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Demikhovsky, Elena via llvm-dev

2016-May-19 14:03 UTC

[llvm-dev] Working on FP SCEV Analysis

> One option would be to extend InductionDescriptor::isInductionPHI in the
vectorizer to directly analyze the PHIs without SCEV support as Sanjoy
suggested.  I *think* that that could be sufficient to handle case B.
I implemented this with FP SCEV and the code looks very structured, including
SCEVExpander. Extending the existing structures without implementing FP SCEV
will be problematic.
And my end goal is to handle case *A*.

-           Elena

From: anemet at apple.com [mailto:anemet at apple.com]
Sent: Thursday, May 19, 2016 07:43
To: Sanjoy Das <sanjoy at playingwithpointers.com>
Cc: Demikhovsky, Elena <elena.demikhovsky at intel.com>; Saito, Hideki
<hideki.saito at intel.com>; llvm-dev <llvm-dev at lists.llvm.org>;
Andrew Trick <atrick at apple.com>
Subject: Re: [llvm-dev] Working on FP SCEV Analysis

On May 18, 2016, at 12:17 PM, Sanjoy Das via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:

Demikhovsky, Elena wrote:
> Even then, I'd personally want to see further evidence of why thecorrect solution is to model the floating point IV in SCEV rather than
find a more powerful way of converting the IV to an integer that
models> the non-integer values taken on by the IV. As an example, if the usecase is the following code with appropriate flags to relax IEEE
semantics so this looks like normal algabra etc:
> for (float f = 0.01f; f < 1.0f; f += 0.01f) ç **A**
...
> I'd rather see us cleverly turn it into:
> float f = 0.01f;
> for (int i = 1; i < 100; i += 1, f += 0.01f) ç **B**
I can later try to enhance IndVarSimplify::handleFloatingPointIV() in
order to convert**A** to **B**.

But **B** is exactly the case I’m starting from. The main IV “i” is
integer. The variable “f” is also considered as IV in this loop.

And this loop is not vectorized because “f” is floating point.

I don’t think that the case **B** is uncommon.

If B is the case we actually care about, I'd say changing SCEV to work with
floating points is an overkill.  How would you expect an SCEVFAddExpr to help
vectorize B, other than tell you what the initial value and the increment is
(and these can be found with a simple value analysis)?

One option would be to extend InductionDescriptor::isInductionPHI in the
vectorizer to directly analyze the PHIs without SCEV support as Sanjoy
suggested.  I *think* that that could be sufficient to handle case B.

Then if we find other pressing cases to handle we can rethink the strategy.

Also the current diagnostics is pretty bad for Hideki’s testcase with TTT as
float.  This is what we currently report with -Rpass-analysis=loop-vectorize:

/tmp/sss.c:3:6: remark: loop not vectorized: value that could not be
      identified as reduction is used outside the loop
      [-Rpass-analysis=loop-vectorize]

I have no clue why we say that the value is used outside the loop.  I think this
should just say that we have a loop-variant value that we couldn’t identify
either as an induction or as a reduction.

Adam

If we're interested in handling complex variants of A directly: computing
trip counts, proving away predicates etc. without translating the loops to use
integer IVs (perhaps because we can't legally do so), then I can see FP-SCEV
as a reasonable implementation strategy, but it looks like the general consensus
is that such cases are rare and generally not worth optimizing?

-- Sanjoy

-*/Elena/*

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Mikhail Zolotukhin via llvm-dev

2016-May-19 22:51 UTC

head link

[llvm-dev] Working on FP SCEV Analysis

> On May 19, 2016, at 7:03 AM, Demikhovsky, Elena via llvm-dev <llvm-dev
at lists.llvm.org> wrote:
> 
> > One option would be to extend InductionDescriptor::isInductionPHI in
the vectorizer to directly analyze the PHIs without SCEV support as Sanjoy
suggested.  I *think* that that could be sufficient to handle case B. <>
>  
> I implemented this with FP SCEV and the code looks very structured,
including SCEVExpander. Extending the existing structures without implementing
FP SCEV will be problematic.
> And my end goal is to handle case *A*.The problem isn't that the code will be bad, but that it might be unused.
SCEV is already quite big, and extending it even further for no (stated so far)
reason sounds questionable.

Owen mentioned an important use case where float is just used because integers
are not supported at all - we definitely should be able to handle such cases,
but it looks like promotion to int is the only thing we need for it. I looked
into a simple example:
#define T float
void foo(T *buf) {
  T i = 0;
#pragma clang loop vectorize(enable)
  for (i = 0; i < 1000; i += 1) {
    buf[(int)i] = i;
  }
}

and we vectorize it now. That said, the generated code for integers is much
cleaner, but I don't think and has anything to do with the IV, as it's
been successfully promoted to an integer IV.

So, as far as I understand, your intention is to handle loops with non-integer
step/start/end value. Why can't we focus on promotable to integer cases,
which can and should be handled even now (if not, we'd rather fix it)?

Thanks,
Michael

PS: My old intel email address isn't accessible any more, so you probably
should remove it from your contacts.
>  
>  
> -           Elena
>  
>  <>From: anemet at apple.com <mailto:anemet at apple.com>
[mailto:anemet at apple.com <mailto:anemet at apple.com>]
> Sent: Thursday, May 19, 2016 07:43
> To: Sanjoy Das <sanjoy at playingwithpointers.com <mailto:sanjoy at
playingwithpointers.com>>
> Cc: Demikhovsky, Elena <elena.demikhovsky at intel.com
<mailto:elena.demikhovsky at intel.com>>; Saito, Hideki
<hideki.saito at intel.com <mailto:hideki.saito at intel.com>>;
llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>>; Andrew Trick <atrick at apple.com <mailto:atrick
at apple.com>>
> Subject: Re: [llvm-dev] Working on FP SCEV Analysis
>  
>  
> On May 18, 2016, at 12:17 PM, Sanjoy Das via llvm-dev <llvm-dev at
lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>  
> 
> 
> Demikhovsky, Elena wrote:
> 
> > Even then, I'd personally want to see further evidence of why the
> correct solution is to model the floating point IV in SCEV rather than
> find a more powerful way of converting the IV to an integer that models
> > the non-integer values taken on by the IV. As an example, if the use
> case is the following code with appropriate flags to relax IEEE
> semantics so this looks like normal algabra etc:
> 
> > for (float f = 0.01f; f < 1.0f; f += 0.01f) ç **A**
> 
> ...
> 
> > I'd rather see us cleverly turn it into:
> 
> > float f = 0.01f;
> 
> > for (int i = 1; i < 100; i += 1, f += 0.01f) ç **B**
> 
> I can later try to enhance IndVarSimplify::handleFloatingPointIV() in
> order to convert**A** to **B**.
> 
> But **B** is exactly the case I’m starting from. The main IV “i” is
> integer. The variable “f” is also considered as IV in this loop.
> 
> And this loop is not vectorized because “f” is floating point.
> 
> I don’t think that the case **B** is uncommon.
> 
> If B is the case we actually care about, I'd say changing SCEV to work
with floating points is an overkill.  How would you expect an SCEVFAddExpr to
help vectorize B, other than tell you what the initial value and the increment
is (and these can be found with a simple value analysis)?
>  
> One option would be to extend InductionDescriptor::isInductionPHI in the
vectorizer to directly analyze the PHIs without SCEV support as Sanjoy
suggested.  I *think* that that could be sufficient to handle case B.
>  
> Then if we find other pressing cases to handle we can rethink the strategy.
>  
> Also the current diagnostics is pretty bad for Hideki’s testcase with TTT
as float.  This is what we currently report with -Rpass-analysis=loop-vectorize:
>  
> /tmp/sss.c:3:6: remark: loop not vectorized: value that could not be
>       identified as reduction is used outside the loop
>       [-Rpass-analysis=loop-vectorize]
>  
> I have no clue why we say that the value is used outside the loop.  I think
this should just say that we have a loop-variant value that we couldn’t identify
either as an induction or as a reduction.
>  
> Adam
> 
> 
> 
> If we're interested in handling complex variants of A directly:
computing trip counts, proving away predicates etc. without translating the
loops to use integer IVs (perhaps because we can't legally do so), then I
can see FP-SCEV as a reasonable implementation strategy, but it looks like the
general consensus is that such cases are rare and generally not worth
optimizing?
> 
> -- Sanjoy
> 
> 
> 
> -*/Elena/*
> 
> ---------------------------------------------------------------------
> Intel Israel (74) Limited
> 
> This e-mail and any attachments may contain confidential material for
> the sole use of the intended recipient(s). Any review or distribution
> by others is strictly prohibited. If you are not the intended
> recipient, please contact the sender and delete all copies.
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
>  
> ---------------------------------------------------------------------
> Intel Israel (74) Limited
> 
> This e-mail and any attachments may contain confidential material for
> the sole use of the intended recipient(s). Any review or distribution
> by others is strictly prohibited. If you are not the intended
> recipient, please contact the sender and delete all copies.
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
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Chandler Carruth via llvm-dev

2016-May-20 02:03 UTC

head link

[llvm-dev] Working on FP SCEV Analysis

On Thu, May 19, 2016 at 7:03 AM Demikhovsky, Elena <
elena.demikhovsky at intel.com> wrote:
> > One option would be to extend InductionDescriptor::isInductionPHI in
the
> vectorizer to directly analyze the PHIs without SCEV support as Sanjoy
> suggested.  I *think* that that could be sufficient to handle case B.
>
>
>
> I implemented this with FP SCEV and the code looks very structured,
> including SCEVExpander. Extending the existing structures without
> implementing FP SCEV will be problematic.
>
> And my end goal is to handle case **A**.
>Ok, but there have been *numerous* requests for an explanation of why this
is important, and that hasn't emerged yet.

We really need to have a clear understanding of the relative importance of
solving these problems in order to understand the best approach.

>
>
>
>
> -          * Elena*
>
>
>
> *From:* anemet at apple.com [mailto:anemet at apple.com]
> *Sent:* Thursday, May 19, 2016 07:43
> *To:* Sanjoy Das <sanjoy at playingwithpointers.com>
> *Cc:* Demikhovsky, Elena <elena.demikhovsky at intel.com>; Saito,
Hideki <
> hideki.saito at intel.com>; llvm-dev <llvm-dev at lists.llvm.org>;
Andrew Trick
> <atrick at apple.com>
>
>
> *Subject:* Re: [llvm-dev] Working on FP SCEV Analysis
>
>
>
>
>
> On May 18, 2016, at 12:17 PM, Sanjoy Das via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>
>
>
>
> Demikhovsky, Elena wrote:
>
> > Even then, I'd personally want to see further evidence of why the
> correct solution is to model the floating point IV in SCEV rather than
> find a more powerful way of converting the IV to an integer that models
> > the non-integer values taken on by the IV. As an example, if the use
> case is the following code with appropriate flags to relax IEEE
> semantics so this looks like normal algabra etc:
>
> > for (float f = 0.01f; f < 1.0f; f += 0.01f) ç **A**
>
> ...
>
> > I'd rather see us cleverly turn it into:
>
> > float f = 0.01f;
>
> > for (int i = 1; i < 100; i += 1, f += 0.01f) ç **B**
>
> I can later try to enhance IndVarSimplify::handleFloatingPointIV() in
> order to convert**A** to **B**.
>
> But **B** is exactly the case I’m starting from. The main IV “i” is
> integer. The variable “f” is also considered as IV in this loop.
>
> And this loop is not vectorized because “f” is floating point.
>
> I don’t think that the case **B** is uncommon.
>
>
> If B is the case we actually care about, I'd say changing SCEV to work
> with floating points is an overkill.  How would you expect an SCEVFAddExpr
> to help vectorize B, other than tell you what the initial value and the
> increment is (and these can be found with a simple value analysis)?
>
>
>
> One option would be to extend InductionDescriptor::isInductionPHI in the
> vectorizer to directly analyze the PHIs without SCEV support as Sanjoy
> suggested.  I *think* that that could be sufficient to handle case B.
>
>
>
> Then if we find other pressing cases to handle we can rethink the strategy.
>
>
>
> Also the current diagnostics is pretty bad for Hideki’s testcase with TTT
> as float.  This is what we currently report with
> -Rpass-analysis=loop-vectorize:
>
>
>
> /tmp/sss.c:3:6: remark: loop not vectorized: value that could not be
>
>       identified as reduction is used outside the loop
>
>       [-Rpass-analysis=loop-vectorize]
>
>
>
> I have no clue why we say that the value is used outside the loop.  I
> think this should just say that we have a loop-variant value that we
> couldn’t identify either as an induction or as a reduction.
>
>
>
> Adam
>
>
>
>
> If we're interested in handling complex variants of A directly:
computing
> trip counts, proving away predicates etc. without translating the loops to
> use integer IVs (perhaps because we can't legally do so), then I can
see
> FP-SCEV as a reasonable implementation strategy, but it looks like the
> general consensus is that such cases are rare and generally not worth
> optimizing?
>
> -- Sanjoy
>
>
>
> -*/Elena/*
>
> ---------------------------------------------------------------------
> Intel Israel (74) Limited
>
> This e-mail and any attachments may contain confidential material for
> the sole use of the intended recipient(s). Any review or distribution
> by others is strictly prohibited. If you are not the intended
> recipient, please contact the sender and delete all copies.
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
>
> ---------------------------------------------------------------------
> Intel Israel (74) Limited
>
> This e-mail and any attachments may contain confidential material for
> the sole use of the intended recipient(s). Any review or distribution
> by others is strictly prohibited. If you are not the intended
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Saito, Hideki via llvm-dev

2016-May-20 17:04 UTC

head link

[llvm-dev] Working on FP SCEV Analysis

To the best of my experience, handling case B (secondary induction) is
must-have, and if I’m not mistaken,
people aren’t opposed to that.

For me, handling case A (primary induction) is “why not?”, but I certainly admit
that that can be very naïve
thinking coming from lack of good understanding on SCEV and their proper usages.
Now, let’s assume we
can postpone discussion about  case A. What is the best approach to handle case
B? Let me summarize
the discussion so far. Hope I didn’t miss anything.

1)
Extend SCEV was the initial approach taken by Elena.
                Elena thinks this solution ”looks very structured”.
                If I’m not mistaken, some people think this is overkill and
overly complicates already complicated SCEV.
                Anyone care to look at the patch Elena came up with?
2)
IndVarSimplify::handleFloatingPointIV  (mentioned by Andy)
                This transforms integer-valued FP (primary) IV into integer IV
and convert.
                Chandler says most of Graphics shading language use case
mentioned by Owen
                should be handled here.
                It certainly has the logic of detecting FP induction, but Andy
punted discussions
                on non-integer valued IV issues to MichaelZ and Adam.
3)
extend InductionDescriptor::isInductionPHI in the vectorizer to directly analyze
the PHIs without SCEV support
(mentioned by Adam)
                If this is the standard way to deal with all secondary
inductions, it certainly looks attractive.
                Elena, would you try doing this and compare with 1)?

Thanks,
Hideki

----------------------
I looked at what our FORTRAN compiler (non-LLVM) does for the following (x, y,
z, and f are float by default typing,
based on names).

       subroutine foo(x, y, z, A, N)
       double precision A(N)
       do f = x, y, z
         A(f) = f
       enddo
       end

The frontend computes the trip count outside of the loop (per language rule) and
uses an integer primary
induction variable (compile-generated) for loop control. So, (future) FORTRAN
usage doesn’t seem to be
a good example for promoting support for case A (= FP primary induction
variable).

From: Chandler Carruth [mailto:chandlerc at google.com]
Sent: Thursday, May 19, 2016 7:03 PM
To: Demikhovsky, Elena <elena.demikhovsky at intel.com>; anemet at
apple.com; Sanjoy Das <sanjoy at playingwithpointers.com>
Cc: Saito, Hideki <hideki.saito at intel.com>; llvm-dev <llvm-dev at
lists.llvm.org>; Andrew Trick <atrick at apple.com>
Subject: Re: [llvm-dev] Working on FP SCEV Analysis

On Thu, May 19, 2016 at 7:03 AM Demikhovsky, Elena <elena.demikhovsky at
intel.com<mailto:elena.demikhovsky at intel.com>>
wrote:> One option would be to extend InductionDescriptor::isInductionPHI in the
vectorizer to directly analyze the PHIs without SCEV support as Sanjoy
suggested.  I *think* that that could be sufficient to handle case B.
I implemented this with FP SCEV and the code looks very structured, including
SCEVExpander. Extending the existing structures without implementing FP SCEV
will be problematic.
And my end goal is to handle case *A*.
Ok, but there have been *numerous* requests for an explanation of why this is
important, and that hasn't emerged yet.

We really need to have a clear understanding of the relative importance of
solving these problems in order to understand the best approach.

-           Elena

From: anemet at apple.com<mailto:anemet at apple.com> [mailto:anemet at
apple.com<mailto:anemet at apple.com>]
Sent: Thursday, May 19, 2016 07:43
To: Sanjoy Das <sanjoy at playingwithpointers.com<mailto:sanjoy at
playingwithpointers.com>>
Cc: Demikhovsky, Elena <elena.demikhovsky at
intel.com<mailto:elena.demikhovsky at intel.com>>; Saito, Hideki
<hideki.saito at intel.com<mailto:hideki.saito at intel.com>>;
llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at
lists.llvm.org>>; Andrew Trick <atrick at apple.com<mailto:atrick at
apple.com>>

Subject: Re: [llvm-dev] Working on FP SCEV Analysis

On May 18, 2016, at 12:17 PM, Sanjoy Das via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:

Demikhovsky, Elena wrote:> Even then, I'd personally want to see further evidence of why thecorrect solution is to model the floating point IV in SCEV rather than
find a more powerful way of converting the IV to an integer that
models> the non-integer values taken on by the IV. As an example, if the usecase is the following code with appropriate flags to relax IEEE
semantics so this looks like normal algabra etc:
> for (float f = 0.01f; f < 1.0f; f += 0.01f) ç **A**
...
> I'd rather see us cleverly turn it into:
> float f = 0.01f;
> for (int i = 1; i < 100; i += 1, f += 0.01f) ç **B**
I can later try to enhance IndVarSimplify::handleFloatingPointIV() in
order to convert**A** to **B**.

But **B** is exactly the case I’m starting from. The main IV “i” is
integer. The variable “f” is also considered as IV in this loop.

And this loop is not vectorized because “f” is floating point.

I don’t think that the case **B** is uncommon.

If B is the case we actually care about, I'd say changing SCEV to work with
floating points is an overkill.  How would you expect an SCEVFAddExpr to help
vectorize B, other than tell you what the initial value and the increment is
(and these can be found with a simple value analysis)?

One option would be to extend InductionDescriptor::isInductionPHI in the
vectorizer to directly analyze the PHIs without SCEV support as Sanjoy
suggested.  I *think* that that could be sufficient to handle case B.

Then if we find other pressing cases to handle we can rethink the strategy.

Also the current diagnostics is pretty bad for Hideki’s testcase with TTT as
float.  This is what we currently report with -Rpass-analysis=loop-vectorize:

/tmp/sss.c:3:6: remark: loop not vectorized: value that could not be
      identified as reduction is used outside the loop
      [-Rpass-analysis=loop-vectorize]

I have no clue why we say that the value is used outside the loop.  I think this
should just say that we have a loop-variant value that we couldn’t identify
either as an induction or as a reduction.

Adam

If we're interested in handling complex variants of A directly: computing
trip counts, proving away predicates etc. without translating the loops to use
integer IVs (perhaps because we can't legally do so), then I can see FP-SCEV
as a reasonable implementation strategy, but it looks like the general consensus
is that such cases are rare and generally not worth optimizing?

-- Sanjoy

-*/Elena/*

---------------------------------------------------------------------
Intel Israel (74) Limited

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recipient, please contact the sender and delete all copies.
_______________________________________________
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llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

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llvm dev - May 2016 - Working on FP SCEV Analysis

[llvm-dev] Working on FP SCEV Analysis

[llvm-dev] Working on FP SCEV Analysis

[llvm-dev] Working on FP SCEV Analysis

[llvm-dev] Working on FP SCEV Analysis