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

[Mai, Haohui]

Nick,

It works pretty cool.

Thank you.

Haohui

On Tue, 2009-06-23 at 23:16 -0700, Nick Lewycky wrote:
> Mai, Haohui wrote:
> > Nick,
> > 
> > It might be a little bit difficult to handle SMax correctly. But is it
> > possible to reduce A+(-A) to 0 in SAddExpr?
> 
> Yes, it should already do that. Here's a quick test I wrote up:
> 
>    $ cat x.ll
>    define i8 @test(i8 %x) {
>      %neg = sub i8 0, %x
>      %sum = add i8 %x, %neg
>      ret i8 %sum
>    }
>    $ llvm-as < x.ll | opt -analyze -scalar-evolution -disable-output
>    Printing analysis 'Scalar Evolution Analysis' for function 'test':
>    Classifying expressions for: test
>            %neg = sub i8 0, %x             ; <i8> [#uses=1]
>      -->  (-1 * %x)
>            %sum = add i8 %x, %neg          ; <i8> [#uses=1]
>      -->  0
>    Determining loop execution counts for: test
> 
> You'll note that %sum is deduced to be 0 and not (%x + (-1 * %x)) which 
> indicates that this optimization is working correctly. If you have a 
> more complicated case where it isn't, feel free to file a bug and cc me.
> 
> Nick
> 
> > On Wed, 2009-06-24 at 01:05 -0500, Mai, Haohui wrote:
> >> 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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