[llvm-commits] [PATCH] ConstantRange::sub

[Nick Lewycky]

On 22 June 2011 14:09, Xi Wang <xi.wang at gmail.com> wrote:

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

Thanks! Committed as r133648.

Nick


>
>
>
> 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
> >
> >
>
>
>
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