[llvm-dev] Working on FP SCEV Analysis

Sanjoy Das via llvm-dev llvm-dev at lists.llvm.org
Wed May 18 12:17:09 PDT 2016

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 

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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