[llvm-dev] [RFC] IR-level Region Annotations

Yonghong Yan via llvm-dev
Fri, 20 Jan 2017 19:21:07 -0000

Xinmin,

outlining turns a parallel program into a sequential one from compiler's
perspective, and that is why most of the parallel-ignorant pass would hurt.
In your IR description for Sanjoy's example, does the current approach of
outlining impacting the way of the IR should be enhanced for parallelism?

For that specific example (or other analysis and optimization SPMD) and
what is implemented in clang, I am not sure whether we are going to change
the frontend so not to outline the parallel region, or allow to perform
certain optimization such as hoisting that alloca in clang which is not
desired I believe. Or annotate the outlined function together with the
intrinsic_a so that hoisting can be performed, in which case the
instrisic_a would like this:

tok = llvm.experimental.intrinsic_a()[ "DIR.PARALLEL"(),
"QUAL.PRIVATE"(i32* %val, i32 %i), "QUAL.NUM_THREADS"(i32 4)
plus info for
OUTLINED_call.

Mehdi,

I think i am asking the same question as you asked me.

Yonghong



>
> On Fri, Jan 20, 2017 at 1:54 PM, Mehdi Amini via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>>
>> > On Jan 20, 2017, at 10:44 AM, Tian, Xinmin via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>> >
>> > Sanjoy, the IR would be like something below. It is ok to hoist
alloca
>> instruction outside the region. There are some small changes in
optimizer
>> to understand region-annotation intrinsic.
>> >
>> > { void main() {
>> >  i32* val = alloca i32
>> >  tok = llvm.experimental.intrinsic_a()[
"DIR.PARALLEL"(),
>> "QUAL.PRIVATE"(i32* val), "QUAL.NUM_THREADS"(i32
4)]
>> >
>> >  int i = omp_get_thread_num();
>> >  compute_something_into_val(val, i);
>> >  a[i] = val;
>> >
>> >
llvm.experimental.intrinsic_b(tok)["DIR.END.PARALLEL"()];
>> > }
>> >
>> > With above representation, we can do privatization and outlining
as
>> below
>> >
>> > { void main() {
>> >  i32* val = alloca i32
>> >  i32* I = alloca 32
>> >  tok = llvm.experimental.intrinsic_a()[
"DIR.PARALLEL"(),
>> "QUAL.PRIVATE"(i32* %val, i32 %i),
"QUAL.NUM_THREADS"(i32 4)]
>> >
>> >  %ii = omp_get_thread_num();
>> >  compute_something_into_val(%val, %i);
>> >  a[i] = %val;
>> >
>> >
llvm.experimental.intrinsic_b(tok)["DIR.END.PARALLEL"()];
>> > }
>> >
>>
>> Here we come to the interesting part: the hoisting of  "i32* I =
alloca
>> 32” above the intrinsics required to update the intrinsics information
>> “QUAL.PRIVATE”.
>> This means that the optimizer has to be aware of it, I’m missing the
>> magic here?
>> I understand that an openmp specific optimization can do it, the
question
>> is how it an openmp agnostic supposed to behave in face of
>> llvm.experimental.intrinsic_a?
>>
>> —
>> Mehdi
>>
>>
>>
>> > 1. create i32* priv_val = alloca i32  %priv_i = ...in the region,
and
>> replace all  %val with %prv_val in the region.
>> > 2. perform outlining.
>> >
>> > Caller code
>> > ....
>> > omp_push_num_threads(4)
>> > omp_fork_call( .... outline_par_region....) ....
>> >
>> > Callee code:
>> > Outlined_par_rgion {
>> >   I32* priv_val = alloca 32
>> >   I32* priv_i = ....
>> >
>> >    Ret
>> > }
>> >
>> > For OpenMP, we do support it at -O0, -O1, -O2 and -O3.  We had to
make
>> sure it runs correctly w/ and w/o optimizations and advanced analysis.
So
>> we need to preserve all source information for BE.
>> > You can take a look our LLVM-HPC paper for a bit some details.
There
>> are still tons of work to be done. Thanks.
>> >
>> > Xinmin
>> >
>> > -----Original Message-----
>> > From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On
Behalf Of
>> Sanjoy Das via llvm-dev
>> > Sent: Thursday, January 19, 2017 10:13 PM
>> > To: Adve, Vikram Sadanand <vadve at illinois.edu>
>> > Cc: llvm-dev <llvm-dev at lists.llvm.org>; llvm-dev-request
at lists.llvm.org
>> > Subject: Re: [llvm-dev] [RFC] IR-level Region Annotations
>> >
>> > Hi Vikram,
>> >
>> > On Thu, Jan 19, 2017 at 9:27 PM, Adve, Vikram Sadanand <
>> vadve at illinois.edu> wrote:
>> >> Hi Sanjoy,
>> >>
>> >> Yes, that's exactly what we have been looking at recently
here, but
>> >> the region tags seem to make it possible to express the
control flow
>> >> as well, so I think we could start with reg ions+metadata, as
Hal and
>> >
>> > I'm not yet convinced that region tags are sufficient to model
exotic
>> control flow.
>> >
>> > (I don't know OpenMP so this is a copy-pasted-edited example)
>> >
>> > Say we have:
>> >
>> > void main() {
>> >  #pragma omp parallel num_threads(4)
>> >  {
>> >    int i = omp_get_thread_num();
>> >    int val;
>> >    compute_something_into_val(&val, i);
>> >    a[i] = val;
>> >  }
>> > }
>> >
>> > I presume the (eventual) intended lowering is something like this
(if
>> the intended lowering is different than this, and avoids the issue
I'm
>> trying to highlight then my point is moot):
>> >
>> > void main() {
>> >  tok = llvm.experimental.intrinsic_a();
>> >
>> >  int i = omp_get_thread_num();
>> >  i32* val = alloca i32
>> >  compute_something_into_val(val, i);
>> >  a[i] = val;
>> >
>> >  llvm.experimental.intrinsic_b(tok);
>> > }
>> >
>> > However, LLVM is free to hoist the alloca to the entry block:
>> >
>> > void main() {
>> >  i32* val = alloca i32
>> >  tok = llvm.experimental.intrinsic_a();
>> >
>> >  int i = omp_get_thread_num();
>> >  compute_something_into_val(val, i);
>> >  a[i] = val;
>> >
>> >  llvm.experimental.intrinsic_b(tok);
>> > }
>> >
>> > and now you have a race between the four parallel forks.
>> >
>> > The problem here is that nothing in the IR expresses that we have
four
>> copies of the region running "at the same time".  In fact,
such a control
>> flow is alien to LLVM today.
>> >
>> > For instance, another evil optimization may turn:
>> >
>> > void main() {
>> >  int a[4];
>> >  #pragma omp parallel num_threads(4)
>> >  {
>> >    int i = omp_get_thread_num();
>> >    int val = compute_something_into_val(i);
>> >    a[i] = val;
>> >  }
>> >
>> >  return a[0] + a[1];
>> > }
>> >
>> > to
>> >
>> > void main() {
>> >  int a[4];
>> >  #pragma omp parallel num_threads(4)
>> >  {
>> >    int i = omp_get_thread_num();
>> >    int val = compute_something_into_val(i);
>> >    a[i] = val;
>> >  }
>> >
>> >  return undef;
>> > }
>> >
>> > since a[i] = val could have initialized at most one element in a.
>> >
>> > Now you could say that the llvm.experimental.intrinsic_a and
>> llvm.experimental.intrinsic_b intrinsics are magic, and even such
"obvious"
>> optimizations are not allowed to happen across them; but then calls to
>> these intrinsics is pretty fundamentally different from
"normal" calls, and
>> are probably best modeled as new instructions.
>> > You're going to have to do the same kind of auditing of passes
either
>> way, and the only extra cost of a new instruction is the extra bitcode
>> reading / writing code.
>> >
>> > I hope I made sense.
>> >
>> > -- Sanjoy
>> >
>> >> Xinmin proposed, and then figure out what needs to be first
class
>> >> instructions.
>> >
>> >>
>> >> --Vikram Adve
>> >>
>> >>
>> >>
>> >>> On Jan 19, 2017, at 11:03 PM, Sanjoy Das <
>> sanjoy at playingwithpointers.com> wrote:
>> >>>
>> >>> Hi,
>> >>>
>> >>> My bias is to use both (b) and (d), since they have
complementary
>> >>> strengths.  We should use (b) for expressing concepts that
can't be
>> >>> semantically modeled as a call or invoke (this branch
takes both its
>> >>> successors), and (d) for expressing things that can be
(this call may
>> >>> never return), and annotation like things (this region
(denoted by
>> >>> def-use of a token) is a reduction).
>> >>>
>> >>> I don't grok OpenMP, but perhaps we can come with one
or two
>> >>> "generalized control flow"-type instructions
that can be used to
>> >>> model the non-call/invoke like semantics we'd like
LLVM to know
>> >>> about, and model the rest with (d)?
>> >>>
>> >>> -- Sanjoy
>> >>>
>> >>> On Thu, Jan 19, 2017 at 8:28 PM, Hal Finkel via llvm-dev
>> >>> <llvm-dev at lists.llvm.org> wrote:
>> >>>>
>> >>>> On 01/19/2017 03:36 PM, Mehdi Amini via llvm-dev
wrote:
>> >>>>
>> >>>>
>> >>>> On Jan 19, 2017, at 1:32 PM, Daniel Berlin <dberlin
at dberlin.org>
>> wrote:
>> >>>>
>> >>>>
>> >>>>
>> >>>>> On Thu, Jan 19, 2017 at 1:12 PM, Mehdi Amini
<mehdi.amini at apple.com>
>> wrote:
>> >>>>>
>> >>>>>
>> >>>>> On Jan 19, 2017, at 12:04 PM, Daniel Berlin
<dberlin at dberlin.org>
>> wrote:
>> >>>>>
>> >>>>>
>> >>>>>
>> >>>>> On Thu, Jan 19, 2017 at 11:46 AM, Mehdi Amini via
llvm-dev
>> >>>>> <llvm-dev at lists.llvm.org> wrote:
>> >>>>>>
>> >>>>>>
>> >>>>>>> On Jan 19, 2017, at 11:36 AM, Adve, Vikram
Sadanand via llvm-dev
>> >>>>>>> <llvm-dev at lists.llvm.org> wrote:
>> >>>>>>>
>> >>>>>>> Hi Johannes,
>> >>>>>>>
>> >>>>>>>> I am especially curious where you get
your data from. Tapir [0]
>> >>>>>>>> (and to some degree PIR [1]) have
shown that,
>> >>>>>>>> counterintuitively, only a few changes
to LLVM passes are
>> >>>>>>>> needed. Tapir was recently used in an
MIT class with a lot of
>> >>>>>>>> students and it seemed to work well
with only minimal changes to
>> >>>>>>>> analysis and especially transformation
passes.
>> >>>>>>>
>> >>>>>>> TAPIR is an elegant, small extension and,
in particular, I think
>> >>>>>>> the idea of asymmetric parallel tasks and
control flow is a
>> >>>>>>> clever way to express parallelism with
serial semantics, as in
>> >>>>>>> Cilk.  Encoding the control flow
extensions as explicit
>> >>>>>>> instructions is orthogonal to that, though
arguably more elegant
>> than using region tags + metadata.
>> >>>>>>>
>> >>>>>>> However, Cilk is a tiny language compared
with the full
>> >>>>>>> complexity of other languages, like
OpenMP.  To take just one
>> >>>>>>> example, TAPIR cannot express the ORDERED
construct of OpenMP.  A
>> >>>>>>> more serious concern, IMO, is that TAPIR
(like Cilk) requires
>> >>>>>>> serial semantics, whereas there are many
parallel languages,
>> OpenMP included, that do not obey that restriction.
>> >>>>>>> Third, OpenMP has *numerous* clauses,
e.g., REDUCTION or PRIVATE,
>> >>>>>>> that are needed because without that,
you’d be dependent on
>> >>>>>>> fundamentally hard compiler analyses to
extract the same
>> >>>>>>> information for satisfactory parallel
performance; realistic
>> >>>>>>> applications cannot depend on the success
of such analyses.
>> >>>>>>
>> >>>>>> I agree with this, but I’m also wondering if
it needs to be first
>> >>>>>> class in the IR?
>> >>>>>> For example we know our alias analysis is very
basic, and C/C++
>> >>>>>> have a higher constraint thanks to their type
system, but we
>> >>>>>> didn’t inject this higher level information
that helps the
>> >>>>>> optimizer as first class IR constructs.
>> >>>>>
>> >>>>>
>> >>>>> FWIW, while i agree with the general point, i
wouldn't use this
>> example.
>> >>>>> Because we pretty much still suffer to this day
because of it (both
>> >>>>> in AA, and devirt, and ...)  :) We can't
always even tell fields
>> >>>>> apart
>> >>>>>
>> >>>>>
>> >>>>> Is it inherent to the infrastructure, i.e. using
metadata instead
>> >>>>> of first class IR construct or is it just a
“quality of
>> implementation” issue?
>> >>>>
>> >>>> Not to derail this conversation:
>> >>>>
>> >>>> IMHO, At some point there is no real difference :)
>> >>>>
>> >>>> Because otherwise, everything is a QOI issue.
>> >>>>
>> >>>> IE if it's super tricky to get metadata that works
well and works
>> >>>> right, doesn't get lost, etc, and that's
inherent to using metadata,
>> >>>> that to me is not a QOI issue.
>> >>>>
>> >>>> So could it be done with metadata? Probably?
>> >>>> But at the same time,  if it had been done with more
first class
>> >>>> constructs, it would have happened years ago  and been
much lower
>> cost.
>> >>>>
>> >>>>
>> >>>> This is what I meant by “inherent to the
infrastructure”, thanks for
>> >>>> clarifying.
>> >>>>
>> >>>>
>> >>>> To clarify, we were proposing metadata that is used as
arguments to
>> >>>> the region-annotation intrinsics. This metadata has
the nice
>> >>>> property that it does not get dropped (so it is just
being used as a
>> >>>> way of encoding whatever data structures are necessary
without
>> predefining a syntactic schema).
>> >>>>
>> >>>> -Hal
>> >>>>
>> >>>>
>> >>>> —
>> >>>> Mehdi
>> >>>>
>> >>>>
>> >>>>
>> >>>>
>> >>>> _______________________________________________
>> >>>> LLVM Developers mailing list
>> >>>> llvm-dev at lists.llvm.org
>> >>>>
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> >>>>
>> >>>>
>> >>>> --
>> >>>> Hal Finkel
>> >>>> Lead, Compiler Technology and Programming Languages
Leadership
>> >>>> Computing Facility Argonne National Laboratory
>> >>>>
>> >>>>
>> >>>> _______________________________________________
>> >>>> LLVM Developers mailing list
>> >>>> llvm-dev at lists.llvm.org
>> >>>>
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> >>>>
>> > _______________________________________________
>> > LLVM Developers mailing list
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>> > _______________________________________________
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>>
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