[llvm-dev] Working on FP SCEV Analysis
Demikhovsky, Elena via llvm-dev
llvm-dev at lists.llvm.org
Tue May 24 12:35:53 PDT 2016
I'm going to post the code to review in a few days. I'm polishing the code and adding more test cases.
Could you, please, upload your patch, it will be interesting to compare.
Thank you.
- Elena
>-----Original Message-----
>From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of
>Amara Emerson via llvm-dev
>Sent: Tuesday, May 24, 2016 18:00
>To: Adam Nemet <anemet at apple.com>
>Cc: llvm-dev <llvm-dev at lists.llvm.org>; Saito, Hideki
><hideki.saito at intel.com>
>Subject: Re: [llvm-dev] Working on FP SCEV Analysis
>
>Adding support for FP inductions through isInductionPHI() is certainly
>possible, I have a relatively small local patch that does exactly that for
>simple fp add-recurrence cases, along with changes to the vectorizer to
>make it aware of FP inductions. It won't get give you the powerful
>reasoning capabilities of SCEV, but for the B-like cases it should work.
>
>Amara
>
>On 20 May 2016 at 19:31, Adam Nemet via llvm-dev <llvm-
>dev at lists.llvm.org> wrote:
>> Hi Hideki,
>>
>> I like this summary overall, thanks. More below.
>>
>> On May 20, 2016, at 10:04 AM, Saito, Hideki <hideki.saito at intel.com>
>wrote:
>>
>>
>> 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.
>>
>>
>> My understanding is that we only need this for *A* not for *B*.
>>
>> On the specific issue of non-integer values, there is simply no
>> attempt made in the code to deal with them. That said, I think it
>> should be possible to compute the trip count and then derive an
>> integer induction variable controlling the loop.
>>
>> 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)?
>>
>>
>> Just to clarify, the code is currently structured to check if the PHI
>> is an add-recurrence that was detected by SCEV. The idea is to add a
>> fall-back to analyze the PHI directly if its type is a float.
>>
>> There is already precedence for such things in LV. We support more
>> reductions (including floating-point) than what SCEV can analyze,
>> therefore RecurrenceDescriptor::AddReductionVar needs to analyze
>PHIs directly.
>>
>> Adam
>>
>>
>> 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> 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/*
>>
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