[llvm-dev] ScalarEvolution questions

[Sanjoy Das via llvm-dev]

Hi Ashutosh,

On Wed, May 9, 2018 at 3:28 AM, Nema, Ashutosh via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> I’m new to ScalarEvolution and wanted to explore its capabilities.
>
> I have following few questions:
>
> 1) How to compare SCEV expressions.
>
> I have a situation to compare two SCEV to find the min among them.
>
> Found an existing function(getMinFromExprs) in LoopAccessAnalysis which
> compares two SCEVs.
>
> getMinFromExprs function finds the diff between two SCEV’s and then checks
> for the negative value to find the minimum.
>
> While checking the negative value it specifically look for SCEVConstant.
>
> Do we have anything to find the min in case of NON-SCEVConstant (with some
> additional information), i.e.:
>
> SCEV1: (sext i32 (-1 + %n.addr.0) to i64)
>
> SCEV2: 0
>
> Extra information “n.addr.0 > 1”

There is ScalarEvolution::isImpliedCond -- using it you can ask the
question "n.addr.0 > 1" implies SCEV1 != SCEV2.  However this is
private to SCEV and I don't think we should expose it as part of
SCEV's public interface.

> 2) How to feed the relational information to SCEV, i.e. “a > b”.
>
> This relational information may not be explicitly available in the program.

Take a look at how SCEV uses assumes.  Given what you've said so far,
perhaps the best fix is to add the extra constraints you have as
assumes and see what SCEV does?

> 3) Is there any way in SCEV to force the compute(i.e. add) by ignore the
> cast(sext, zext, truc)
>
> SCEV: (2 + (sext i32 (-1 + %n.addr.0) to i64))
>
> After force addition expecting something like:  “(1 + %n.addr.0)”

You can't ask SCEV to do this directly, but you can rewrite SCEV
expressions to substitute subexpressions in any way you like.  E.g.
see SCEVInitRewriter.

-- Sanjoy