[LLVMdev] RFC: ThinLTO Impementation Plan

Sean Silva
Mon, 18 May 2015 21:47:26 -0000

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.
>

What is your definition of "the wild" when it comes to build systems?
It
sounds like your current definition is "autotools/make builds on typical
GNU/Linux distros".

Merely having the linker do a "parallel make invocation" as your
proposal
state already presents a huge barrier to integration with almost any build
system, far, far beyond any issues of wrapping bitcode. As your proposal
admits, you are having to make this part custom for your own internal build
system. The same is going to be true of every build system.

If you are serious about "enable easy integration with any build system in
the wild", I would recommend at the very least considering the use cases of
a couple different common[1] -G settings of LLVM's own CMake build
(-GNinja, -G'Visual Studio', -G'Xcode'). From my experience with
"the wild"
of build systems, I would suggest simply dropping any pretense of having
this work with any build system with any degree of ease.

[1] http://llvm.org/docs/GettingStarted.html - "common generators are"

-- Sean Silva


>
> 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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