[LLVMdev] Handling SMax(N, N - constInt) in Scalar Evolution pass

[Nick Lewycky]

Mai, Haohui wrote:
> Hi all,
> 
> I'm working on a project which tries to prove an access to an array is
> safe. For example,
> 
> int foo(int s) {
>   int * p = malloc(s * sizeof int);
>   ...
>   int q = p[s - 2];
> }
> 
> then the access of p[s - 2] always stays in bound.
> 
> I implemented a prototype using the Scalar Evolution pass. Here are the
> pseudo-code of the implementation:
> 
>   const SCEV * offset = SE->getMinusSCEV(SE->getSCEV(GEP), GEPBase);
>   const SCEV * bounds = SE->getSCEV(objSize);
>  
>   if (SE->getSMaxExpr(offset, bounds) == bounds) {
>     ++safeGEPs;
>   }
> 
> But it turns out that SCEVSMaxExpr does not handle the case of SMax(N,
> N-2).

Consider 8-bit integers and N = -127. N-1 equals INT_MIN and N-2 then is 
equal to INT_MAX, or 127. In that case, the SMax would equal N-2, not N.

In other cases (like N = 2) the SMax would equal N, not N-2.

Because of this, we cannot reduce this SMax any further. Your suggestion 
that "SMax(A, B) == SMax(A-B, 0) + B" is incorrect.

Nick

> My question is, is there a plan to support something like this, or is it
> possible to do some refactoring to enhance the capability of Scalar
> Evolution? I notice that Scalar Evolution, Instruction Combining and
> Memory Dependence Analysis require sort of evaluating symbolic
> expression like this.
> 
> For this case SMax(A, B) is equivalent to SMax(A-B,0) + B, instruction
> combining handles  sophisticated expressions like (A+B)-B pretty well.
> It would be great if Scalar Evolution can support this.
> 
> Any comments are appreciated.
> 
> Cheers,
> 
> Haohui
> 
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