[LLVMdev] RFC: ThinLTO Impementation Plan

Wed May 13 23:23:53 PDT 2015

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?

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
> >
> > _______________________________________________
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> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
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>
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