[LLVMdev] RFC: ThinLTO Impementation Plan

Eric Christopher echristo at gmail.com
Thu May 14 14:46:27 PDT 2015 

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 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?

-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
>
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