[LLVMdev] RFC: ThinLTO Impementation Plan

[Xinliang David Li]

On Thu, May 14, 2015 at 2:46 PM, Eric Christopher <echristo at gmail.com>
wrote:

>
>
> On Thu, May 14, 2015 at 2:31 PM Teresa Johnson <tejohnson at google.com>
> wrote:
>
>> 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.
>>
>>
> I think this is what should happen anyhow. ld -r that doesn't do a partial
> link is misleading.
>
> > 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.
>>
>>
> We have tools that do understand native object files and it's pretty easy
> to use the libraries that they're built upon.
>
>
>> 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 don't think that natively wrapped bitcode gets you as much as you think
> it does anyhow, unless you're duplicating a lot of information (ar, as
> discussed earlier, aside). I'm not too worried about the build system as
> far as a wrapping mechanism
>

Do not under estimate the importance of build system integration. Tools
used in the build can include ar, nm, ranlib, objcopy, strip, etc. The
latest binutils  support plugin for ar, nm and ranlib, but not others.
objcopy can actually change visibility of symbols. It is conceivably easy
to support this with native object wrapper (i.e. propagate the visibility
change at IR reading time), but it is unclear wether existing plugin
interfaces are enough for it.


> and I think more traditional LTO schemes with LLVM have just used
> bitcode/IR output as an input to the LTO link step. I think what we're
> talking about here is the best way to encode the data that thin lto
> needs/wants in order to handle summary information etc right?
>

Not entirely. We want both functionality and usability. Easy integration
with build is considered as the usability.  Asking users to use wrapper
tools in order to pass plugin path is not something I consider as being
highly usable -- but I can be convinced the other way :)

thanks,

David



> -eric
>
>
>> >
>> > 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
>>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
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