[LLVMdev] RFC: ThinLTO Impementation Plan

[Eric Christopher]

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.

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
> >
>
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