[llvm-dev] Wrong Range of SCEV for URem

[Hongbin Zheng via llvm-dev]

I feel that we maybe able to pattern match "%x -<nuw> ((%x udiv %y)
*<nuw> %y)" to infer that the results should be smaller than %y. But
not sure if this approach could scale well

On Tue, Jun 25, 2019 at 8:50 PM Sanjoy Das via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> 1. There is nothing wrong about getUnsignedRange returning a range
> with a negative value.  "Signed" and "Unsigned" are merely hints, the
> underlying ConstantRange representation does not have a concept of
> sign.  [A, B) represents all the values enumerated by "for (I = A; I
> != B; I++ /*wrapping addition*/) { yield(I); }"
>
> 2. I would guess this is because SCEV does not directly represent urem
> but instead represents it as "%x urem %y == %x -<nuw> ((%x udiv %y)
> *<nuw> %y)".  This makes it difficult for SCEV to infer that the X
> urem 3 is u< 3.
>
> On Wed, Jun 12, 2019 at 9:45 AM Tingyuan LIANG via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > Dear all,
> >
> >     Hi! I noticed an interesting situation when using getUnsignedRange and getSignedRange of SCEV for URem instruction.
> >     Here is an example with 2 IR instructions:
> >
> >          %rem.lhs.trunc = trunc i32 %i15.082 to i8 -->  getUnsignedRange --> [1,50)
> >          %rem81 = urem i8 %rem.lhs.trunc, 3  -->  getUnsignedRange --> [-47,50)
> >
> >     The problems are:
> >         1) From my perspective, the getUnsignedRange() function should return non-negative range but it seems not so in the example. Is there anything wrong?
> >         2) The range of the IR %rem81 should be [0,3), considering the equation: X%3 < 3?
> >
> >     Thanks in advance for your time and suggestions!
> >
> >
> > Best regards,
> > ------------------------------------------
> > Tingyuan LIANG
> > MPhil Student
> > Department of Electronic and Computer Engineering
> > The Hong Kong University of Science and Technology
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