[LLVMdev] ConstantRange::sub

[Nick Lewycky]

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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