llvm dev - Jan 2017 - [RFC] IR-level Region Annotations

> On Jan 13, 2017, at 9:41 AM, Hal Finkel <hfinkel at anl.gov> wrote:
> 
> 
> On 01/13/2017 12:29 AM, Mehdi Amini wrote:
>> 
>>> On Jan 12, 2017, at 5:02 PM, Hal Finkel <hfinkel at anl.gov
<mailto:hfinkel at anl.gov>> wrote:
>>> On 01/12/2017 06:20 PM, Reid Kleckner via llvm-dev wrote:
>>> 
>>>> On Wed, Jan 11, 2017 at 8:13 PM, Mehdi Amini <mehdi.amini at
apple.com <mailto:mehdi.amini at apple.com>> wrote:
>>>> Can you elaborate why? I’m curious.
>>>> 
>>>> The con of proposal c was that many passes would need to learn
about many region intrinsics. With tokens, you only need to teach all passes
about tokens, which they should already know about because WinEH and other
things use them.
>>>> 
>>>> With tokens, we can add as many region-introducing intrinsics
as makes sense without any additional cost to the middle end. We don't need
to make one omnibus region intrinsic set that describes every parallel loop
annotation scheme supported by LLVM. Instead we would factor things according to
other software design considerations.
>>> 
>>> I think that, unless we allow frontends to add their own intrinsics
without recompiling LLVM, this severely restricts the usefulness of this
feature.
>> 
>> I’m not convinced that “building a frontend without recompiling LLVM
while injecting custom passes” is a strong compelling use-case, i.e. can you
explain why requiring such use-case/frontends to rebuild LLVM is so limiting?
> 
> I don't understand your viewpoint. Many frontends either compose their
own pass pipelines or use the existing extension-point mechanism. Some
frontends, Chapel for example, can insert code using custom address spaces and
then insert passes later to turn accesses using pointers to those address spaces
into runtime calls. This is the kind of design we'd like to support, without
forcing frontends to use custom versions of LLVM, but with annotated regions
instead of just with address spaces.
I think we’re talking about two different things here: you mentioned originally
“without recompiling LLVM”, which I don’t see as major blocker, while now you’re
now clarifying I think that you’re more concerned about putting a requirement on
a *custom* LLVM, as in “it wouldn’t work with the source from a vanilla upstream
LLVM”, which I agree is a different story.

That said, it extends the point from the other email (in parallel) about the
semantics of the intrinsics: while your solution allows these frontend to reuse
the intrinsics, it means that upstream optimization have to consider such
intrinsics as optimization barrier because their semantic is unknown.


— 
Mehdi
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On 01/13/2017 12:11 PM, Mehdi Amini wrote:
>
>> On Jan 13, 2017, at 9:41 AM, Hal Finkel <hfinkel at anl.gov 
>> <mailto:hfinkel at anl.gov>> wrote:
>>
>>
>> On 01/13/2017 12:29 AM, Mehdi Amini wrote:
>>>
>>>> On Jan 12, 2017, at 5:02 PM, Hal Finkel <hfinkel at anl.gov 
>>>> <mailto:hfinkel at anl.gov>> wrote:
>>>>
>>>> On 01/12/2017 06:20 PM, Reid Kleckner via llvm-dev wrote:
>>>>
>>>>> On Wed, Jan 11, 2017 at 8:13 PM, Mehdi Amini 
>>>>> <mehdi.amini at apple.com <mailto:mehdi.amini at
apple.com>> wrote:
>>>>>
>>>>>     Can you elaborate why? I’m curious.
>>>>>
>>>>>
>>>>> The con of proposal c was that many passes would need to
learn
>>>>> about many region intrinsics. With tokens, you only need to
teach
>>>>> all passes about tokens, which they should already know
about
>>>>> because WinEH and other things use them.
>>>>>
>>>>> With tokens, we can add as many region-introducing
intrinsics as
>>>>> makes sense without any additional cost to the middle end.
We
>>>>> don't need to make one omnibus region intrinsic set
that describes
>>>>> every parallel loop annotation scheme supported by LLVM.
Instead
>>>>> we would factor things according to other software design 
>>>>> considerations.
>>>>
>>>> I think that, unless we allow frontends to add their own
intrinsics
>>>> without recompiling LLVM, this severely restricts the
usefulness of
>>>> this feature.
>>>
>>> I’m not convinced that “building a frontend without recompiling
LLVM
>>> while injecting custom passes” is a strong compelling use-case,
i.e.
>>> can you explain why requiring such use-case/frontends to rebuild 
>>> LLVM is so limiting?
>>
>> I don't understand your viewpoint. Many frontends either compose 
>> their own pass pipelines or use the existing extension-point 
>> mechanism. Some frontends, Chapel for example, can insert code using 
>> custom address spaces and then insert passes later to turn accesses 
>> using pointers to those address spaces into runtime calls. This is 
>> the kind of design we'd like to support, without forcing frontends
to
>> use custom versions of LLVM, but with annotated regions instead of 
>> just with address spaces.
>
> I think we’re talking about two different things here: you mentioned 
> originally “without recompiling LLVM”, which I don’t see as major 
> blocker, while now you’re now clarifying I think that you’re more 
> concerned about putting a requirement on a *custom* LLVM, as in “it 
> wouldn’t work with the source from a vanilla upstream LLVM”, which I 
> agree is a different story.
>
> That said, it extends the point from the other email (in parallel) 
> about the semantics of the intrinsics: while your solution allows 
> these frontend to reuse the intrinsics, it means that upstream 
> optimization have to consider such intrinsics as optimization barrier 
> because their semantic is unknown.
I see no reason why this needs to be true (at least so long as you're 
willing to accept a certain amount of "as if" parallelism). Moreover,
if
it is true, then we'll lose the benefits of, for example, being able to 
hoist scalar loads out of parallel loops. We might need to include 
dependencies on "inaccessible memory", so cover natural runtime 
dependencies by default (this can be refined with custom AA logic), but 
that is not a complete code-motion barrier. Memory being explicitly 
managed will end up as arguments to the region intrinsics, so we'll 
automatically get more-fine-grained information.

  -Hal
>
>
> —
> Mehdi
-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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> On Jan 20, 2017, at 6:59 AM, Hal Finkel <hfinkel at anl.gov> wrote:
> 
> On 01/13/2017 12:11 PM, Mehdi Amini wrote:
>> 
>>> On Jan 13, 2017, at 9:41 AM, Hal Finkel <hfinkel at anl.gov
<mailto:hfinkel at anl.gov>> wrote:
>>> 
>>> 
>>> On 01/13/2017 12:29 AM, Mehdi Amini wrote:
>>>> 
>>>>> On Jan 12, 2017, at 5:02 PM, Hal Finkel <hfinkel at
anl.gov <mailto:hfinkel at anl.gov>> wrote:
>>>>> On 01/12/2017 06:20 PM, Reid Kleckner via llvm-dev wrote:
>>>>> 
>>>>>> On Wed, Jan 11, 2017 at 8:13 PM, Mehdi Amini
<mehdi.amini at apple.com <mailto:mehdi.amini at apple.com>> wrote:
>>>>>> Can you elaborate why? I’m curious.
>>>>>> 
>>>>>> The con of proposal c was that many passes would need
to learn about many region intrinsics. With tokens, you only need to teach all
passes about tokens, which they should already know about because WinEH and
other things use them.
>>>>>> 
>>>>>> With tokens, we can add as many region-introducing
intrinsics as makes sense without any additional cost to the middle end. We
don't need to make one omnibus region intrinsic set that describes every
parallel loop annotation scheme supported by LLVM. Instead we would factor
things according to other software design considerations.
>>>>> 
>>>>> I think that, unless we allow frontends to add their own
intrinsics without recompiling LLVM, this severely restricts the usefulness of
this feature.
>>>> 
>>>> I’m not convinced that “building a frontend without recompiling
LLVM while injecting custom passes” is a strong compelling use-case, i.e. can
you explain why requiring such use-case/frontends to rebuild LLVM is so
limiting?
>>> 
>>> I don't understand your viewpoint. Many frontends either
compose their own pass pipelines or use the existing extension-point mechanism.
Some frontends, Chapel for example, can insert code using custom address spaces
and then insert passes later to turn accesses using pointers to those address
spaces into runtime calls. This is the kind of design we'd like to support,
without forcing frontends to use custom versions of LLVM, but with annotated
regions instead of just with address spaces.
>> 
>> I think we’re talking about two different things here: you mentioned
originally “without recompiling LLVM”, which I don’t see as major blocker, while
now you’re now clarifying I think that you’re more concerned about putting a
requirement on a *custom* LLVM, as in “it wouldn’t work with the source from a
vanilla upstream LLVM”, which I agree is a different story.
>> 
>> That said, it extends the point from the other email (in parallel)
about the semantics of the intrinsics: while your solution allows these frontend
to reuse the intrinsics, it means that upstream optimization have to consider
such intrinsics as optimization barrier because their semantic is unknown.
> 
> I see no reason why this needs to be true (at least so long as you're
willing to accept a certain amount of "as if" parallelism).
Sorry, I didn’t quite get that?
> Moreover, if it is true, then we'll lose the benefits of, for example,
being able to hoist scalar loads out of parallel loops. We might need to include
dependencies on "inaccessible memory", so cover natural runtime
dependencies by default (this can be refined with custom AA logic), but that is
not a complete code-motion barrier. Memory being explicitly managed will end up
as arguments to the region intrinsics, so we'll automatically get
more-fine-grained information.
Sanjoy gave an example of the kind of optimization that can break the semantic:
http://lists.llvm.org/pipermail/llvm-dev/2017-January/109302.html
<http://lists.llvm.org/pipermail/llvm-dev/2017-January/109302.html> ; I
haven’t yet seen an explanation about how this is addressed?

I’m not sure how you imagine going around the optimization barrier that goes
with “this intrinsic has an unknown semantic that can impact the control flow of
the program implicitly”, unless it acts as a “hint” only (but I don’t believe it
is the direction?).

— 
Mehdi
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