[LLVMdev] RFC: ThinLTO Impementation Plan

Thu May 14 07:30:48 PDT 2015

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