[llvm-dev] Wrong Range of SCEV for URem

[Sanjoy Das via llvm-dev]

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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