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

Sanjoy Das via llvm-dev
Fri, 20 Jan 2017 19:35:35 -0000

Hi,

I'm going to club together some responses.

I agree that outlining function sub-bodies and passing in the function
pointers to said outlined bodies to OpenMP helpers lets us correctly
implement the semantics we need.  However, unless I severely
misunderstood the thread, I thought the key idea was to move *away*
from that representation and towards a representation that _allows_
optimization?

My problem with representing parallel regions with
intrinsic-denoted-regions is that we're lying to the optimizer about
what the code actually does.  Calls, even to intrinsics, can "at
worst" do some combination of the following:

 - Write to and read from arbitrary memory
 - Have UB (but we're allowed to pretend that they don't)
 - Throw an exception
 - Never return, either by infinite looping or by calling exit(0)
 - Have memory synchronization operations, like fences, atomic loads,
   stores etc.
 - Have side effects like IO, volatile writes

If an intrinsic's behavior can be explained by some subset of the
above, then you should not need to edit any pass to preserve
_correctness_ -- all optimization passes (today) conservatively assume
that calls that they don't understand have all of the behaviors
outlined above.

However, if to preserve *correctness* you have to edit optimization
passes and teach them that certain intrinsic calls have behavior
*outside* the set mentioned above then the instruction really does not
have "call semantics".  `call @llvm.experimental.region_begin()` is
really a fundamentally new instruction masquerading as an intrinsic,
and it is probably better to call a spade a spade and represent it as
a new instruction.

The setting for the examples I gave was not that "here is a case we
need to get right".  The setting was that "here is a *symptom* that
shows that we've lied to the optimizer".  We can go ahead and fix all
the symptoms by adding bailouts to the respective passes, but that
does not make us immune to passes that we don't know about
e.g. downstream passes, passes that will be added later.  It also puts
us in a weird spot around semantics of call instructions.

-- Sanjoy

On Fri, Jan 20, 2017 at 11:22 AM, Tian, Xinmin via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Yonghong, In our implementation (not open sourced), we don’t do outlining
> the Front-End.  See my previous reply to Medhi’s email.
>
>
>
> Xinmin
>
>
>
> From: Yonghong Yan [mailto:yan at oakland.edu]
> Sent: Friday, January 20, 2017 11:18 AM
> To: Mehdi Amini
> Cc: Tian, Xinmin; llvm-dev at lists.llvm.org; llvm-dev-request at
lists.llvm.org;
> Adve, Vikram Sadanand
> Subject: Re: [llvm-dev] [RFC] IR-level Region Annotations
>
>
>
> 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
>>>>>
>> _______________________________________________
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