[LLVMdev] RFC: ThinLTO Impementation Plan

Dave Bozier
Fri, 15 May 2015 17:07:22 -0000

> There is no need for emitting the full symtab. I checked the overhead
with a huge internal C++ source. The overhead of symtab + str table
compared with byte code with debug is about 3%.

It's still sizable and could be noticeable if thinLTO can deliver compile
times that closer to what resembles builds without LTO as your results
suggest.

> More importantly, it is also possible to use the symtab also for
index/summary purpose, which makes the space usage completely
'unwasted'.
That gets into the details which will follow when patches are in.

There is symbol information in both the native object symbol table and the
bitcode file? isn't that waste? I understand the reasons for using the
native object wrapper (compatibility with other tools) and happy with that.
But I'd also like to see the option for function index/summary data to be
produced without the wrapper, so that bitcode aware tools do not need to
use this wrapped format. If you mix the native object wrapper symbol
information with the function/index summary data then that would end up
being impossible.

Also won't having the native object data with the function index/summary
have a cost on testing for all of the supported native object formats?

On Fri, May 15, 2015 at 4:26 PM, Xinliang David Li <xinliangli at
gmail.com>
wrote:

> There is no need for emitting the full symtab. I checked the overhead with
> a huge internal C++ source. The overhead of symtab + str table compared
> with byte code with debug is about 3%.
>
> More importantly, it is also possible to use the symtab also for
> index/summary purpose, which makes the space usage completely
'unwasted'.
> That gets into the details which will follow when patches are in.
>
> David
>
> On Fri, May 15, 2015 at 5:11 AM, Dave Bozier <seifsta at gmail.com>
wrote:
>
>> > Are you sure about the additional I/O? With native symtab,
existing
>> tools just need to read those, while plugin based approach needs to
read
>> bit code section to feedback symbols to the tool.
>>
>> The additional I/O will be quite big if you are going to emit the full
>> symbol table. Looking at some of our real world links the symbol table
and
>> string tables of all the inputs seen by the linker add up to about 50 -
>> 100mb.
>>
>> On Thu, May 14, 2015 at 10:28 PM, Xinliang David Li <xinliangli at
gmail.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.
>>>>
>>>
>>> What do we want plugin to do anything here?  We just need the
linker to
>>> concatenate the bitcode sections and produce a combined bitcode
file.
>>>
>>>
>>>>
>>>>
>>>>> > Agreed. The ar situation is interesting because one
thing we
>>>>> discussed after
>>>>> > you wandered off was just adding a ToC section to
bitcode as it is
>>>>> and then
>>>>> > having the tools handle that. Would seem to accomplish
at least the
>>>>> goals as
>>>>> > I've seen them up to this point without worrying
too much.
>>>>>
>>>>> The ToC section is a way we can encode the function
index/summary into
>>>>> bitcode, but won't help integrate with existing tools.
The main issue
>>>>> we are trying to solve is integrating transparently with
existing
>>>>> binutils tools in use in our build system and probably
elsewhere.
>>>>>
>>>>>
>>>> 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.
>>>>
>>>> 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
>>>>
>>>
>>> Are you sure about the additional I/O? With native symtab, existing
>>> tools just need to read those, while plugin based approach needs to
read
>>> bit code section to feedback symbols to the tool.
>>>
>>>
>>>>  - we still have to update some tools to work at all
>>>>
>>>
>>> If any, it will be minimal.
>>>
>>>
>>>>
>>>> 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.
>>>>
>>>>
>>> I am fine making llvm tools work with it, but we should not
>>> require/force user using them. I think this is an orthogonal
feature.
>>>
>>> David
>>>
>>>
>>>
>>>
>>>> >
>>>>> > 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
>>>>>
>>>>
>>>> _______________________________________________
>>>> 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
>>>
>>>
>>
>
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