[LLVMdev] RFC: ThinLTO Impementation Plan

Xinliang David Li
Thu, 14 May 2015 16:26:53 -0000

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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