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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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
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/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 > > _______________________________________________ > > LLVM Developers mailing list > > llvm-dev at lists.llvm.org > > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev