[llvm-dev] Working on FP SCEV Analysis

via llvm-dev llvm-dev at lists.llvm.org
Mon May 16 17:12:43 PDT 2016

One thought I’ve had about this in the past —

SCEV is useful for proving two expressions are equivalent (in general, not just specifically for loop induction variables). This gets a little bit trickier with fast-math; for example, suppose two expressions are equivalent, but *only* with reassociativity allowed?

This would be useful to make N-ary reassociate or similar algorithms work with float, as it currently uses SCEV to prove equivalence, but fast-math makes it become much trickier (and without fast-math, there’s not much you can prove).


> On May 16, 2016, at 6:07 AM, Demikhovsky, Elena via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
> Hi,
> I’m working now on extending SCEV Analysis and adding FP support.
> At the beginning, I want to vectorize this loop:
> float fp_inc;
>   float x = init;
>   for (int i=0;i<N;i++){
>     A[i] = x;
>     x += fp_inc; // Loop invariant variable or constant
>   }
> In this loop “x” is a FP induction variable. But it is not the “primary” induction and loop trip count is still depends on integer induction “i”.
> In the future, I plan to work on “primary” FP inductions.
> Intel compiler vectorizes FP-induction loops, GCC does not.
> I wanted to hear that community does not have any principal objections for this work.
> Thank you.
> Elena
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