ScalarEvolution questions

Hi,

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”

  1. How to feed the relational information to SCEV, i.e. “a > b”.

This relational information may not be explicitly available in the program.

  1. 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)”

Any help by answer or pointers will be really useful.

Thanks,

Ashutosh

Hi Ashutosh,

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

Thanks Sanjoy, I'll look into this.

Hi Sanjoy,

Your inputs really helped.
  
Using “isImpliedCond”, able to relate and find the min for cases like:

SCEV1: (-1 + (sext i32 %n.addr.058 to i64))<nsw>
SCEV2: 0
Extra-Condition: (n.addr.058 > 7)
Result: ‘0’ (SCEV2)

I have another case where trying to find min between two SCEVs under an extra condition.
“isImpliedCond” is not helping for below case, may be I’m missing something:

SCEV1: (1 + (-1 * (sext i32 %n.addr.058 to i64)))<nsw>
SCEV2: 0
Extra-Condition: (n.addr.058 > 7)
In this case expecting result as SCEV1.

This is how using “isImpliedCond”:
Result = SE->isImpliedCond(ICmpInst::ICMP_SGT, B, A, ICmpInst::ICMP_SGT, Var, Const);
Inputs:

Hi Sanjoy,

Your inputs really helped.

Using “isImpliedCond”, able to relate and find the min for cases like:

SCEV1: (-1 + (sext i32 %n.addr.058 to i64))<nsw>
SCEV2: 0
Extra-Condition: (n.addr.058 > 7)
Result: ‘0’ (SCEV2)

I have another case where trying to find min between two SCEVs under an extra condition.
“isImpliedCond” is not helping for below case, may be I’m missing something:

SCEV1: (1 + (-1 * (sext i32 %n.addr.058 to i64)))<nsw>
SCEV2: 0
Extra-Condition: (n.addr.058 > 7)
In this case expecting result as SCEV1.

This is how using “isImpliedCond”:
Result = SE->isImpliedCond(ICmpInst::ICMP_SGT, B, A, ICmpInst::ICMP_SGT, Var, Const);
Inputs:
A: (1 + (-1 * (sext i32 %n.addr.058 to i64)))<nsw>
B: 0
Var: %n.addr.058
Const: 7

In this case "isImpliedCond" simply returns false.

In this case isImpliedCond is probably just being imprecise. We could
make it more precise though, but can you give us some detail on what
you're actually trying to do?

-- Sanjoy

Hi Sanjoy,

I have a situation where a collection of SCEV's is required to be compared, to find the minimum & maximum. This comparison is with extra information/constraint which we have derived with some analysis. The extra constraint is in the form of "var1 > var2".

I'll look into 'isImpliedCond' and try to improve it for the case mentioned in previous mail.

Thanks,
Ashutosh