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

[Mai, Haohui]

On Tue, 2009-06-23 at 22:55 -0700, Nick Lewycky wrote:
> 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

It seems that there are codes in Scalar Evolution to handle this case.
Many operations in scalar evolution only happen when SCEV can be sign
extended.

> 
> > 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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> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
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> > 
> 
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