llvm dev - May 2015 - [LLVMdev] RFC: ThinLTO Impementation Plan

On Thu, May 14, 2015 at 2:46 PM, Eric Christopher <echristo at gmail.com>
wrote:
>
>
> On Thu, May 14, 2015 at 2:31 PM Teresa Johnson <tejohnson at
google.com>
> wrote:
>
>> On Thu, May 14, 2015 at 2:09 PM, Eric Christopher <echristo at
gmail.com>
>> wrote:
>> >
>> >
>> > On Thu, May 14, 2015 at 1:35 PM Teresa Johnson <tejohnson at
google.com>
>> wrote:
>> >>
>> >> On Thu, May 14, 2015 at 1:18 PM, Eric Christopher <echristo
at gmail.com>
>> >> wrote:
>> >> >
>> >> >
>> >> > On Thu, May 14, 2015 at 1:11 PM David Blaikie
<dblaikie at gmail.com>
>> >> > wrote:
>> >> >>
>> >> >> On Thu, May 14, 2015 at 12:53 PM, Eric Christopher
<
>> echristo at gmail.com>
>> >> >> wrote:
>> >> >>>
>> >> >>>
>> >> >>>
>> >> >>> On Thu, May 14, 2015 at 11:34 AM Daniel Berlin
<
>> dberlin at dberlin.org>
>> >> >>> wrote:
>> >> >>>>
>> >> >>>> On Thu, May 14, 2015 at 11:14 AM, Eric
Christopher
>> >> >>>> <echristo at gmail.com>
>> >> >>>> wrote:
>> >> >>>> > I'm not sure this is a particularly
great assumption to make.
>> >> >>>>
>> >> >>>> Which part?
>> >> >>>
>> >> >>>
>> >> >>> The binutils part :)
>> >> >>>
>> >> >>>>
>> >> >>>>
>> >> >>>> >  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 think you may have misunderstood
>> >> >>>> His point was exactly that they want to be
transparent to *all of*
>> >> >>>> these
>> >> >>>> tools.
>> >> >>>> You are saying "we should be friendly to
everyone". He is saying
>> the
>> >> >>>> same thing.
>> >> >>>> We should be friendly to everyone. The
friendly way to do this is
>> to
>> >> >>>> not require all of these tools build plugins
to handle bitcode.
>> >> >>>>
>> >> >>>> Hence, elf-wrapped bitcode.
>> >> >>>
>> >> >>>
>> >> >>> Oh, I understood. I just don't know that I
agree. To do anything
>> with
>> >> >>> the
>> >> >>> tools will require some knowledge of bitcode
anyhow or need the
>> >> >>> plugin. I'm
>> >> >>> saying that as a baseline start we should look at
how to do this
>> using
>> >> >>> the
>> >> >>> tools we've got rather than wrapping things
for no real gain.
>> >> >>
>> >> >>
>> >> >> That doesn't seem strictly true - the ar
situation (which I'm lead
>> to
>> >> >> believe is in use in our build system & others,
one would assume).
>> With
>> >> >> the
>> >> >> symbol table included as proposed, ar can be used
without any
>> knowledge
>> >> >> of
>> >> >> the bitcode or need for a plugin.
>> >> >>
>> >> >
>> >> > For some bits, sure. Optimizing for ar seems a bit silly,
why not 'ld
>> >> > -r'?
>> >>
>> >> But as mentioned, ld -r can work on native object wrapped
bitcode
>> >> without a plugin as well.
>> >>
>> >
>> > How? It's not like any partial linking is going to go on
inside the
>> bitcode
>> > if the linker doesn't understand bitcode.
>>
>> It allows us to delay the actual linking until the full link step,
>> thereby enabling ThinLTO on those modules.
>>
>> As we discussed offline, the current ld -r behavior with the plugin is
>> to compile all the way down to machine code. The alternative if we use
>> straight bitcode is to tell the plugin to stop early after combining
>> the bitcode and emit bitcode back out, with the thinlto function info
>> also combined.
>>
>>
> I think this is what should happen anyhow. ld -r that doesn't do a
partial
> link is misleading.
>
> > Right. I'm not entirely sure what use we're going to see in
the existing
>> > tools that we want to encompass here. There's some of it for
convenience
>> > (i.e. nm etc for developers), but they can use a tool that
understands
>> > bitcode and we can make the existing llvm tools suffice for these
needs.
>>
>> My understanding from our discussion is that the llvm versions of
>> those tools do not accept native object files, so that is not
>> something that will work in the short term.
>>
>>
> We have tools that do understand native object files and it's pretty
easy
> to use the libraries that they're built upon.
>
>
>> The best alternative to native wrapped bitcode seems to be relying on
>> the plugin (and changing its behavior for ld -r). Which means that the
>> ability to use some of the tools from the native toolchain out of the
>> box, and build systems such as ours have to be taught to use it.
>>
>>
> I don't think that natively wrapped bitcode gets you as much as you
think
> it does anyhow, unless you're duplicating a lot of information (ar, as
> discussed earlier, aside). I'm not too worried about the build system
as
> far as a wrapping mechanism
>
Do not under estimate the importance of build system integration. Tools
used in the build can include ar, nm, ranlib, objcopy, strip, etc. The
latest binutils  support plugin for ar, nm and ranlib, but not others.
objcopy can actually change visibility of symbols. It is conceivably easy
to support this with native object wrapper (i.e. propagate the visibility
change at IR reading time), but it is unclear wether existing plugin
interfaces are enough for it.

> and I think more traditional LTO schemes with LLVM have just used
> bitcode/IR output as an input to the LTO link step. I think what we're
> talking about here is the best way to encode the data that thin lto
> needs/wants in order to handle summary information etc right?
>
Not entirely. We want both functionality and usability. Easy integration
with build is considered as the usability.  Asking users to use wrapper
tools in order to pass plugin path is not something I consider as being
highly usable -- but I can be convinced the other way :)

thanks,

David


> -eric
>
>
>> >
>> > I think the way of looking at this is that we can:
>> >
>> > a) go with wrapping things in native object formats, this means
>> >  - some tools continue to work at the cost of additional I/O and
space
>> at
>> > compile/link time
>> >  - we still have to update some tools to work at all
>> >
>> > b) we extend those tools/our own tools and have them be drop in
>> replacements
>> > to the existing tools. They'll understand the bitcode format
natively,
>> > they'll be smaller, and we'll be able to push the state of
the art in
>> > tooling/analysis a bit more in the future without having to rework
thin
>> lto.
>> >
>> > It's basically a set of trade-offs and for llvm we've
historically gone
>> the
>> > b direction.
>> >
>> >> >
>> >> > At any rate, I think this aspect of the proposal needs a
bit of
>> >> > discussion
>> >> > and some mapping out of the pros and cons here.
>> >>
>> >> Sure, we can continue to discuss and I will try to lay out the
>> pros/cons.
>> >
>> >
>> > Excellent.
>> >
>> > -eric
>> >
>> >>
>> >>
>> >> Teresa
>> >>
>> >> >
>> >> > -eric
>> >> >
>> >> >>>
>> >> >>> I've talked to Teresa a bit offline and
we're going to talk more
>> later
>> >> >>> (and discuss on the list), but there are some
discussions about
>> how to
>> >> >>> make
>> >> >>> this work either with just bitcode/llvm tools and
so not requiring
>> >> >>> integration on all platforms. The latter is what
I consider as
>> >> >>> particularly
>> >> >>> friendly :)
>> >> >>>
>> >> >>> -eric
>> >> >>>
>> >> >>>>
>> >> >>>>
>> >> >>>>
>> >> >>>> > 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
>> >> >>>> >>
>> >> >>>> >>
>> >> >>>> >
>> >> >>>> >
_______________________________________________
>> >> >>>> > 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
>> >> >>>
>> >> >
>> >> > _______________________________________________
>> >> > 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
>>
>>
>>
>> --
>> 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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(resend a bounced message).
>
>> I don't think that natively wrapped bitcode gets you as much as you
think
>> it does anyhow, unless you're duplicating a lot of information (ar,
as
>> discussed earlier, aside). I'm not too worried about the build
system as
>> far as a wrapping mechanism
>>
>
Do not under estimate the importance of build system integration. Tools
used in the build can include ar, nm, ranlib, objcopy, strip, etc. The
latest binutils support plugin for ar, nm and ranlib, but not others.
objcopy can actually change visibility of symbols. It is conceivably easy
to support this with native object wrapper (i.e. propagate the visibility
change at IR reading time), but it is unclear wether existing plugin
interfaces are enough for it.
>
>
>> and I think more traditional LTO schemes with LLVM have just used
>> bitcode/IR output as an input to the LTO link step. I think what
we're
>> talking about here is the best way to encode the data that thin lto
>> needs/wants in order to handle summary information etc right?
>>
>
Not entirely. We want both functionality and usability. Easy integration
with build is considered as the usability.  Asking users to use wrapper
tools in order to pass plugin path is not something I consider as being
highly usable -- but I can be convinced the other way :)

David
>
>
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>
>
>> I don't think that natively wrapped bitcode gets you as much as you
think
>> it does anyhow, unless you're duplicating a lot of information (ar,
as
>> discussed earlier, aside). I'm not too worried about the build
system as
>> far as a wrapping mechanism
>>
>
> Do not under estimate the importance of build system integration. Tools
>
I'm pretty sure I'm never going to underestimate build system
integration.
:)

> used in the build can include ar, nm, ranlib, objcopy, strip, etc. The
> latest binutils  support plugin for ar, nm and ranlib, but not others.
> objcopy can actually change visibility of symbols. It is conceivably easy
> to support this with native object wrapper (i.e. propagate the visibility
> change at IR reading time), but it is unclear wether existing plugin
> interfaces are enough for it.
>
>
Others have replied to this in more detail, but I'd like to understand more
the compilation tools pipeline you're talking about here where you need
this particular functionality. Also, the engineering tradeoffs in looking
at the plugin interfaces rather than this.

-eric

and I think more traditional LTO schemes with LLVM have just
used
>> bitcode/IR output as an input to the LTO link step. I think what
we're
>> talking about here is the best way to encode the data that thin lto
>> needs/wants in order to handle summary information etc right?
>>
>
> Not entirely. We want both functionality and usability. Easy integration
> with build is considered as the usability.  Asking users to use wrapper
> tools in order to pass plugin path is not something I consider as being
> highly usable -- but I can be convinced the other way :)
>
> thanks,
>
> David
>
>
>
>> -eric
>>
>>
>>> >
>>> > I think the way of looking at this is that we can:
>>> >
>>> > a) go with wrapping things in native object formats, this
means
>>> >  - some tools continue to work at the cost of additional I/O
and space
>>> at
>>> > compile/link time
>>> >  - we still have to update some tools to work at all
>>> >
>>> > b) we extend those tools/our own tools and have them be drop
in
>>> replacements
>>> > to the existing tools. They'll understand the bitcode
format natively,
>>> > they'll be smaller, and we'll be able to push the
state of the art in
>>> > tooling/analysis a bit more in the future without having to
rework
>>> thin lto.
>>> >
>>> > It's basically a set of trade-offs and for llvm we've
historically
>>> gone the
>>> > b direction.
>>> >
>>> >> >
>>> >> > At any rate, I think this aspect of the proposal
needs a bit of
>>> >> > discussion
>>> >> > and some mapping out of the pros and cons here.
>>> >>
>>> >> Sure, we can continue to discuss and I will try to lay out
the
>>> pros/cons.
>>> >
>>> >
>>> > Excellent.
>>> >
>>> > -eric
>>> >
>>> >>
>>> >>
>>> >> Teresa
>>> >>
>>> >> >
>>> >> > -eric
>>> >> >
>>> >> >>>
>>> >> >>> I've talked to Teresa a bit offline and
we're going to talk more
>>> later
>>> >> >>> (and discuss on the list), but there are some
discussions about
>>> how to
>>> >> >>> make
>>> >> >>> this work either with just bitcode/llvm tools
and so not requiring
>>> >> >>> integration on all platforms. The latter is
what I consider as
>>> >> >>> particularly
>>> >> >>> friendly :)
>>> >> >>>
>>> >> >>> -eric
>>> >> >>>
>>> >> >>>>
>>> >> >>>>
>>> >> >>>>
>>> >> >>>> > 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
>>> >> >>>> >>
>>> >> >>>> >>
>>> >> >>>> >
>>> >> >>>> >
_______________________________________________
>>> >> >>>> > 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
>>> >> >>>
>>> >> >
>>> >> > _______________________________________________
>>> >> > 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
>>>
>>>
>>>
>>> --
>>> 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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On Fri, May 15, 2015 at 1:14 PM, Eric Christopher <echristo at gmail.com>
wrote:
>
>>> I don't think that natively wrapped bitcode gets you as much as
you
>>> think it does anyhow, unless you're duplicating a lot of
information (ar,
>>> as discussed earlier, aside). I'm not too worried about the
build system as
>>> far as a wrapping mechanism
>>>
>>
>> Do not under estimate the importance of build system integration. Tools
>>
>
> I'm pretty sure I'm never going to underestimate build system
integration.
> :)
>
>
>> used in the build can include ar, nm, ranlib, objcopy, strip, etc. The
>> latest binutils  support plugin for ar, nm and ranlib, but not others.
>> objcopy can actually change visibility of symbols. It is conceivably
easy
>> to support this with native object wrapper (i.e. propagate the
visibility
>> change at IR reading time), but it is unclear wether existing plugin
>> interfaces are enough for it.
>>
>>
>
> Others have replied to this in more detail, but I'd like to understand
> more the compilation tools pipeline you're talking about
>

This is just an example I have seen from a highly curated build system.

The engineering solution to be taken depends on the final goal and target
users. If we are only targeting thinLTO for a small set of users who does
have good handle on their build systems, it might be a different decision.
ThinLTO has the goal to be a drop-in replacement for O2 (basically enabling
CMO everywhere), the requirement will be quite different. Making it harder
to use won't help that goal. This basically means we need to cut down the
dependency on plugins -- which can be controlled under an option.

here where you need this particular functionality. Also, the
engineering
> tradeoffs in looking at the plugin interfaces rather than this.
>
With native object wrapper, we almost get it (the objcopy example) for
free. Symbol attributes can be changed in the final link step, so the
combined index/summary will contain it and be used in backend compilation
anyway.

thanks,

David


>
>
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