[LLVMdev] [PATCH] ConstantRange::sub

[Xi Wang]

Here goes the patch along with a test for ConstantRange::sub.  Thanks.

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On Jun 22, 2011, at 3:57 PM, Nick Lewycky wrote:

> On 22 June 2011 12:51, Xi Wang <xi.wang at gmail.com> wrote:
> Sure.  I will submit a patch.
> 
> BTW, what's the difference between the bounds I was expecting
> 
> APInt NewLower = getLower() - Other.getUpper() + 1;
> APInt NewUpper = getUpper() - Other.getLower();
> 
> and the two you mentioned
> 
> NewLower = Lower - (Upper-1)
> NewUpper = (Upper-1) - Lower + 1
> 
> They look equivalent to me.  Did I miss anything?  Thanks.
> 
> ... they are. Sorry, I was just surprised to see the +1 on the NewLower (it always goes on the NewUpper instead, right), but I didn't actually simplify the expressions.
> 
> It sounds to me like you've got this one handled. :-)
> 
> Nick
>  
> 
> - xi
> 
> On Jun 22, 2011, at 2:39 PM, Nick Lewycky wrote:
> 
> > Thanks, I think you've found a serious bug!
> >
> > Would you be willing to fix it? Please add a test to unittests/Support/ConstantRangeTest.cpp and then mail llvm-commits with the patch to fix it and add the test.
> >
> > On 20 June 2011 23:09, Xi Wang <xi.wang at gmail.com> wrote:
> > Hi,
> >
> > I have a question about ConstantRange::sub(const ConstantRange &Other) at lib/Support/ConstantRange.cpp:524.  The code computes the new bounds as follows.
> >
> >  APInt NewLower = getLower() - Other.getLower();
> >  APInt NewUpper = getUpper() - Other.getUpper() + 1;
> >
> > Could someone explain this to me?  I was expecting something like
> >
> >  APInt NewLower = getLower() - Other.getUpper() + 1;
> >  APInt NewUpper = getUpper() - Other.getLower();
> >
> > These aren't quite right, I think it should be:
> >
> > NewLower = Lower - (Upper-1)
> > NewUpper = (Upper-1) - Lower + 1
> >
> > Constant ranges are stored half-open, [lower, upper) which means that the upper value is one past the end of the range. I often think of the formula as newmax = max - min --> newupper - 1 = ((getUpper() - 1) - Other.getLower(). min = lower, while max = upper - 1.
> >
> > Nick
> 
>