[LLVMdev] ConstantRange::sub

[Xi Wang]

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.

- 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