[LLVMdev] RFC: ThinLTO Impementation Plan

Eric Christopher echristo at gmail.com
Thu May 14 11:14:52 PDT 2015 

I'm not sure this is a particularly great assumption to make. 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 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
>>
>
>
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