[LLVMdev] SIV tests in LoopDependence Analysis, Sanjoy's patch

[Preston Briggs]

Wait Are you really setting isGT to true if the distance is less than zero?
If so, I think your code is ok, but the naming could use a rework.

Preston

On Thu, Apr 5, 2012 at 4:09 PM, Preston Briggs <preston.briggs at gmail.com>wrote:

> Hi Sanjoy,
>
> Reading through LoopDependenceAnalysis::analyseStrongSIV(), I noticed one
> problem and one confusion.
>
> My confusion related to your naming of the two instructions as A and B.
> It's consistent all through LoopDependenceAnalysis. I'd prefer something
> like source and destination, so I can keep track of which is which. It
> didn't matter so much when you were simply proving or disproving
> dependence, but when you compute direction, it's suddenly crucial.
>
> The problem is the computation of direction from distance. The code says:
>
> if (distance->isZero())
>   S->Direction = Subscript::EQ;
> else if (isGT)
>   S->Direction = Subscript::GT;
> else
>   S->Direction = Subscript::LT;
>
>
> While it looks sensible, it's incorrect. Correct is
>
>
> if (distance->isZero())
>   S->Direction = Subscript::EQ;
> else if (isGT)
>   S->Direction = Subscript::LT; // !!
> else
>   S->Direction = Subscript::GT; // !!
>
>
> The Delta test paper (Section 1.3) and the 1st printing of Allen & Kennedy
> (Definition 2.10) are similarly incorrect.
> See http://www.elsevierdirect.com/companion.jsp?ISBN=9781558602861
> particularly the replacement for page 46.
>
> I'm also confused about the math, but I'll keep working on it.
>
> Thanks,
> Preston
>
>
>
> On Sun, Mar 25, 2012 at 9:49 PM, Sanjoy Das <
> sanjoy at playingwithpointers.com> wrote:
> > Hi Hal, Preston!
> >
> > Sorry for the delay!  Got busy with some offline work.
> >
> > I've worked on my previous code to calculate direction and distance
> > vectors whenever possible (strong SIV, basically).  I think the
> > current code is much clearer and would like your opinions on it.
> >
> > I have attached the patch and also pushed to the github repo I
> > mentioned [1].
> >
> > Thanks!
> >
> > [1] https://github.com/sanjoy/llvm/tree/lda
> > --
> > Sanjoy Das.
> > http://playingwithpointers.com
>
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