[LLVMdev] ConstantRange::sub

[Nick Lewycky]

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
>
>
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