[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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