallel lazy importing
>> >>>> >>> >> >> > performed
>> >>>> >>> >> >> > later. ThinLTO aims to be scalable like a regular O2
>> >>>> >>> >> >> > build,
>> >>>> >>> >> >> > enabling
>> >>>> >>> >> >> > CMO on machines without large memory configurations,
>> >>>> >>> >> >> > while
>> >>>> >>> >> >> > also
>> >>>> >>> >> >> > integrating well with distributed build systems. Results
>> >>>> >>> >> >> > from
>> >>>> >>> >> >> > early
>> >>>> >>> >> >> > prototyping on SPEC cpu2006 C++ benchmarks are in line
>> >>>> >>> >> >> > with
>> >>>> >>> >> >> > expectations that ThinLTO can scale like O2 while
>> >>>> >>> >> >> > enabling
>> >>>> >>> >> >> > much
>> >>>> >>> >> >> > of
>> >>>> >>> >> >> > the
>> >>>> >>> >> >> > CMO performed during a full LTO build.
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > A ThinLTO build is divided into 3 phases, which are
>> >>>> >>> >> >> > referred
>> >>>> >>> >> >> > to
>> >>>> >>> >> >> > in
>> >>>> >>> >> >> > the
>> >>>> >>> >> >> > following implementation plan:
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > phase-1: IR and Function Summary Generation (-c compile)
>> >>>> >>> >> >> > phase-2: Thin Linker Plugin Layer (thin archive linker
>> >>>> >>> >> >> > step)
>> >>>> >>> >> >> > phase-3: Parallel Backend with Demand-Driven Importing
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > Implementation Plan
>> >>>> >>> >> >> > ================
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > This section gives a high-level breakdown of the ThinLTO
>> >>>> >>> >> >> > support
>> >>>> >>> >> >> > that
>> >>>> >>> >> >> > will be added, in roughly the order that the patches
>> >>>> >>> >> >> > would
>> >>>> >>> >> >> > be
>> >>>> >>> >> >> > staged.
>> >>>> >>> >> >> > The patches are divided into three stages. The first
>> >>>> >>> >> >> > stage
>> >>>> >>> >> >> > contains a
>> >>>> >>> >> >> > minimal amount of preparation work that is not
>> >>>> >>> >> >> > ThinLTO-specific.
>> >>>> >>> >> >> > The
>> >>>> >>> >> >> > second stage contains most of the infrastructure for
>> >>>> >>> >> >> > ThinLTO,
>> >>>> >>> >> >> > which
>> >>>> >>> >> >> > will be off by default. The third stage includes
>> >>>> >>> >> >> > enhancements/improvements/tunings that can be performed
>> >>>> >>> >> >> > after the
>> >>>> >>> >> >> > main
>> >>>> >>> >> >> > ThinLTO infrastructure is in.
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > The second and third implementation stages will initially
>> >>>> >>> >> >> > be
>> >>>> >>> >> >> > very
>> >>>> >>> >> >> > volatile, requiring a lot of iterations and tuning with
>> >>>> >>> >> >> > large
>> >>>> >>> >> >> > apps to
>> >>>> >>> >> >> > get stabilized. Therefore it will be important to do fast
>> >>>> >>> >> >> > commits
>> >>>> >>> >> >> > for
>> >>>> >>> >> >> > these implementation stages.
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > 1. Stage 1: Preparation
>> >>>> >>> >> >> > -------------------------------
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > The first planned sets of patches are enablers for
>> >>>> >>> >> >> > ThinLTO
>> >>>> >>> >> >> > work:
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > a. LTO directory structure:
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > Restructure the LTO directory to remove circular
>> >>>> >>> >> >> > dependence
>> >>>> >>> >> >> > when
>> >>>> >>> >> >> > ThinLTO pass added. Because ThinLTO is being implemented
>> >>>> >>> >> >> > as
>> >>>> >>> >> >> > a SCC
>> >>>> >>> >> >> > pass
>> >>>> >>> >> >> > within Transforms/IPO, and leverages the LTOModule class
>> >>>> >>> >> >> > for
>> >>>> >>> >> >> > linking
>> >>>> >>> >> >> > in functions from modules, IPO then requires the LTO
>> >>>> >>> >> >> > library.
>> >>>> >>> >> >> > This
>> >>>> >>> >> >> > creates a circular dependence between LTO and IPO. To
>> >>>> >>> >> >> > break
>> >>>> >>> >> >> > that,
>> >>>> >>> >> >> > we
>> >>>> >>> >> >> > need to split the lib/LTO directory/library into
>> >>>> >>> >> >> > lib/LTO/CodeGen
>> >>>> >>> >> >> > and
>> >>>> >>> >> >> > lib/LTO/Module, containing LTOCodeGenerator and
>> >>>> >>> >> >> > LTOModule,
>> >>>> >>> >> >> > respectively. Only LTOCodeGenerator has a dependence on
>> >>>> >>> >> >> > IPO,
>> >>>> >>> >> >> > removing
>> >>>> >>> >> >> > the circular dependence.
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > b. ELF wrapper generation support:
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > Implement ELF wrapped bitcode writer. In order to more
>> >>>> >>> >> >> > easily
>> >>>> >>> >> >> > interact
>> >>>> >>> >> >> > with tools such as $AR, $NM, and “$LD -r” we plan to emit
>> >>>> >>> >> >> > the
>> >>>> >>> >> >> > phase-1
>> >>>> >>> >> >> > bitcode wrapped in ELF via the .llvmbc section, along
>> >>>> >>> >> >> > with a
>> >>>> >>> >> >> > symbol
>> >>>> >>> >> >> > table. The goal is both to interact with these tools
>> >>>> >>> >> >> > without
>> >>>> >>> >> >> > requiring
>> >>>> >>> >> >> > a plugin, and also to avoid doing partial LTO/ThinLTO
>> >>>> >>> >> >> > across
>> >>>> >>> >> >> > files
>> >>>> >>> >> >> > linked with “$LD -r” (i.e. the resulting object file
>> >>>> >>> >> >> > should
>> >>>> >>> >> >> > still
>> >>>> >>> >> >> > contain ELF-wrapped bitcode to enable ThinLTO at the full
>> >>>> >>> >> >> > link
>> >>>> >>> >> >> > step).
>> >>>> >>> >> >> > I will send a separate design document for these changes,
>> >>>> >>> >> >> > but the
>> >>>> >>> >> >> > following is a high-level overview.
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > Support was added to LLVM for reading ELF-wrapped bitcode
>> >>>> >>> >> >> > (http://reviews.llvm.org/rL218078), but there does not
>> >>>> >>> >> >> > yet
>> >>>> >>> >> >> > exist
>> >>>> >>> >> >> > support in LLVM/Clang for emitting bitcode wrapped in
>> >>>> >>> >> >> > ELF. I
>> >>>> >>> >> >> > plan
>> >>>> >>> >> >> > to
>> >>>> >>> >> >> > add support for optionally generating bitcode in an ELF
>> >>>> >>> >> >> > file
>> >>>> >>> >> >> > containing a single .llvmbc section holding the bitcode.
>> >>>> >>> >> >> > Specifically,
>> >>>> >>> >> >> > the patch would add new options “emit-llvm-bc-elf”
>> >>>> >>> >> >> > (object
>> >>>> >>> >> >> > file)
>> >>>> >>> >> >> > and
>> >>>> >>> >> >> > corresponding “emit-llvm-elf” (textual assembly code
>> >>>> >>> >> >> > equivalent).
>> >>>> >>> >> >> > Eventually these would be automatically triggered under
>> >>>> >>> >> >> > “-fthinlto
>> >>>> >>> >> >> > -c”
>> >>>> >>> >> >> > and “-fthinlto -S”, respectively.
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > Additionally, a symbol table will be generated in the ELF
>> >>>> >>> >> >> > file,
>> >>>> >>> >> >> > holding the function symbols within the bitcode. This
>> >>>> >>> >> >> > facilitates
>> >>>> >>> >> >> > handling archives of the ELF-wrapped bitcode created with
>> >>>> >>> >> >> > $AR,
>> >>>> >>> >> >> > since
>> >>>> >>> >> >> > the archive will have a symbol table as well. The archive
>> >>>> >>> >> >> > symbol
>> >>>> >>> >> >> > table
>> >>>> >>> >> >> > enables gold to extract and pass to the plugin the
>> >>>> >>> >> >> > constituent
>> >>>> >>> >> >> > ELF-wrapped bitcode files. To support the concatenated
>> >>>> >>> >> >> > llvmbc
>> >>>> >>> >> >> > section
>> >>>> >>> >> >> > generated by “$LD -r”, some handling needs to be added to
>> >>>> >>> >> >> > gold
>> >>>> >>> >> >> > and to
>> >>>> >>> >> >> > the backend driver to process each original module’s
>> >>>> >>> >> >> > bitcode.
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > The function index/summary will later be added as a
>> >>>> >>> >> >> > special
>> >>>> >>> >> >> > ELF
>> >>>> >>> >> >> > section alongside the .llvmbc sections.
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > 2. Stage 2: ThinLTO Infrastructure
>> >>>> >>> >> >> > ----------------------------------------------
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > The next set of patches adds the base implementation of
>> >>>> >>> >> >> > the
>> >>>> >>> >> >> > ThinLTO
>> >>>> >>> >> >> > infrastructure, specifically those required to make
>> >>>> >>> >> >> > ThinLTO
>> >>>> >>> >> >> > functional
>> >>>> >>> >> >> > and generate correct but not necessarily high-performing
>> >>>> >>> >> >> > binaries. It
>> >>>> >>> >> >> > also does not include support to make debug support under
>> >>>> >>> >> >> > -g
>> >>>> >>> >> >> > efficient
>> >>>> >>> >> >> > with ThinLTO.
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > a. Clang/LLVM/gold linker options:
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > An early set of clang/llvm patches is needed to provide
>> >>>> >>> >> >> > options
>> >>>> >>> >> >> > to
>> >>>> >>> >> >> > enable ThinLTO (off by default), so that the rest of the
>> >>>> >>> >> >> > implementation can be disabled by default as it is added.
>> >>>> >>> >> >> > Specifically, clang options -fthinlto (used instead of
>> >>>> >>> >> >> > -flto)
>> >>>> >>> >> >> > will
>> >>>> >>> >> >> > cause clang to invoke the phase-1 emission of LLVM
>> >>>> >>> >> >> > bitcode
>> >>>> >>> >> >> > and
>> >>>> >>> >> >> > function summary/index on a compile step, and pass the
>> >>>> >>> >> >> > appropriate
>> >>>> >>> >> >> > option to the gold plugin on a link step. The -thinlto
>> >>>> >>> >> >> > option
>> >>>> >>> >> >> > will be
>> >>>> >>> >> >> > added to the gold plugin and llvm-lto tool to launch the
>> >>>> >>> >> >> > phase-2
>> >>>> >>> >> >> > thin
>> >>>> >>> >> >> > archive step. The -thinlto option will also be added to
>> >>>> >>> >> >> > the
>> >>>> >>> >> >> > ‘opt’
>> >>>> >>> >> >> > tool
>> >>>> >>> >> >> > to invoke it as a phase-3 parallel backend instance.
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > b. Thin-archive linking support in Gold plugin and
>> >>>> >>> >> >> > llvm-lto:
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > Under the new plugin option (see above), the plugin needs
>> >>>> >>> >> >> > to
>> >>>> >>> >> >> > perform
>> >>>> >>> >> >> > the phase-2 (thin archive) link which simply emits a
>> >>>> >>> >> >> > combined
>> >>>> >>> >> >> > function
>> >>>> >>> >> >> > map from the linked modules, without actually performing
>> >>>> >>> >> >> > the
>> >>>> >>> >> >> > normal
>> >>>> >>> >> >> > link. Corresponding support should be added to the
>> >>>> >>> >> >> > standalone
>> >>>> >>> >> >> > llvm-lto
>> >>>> >>> >> >> > tool to enable testing/debugging without involving the
>> >>>> >>> >> >> > linker and
>> >>>> >>> >> >> > plugin.
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > c. ThinLTO backend support:
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > Support for invoking a phase-3 backend invocation
>> >>>> >>> >> >> > (including
>> >>>> >>> >> >> > importing) on a module should be added to the ‘opt’ tool
>> >>>> >>> >> >> > under
>> >>>> >>> >> >> > the
>> >>>> >>> >> >> > new
>> >>>> >>> >> >> > option. The main change under the option is to
>> >>>> >>> >> >> > instantiate a
>> >>>> >>> >> >> > Linker
>> >>>> >>> >> >> > object used to manage the process of linking imported
>> >>>> >>> >> >> > functions
>> >>>> >>> >> >> > into
>> >>>> >>> >> >> > the module, efficient read of the combined function map,
>> >>>> >>> >> >> > and
>> >>>> >>> >> >> > enable
>> >>>> >>> >> >> > the ThinLTO import pass.
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > d. Function index/summary support:
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > This includes infrastructure for writing and reading the
>> >>>> >>> >> >> > function
>> >>>> >>> >> >> > index/summary section. As noted earlier this will be
>> >>>> >>> >> >> > encoded
>> >>>> >>> >> >> > in a
>> >>>> >>> >> >> > special ELF section within the module, alongside the
>> >>>> >>> >> >> > .llvmbc
>> >>>> >>> >> >> > section
>> >>>> >>> >> >> > containing the bitcode. The thin archive generated by
>> >>>> >>> >> >> > phase-2 of
>> >>>> >>> >> >> > ThinLTO simply contains all of the function index/summary
>> >>>> >>> >> >> > sections
>> >>>> >>> >> >> > across the linked modules, organized for efficient
>> >>>> >>> >> >> > function
>> >>>> >>> >> >> > lookup.
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > Each function available for importing from the module
>> >>>> >>> >> >> > contains an
>> >>>> >>> >> >> > entry in the module’s function index/summary section and
>> >>>> >>> >> >> > in
>> >>>> >>> >> >> > the
>> >>>> >>> >> >> > resulting combined function map. Each function entry
>> >>>> >>> >> >> > contains
>> >>>> >>> >> >> > that
>> >>>> >>> >> >> > function’s offset within the bitcode file, used to
>> >>>> >>> >> >> > efficiently
>> >>>> >>> >> >> > locate
>> >>>> >>> >> >> > and quickly import just that function. The entry also
>> >>>> >>> >> >> > contains
>> >>>> >>> >> >> > summary
>> >>>> >>> >> >> > information (e.g. basic information determined during
>> >>>> >>> >> >> > parsing
>> >>>> >>> >> >> > such as
>> >>>> >>> >> >> > the number of instructions in the function), that will be
>> >>>> >>> >> >> > used to
>> >>>> >>> >> >> > help
>> >>>> >>> >> >> > guide later import decisions. Because the contents of
>> >>>> >>> >> >> > this
>> >>>> >>> >> >> > section
>> >>>> >>> >> >> > will change frequently during ThinLTO tuning, it should
>> >>>> >>> >> >> > also
>> >>>> >>> >> >> > be
>> >>>> >>> >> >> > marked
>> >>>> >>> >> >> > with a version id for backwards compatibility or version
>> >>>> >>> >> >> > checking.
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > e. ThinLTO importing support:
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > Support for the mechanics of importing functions from
>> >>>> >>> >> >> > other
>> >>>> >>> >> >> > modules,
>> >>>> >>> >> >> > which can go in gradually as a set of patches since it
>> >>>> >>> >> >> > will
>> >>>> >>> >> >> > be
>> >>>> >>> >> >> > off by
>> >>>> >>> >> >> > default. Separate patches can include:
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > - BitcodeReader changes to use function index to
>> >>>> >>> >> >> > import/deserialize
>> >>>> >>> >> >> > single function of interest (small changes, leverages
>> >>>> >>> >> >> > existing
>> >>>> >>> >> >> > lazy
>> >>>> >>> >> >> > streamer support).
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > - Minor LTOModule changes to pass the ThinLTO function to
>> >>>> >>> >> >> > import
>> >>>> >>> >> >> > and
>> >>>> >>> >> >> > its index into bitcode reader.
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > - Marking of imported functions (for use in
>> >>>> >>> >> >> > ThinLTO-specific
>> >>>> >>> >> >> > symbol
>> >>>> >>> >> >> > linking and global DCE, for example). This can be
>> >>>> >>> >> >> > in-memory
>> >>>> >>> >> >> > initially,
>> >>>> >>> >> >> > but IR support may be required in order to support
>> >>>> >>> >> >> > streaming
>> >>>> >>> >> >> > bitcode
>> >>>> >>> >> >> > out and back in again after importing.
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > - ModuleLinker changes to do ThinLTO-specific symbol
>> >>>> >>> >> >> > linking
>> >>>> >>> >> >> > and
>> >>>> >>> >> >> > static promotion when necessary. The linkage type of
>> >>>> >>> >> >> > imported
>> >>>> >>> >> >> > functions changes to AvailableExternallyLinkage, for
>> >>>> >>> >> >> > example.
>> >>>> >>> >> >> > Statics
>> >>>> >>> >> >> > must be promoted in certain cases, and renamed in
>> >>>> >>> >> >> > consistent
>> >>>> >>> >> >> > ways.
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > - GlobalDCE changes to support removing imported
>> >>>> >>> >> >> > functions
>> >>>> >>> >> >> > that
>> >>>> >>> >> >> > were
>> >>>> >>> >> >> > not inlined (very small changes to existing pass logic).
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > f. ThinLTO Import Driver SCC pass:
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > Adds Transforms/IPO/ThinLTO.cpp with framework for doing
>> >>>> >>> >> >> > ThinLTO
>> >>>> >>> >> >> > via
>> >>>> >>> >> >> > an SCC pass, enabled only under -fthinlto options. The
>> >>>> >>> >> >> > pass
>> >>>> >>> >> >> > includes
>> >>>> >>> >> >> > utilizing the thin archive (global function
>> >>>> >>> >> >> > index/summary),
>> >>>> >>> >> >> > import
>> >>>> >>> >> >> > decision heuristics, invocation of LTOModule/ModuleLinker
>> >>>> >>> >> >> > routines
>> >>>> >>> >> >> > that perform the import, and any necessary callgraph
>> >>>> >>> >> >> > updates
>> >>>> >>> >> >> > and
>> >>>> >>> >> >> > verification.
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > g. Backend Driver:
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > For a single node build, the gold plugin can simply write
>> >>>> >>> >> >> > a
>> >>>> >>> >> >> > makefile
>> >>>> >>> >> >> > and fork the parallel backend instances directly via
>> >>>> >>> >> >> > parallel
>> >>>> >>> >> >> > make.
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > 3. Stage 3: ThinLTO Tuning and Enhancements
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > ----------------------------------------------------------------
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > This refers to the patches that are not required for
>> >>>> >>> >> >> > ThinLTO
>> >>>> >>> >> >> > to
>> >>>> >>> >> >> > work,
>> >>>> >>> >> >> > but rather to improve compile time, memory, run-time
>> >>>> >>> >> >> > performance
>> >>>> >>> >> >> > and
>> >>>> >>> >> >> > usability.
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > a. Lazy Debug Metadata Linking:
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > The prototype implementation included lazy importing of
>> >>>> >>> >> >> > module-level
>> >>>> >>> >> >> > metadata during the ThinLTO pass finalization (i.e. after
>> >>>> >>> >> >> > all
>> >>>> >>> >> >> > function
>> >>>> >>> >> >> > importing is complete). This actually applies to all
>> >>>> >>> >> >> > module-level
>> >>>> >>> >> >> > metadata, not just debug, although it is the largest.
>> >>>> >>> >> >> > This
>> >>>> >>> >> >> > can be
>> >>>> >>> >> >> > added as a separate set of patches. Changes to
>> >>>> >>> >> >> > BitcodeReader,
>> >>>> >>> >> >> > ValueMapper, ModuleLinker
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > b. Import Tuning:
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > Tuning the import strategy will be an iterative process
>> >>>> >>> >> >> > that
>> >>>> >>> >> >> > will
>> >>>> >>> >> >> > continue to be refined over time. It involves several
>> >>>> >>> >> >> > different
>> >>>> >>> >> >> > types
>> >>>> >>> >> >> > of changes: adding support for recording additional
>> >>>> >>> >> >> > metrics
>> >>>> >>> >> >> > in
>> >>>> >>> >> >> > the
>> >>>> >>> >> >> > function summary, such as profile data and optional
>> >>>> >>> >> >> > heavier-weight
>> >>>> >>> >> >> > IPA
>> >>>> >>> >> >> > analyses, and tuning the import heuristics based on the
>> >>>> >>> >> >> > summary
>> >>>> >>> >> >> > and
>> >>>> >>> >> >> > callsite context.
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > c. Combined Function Map Pruning:
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > The combined function map can be pruned of functions that
>> >>>> >>> >> >> > are
>> >>>> >>> >> >> > unlikely
>> >>>> >>> >> >> > to benefit from being imported. For example, during the
>> >>>> >>> >> >> > phase-2
>> >>>> >>> >> >> > thin
>> >>>> >>> >> >> > archive plug step we can safely omit large and (with
>> >>>> >>> >> >> > profile
>> >>>> >>> >> >> > data)
>> >>>> >>> >> >> > cold functions, which are unlikely to benefit from being
>> >>>> >>> >> >> > inlined.
>> >>>> >>> >> >> > Additionally, all but one copy of comdat functions can be
>> >>>> >>> >> >> > suppressed.
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > d. Distributed Build System Integration:
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > For a distributed build system, the gold plugin should
>> >>>> >>> >> >> > write
>> >>>> >>> >> >> > the
>> >>>> >>> >> >> > parallel backend invocations into a makefile, including
>> >>>> >>> >> >> > the
>> >>>> >>> >> >> > mapping
>> >>>> >>> >> >> > from the IR file to the real object file path, and exit.
>> >>>> >>> >> >> > Additional
>> >>>> >>> >> >> > work needs to be done in the distributed build system
>> >>>> >>> >> >> > itself
>> >>>> >>> >> >> > to
>> >>>> >>> >> >> > distribute and dispatch the parallel backend jobs to the
>> >>>> >>> >> >> > build
>> >>>> >>> >> >> > cluster.
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > e. Dependence Tracking and Incremental Compiles:
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > In order to support build systems that stage from local
>> >>>> >>> >> >> > disks or
>> >>>> >>> >> >> > network storage, the plugin will optionally support
>> >>>> >>> >> >> > computation
>> >>>> >>> >> >> > of
>> >>>> >>> >> >> > dependent sets of IR files that each module may import
>> >>>> >>> >> >> > from.
>> >>>> >>> >> >> > This
>> >>>> >>> >> >> > can
>> >>>> >>> >> >> > be computed from profile data, if it exists, or from the
>> >>>> >>> >> >> > symbol
>> >>>> >>> >> >> > table
>> >>>> >>> >> >> > and heuristics if not. These dependence sets also enable
>> >>>> >>> >> >> > support
>> >>>> >>> >> >> > for
>> >>>> >>> >> >> > incremental backend compiles.
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > --
>> >>>> >>> >> >> > Teresa Johnson | Software Engineer | tejohnson at google.com
>> >>>> >>> >> >> > |
>> >>>> >>> >> >> > 408-460-2413
>> >>>> >>> >> >> >
>> >>>> >>> >> >> > _______________________________________________
>> >>>> >>> >> >> > LLVM Developers mailing list
>> >>>> >>> >> >> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> >>>> >>> >> >> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> >>>> >>> >> >>
>> >>>> >>> >> >> _______________________________________________
>> >>>> >>> >> >> LLVM Developers mailing list
>> >>>> >>> >> >> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> >>>> >>> >> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> >>>> >>> >> >
>> >>>> >>> >> >
>> >>>> >>> >>
>> >>>> >>> >>
>> >>>> >>> >>
>> >>>> >>> >> --
>> >>>> >>> >> Teresa Johnson | Software Engineer | tejohnson at google.com |
>> >>>> >>> >> 408-460-2413
>> >>>> >>> >>
>> >>>> >>> >> _______________________________________________
>> >>>> >>> >> LLVM Developers mailing list
>> >>>> >>> >> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> >>>> >>> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> >>>> >>>
>> >>>> >>>
>> >>>> >>>
>> >>>> >>> --
>> >>>> >>> Teresa Johnson | Software Engineer | tejohnson at google.com |
>> >>>> >>> 408-460-2413
>> >>>> >>
>> >>>> >>
>> >>>> >
>> >>>> > _______________________________________________
>> >>>> > LLVM Developers mailing list
>> >>>> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> >>>> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> >>>> >
>> >>>
>> >>>
>> >>> _______________________________________________
>> >>> LLVM Developers mailing list
>> >>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> >>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> >>>
>> >
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> >
>>
>>
>>
>> --
>> Teresa Johnson | Software Engineer | tejohnson at google.com | 408-460-2413



-- 
Teresa Johnson | Software Engineer | tejohnson at google.com | 408-460-2413