[llvm-dev] [SCEV] Mul/UDiv folding

[Sanjoy Das via llvm-dev]

Hi Alexandre,

On Wed, Sep 6, 2017 at 7:44 AM, Alexandre Isoard via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> I was looking into the expression folding strategies of SCEV and found out
> that we don't fold multiplication and divisions:
>
> define void @test12(i32) {
> entry:
>   %1 = udiv i32 %0, 123
>   %2 = mul i32 %1, 123
>   %3 = udiv i32 %2, 123
>   %4 = mul i32 %3, 123
>   ret void
> }
>
> Will give:
>
>   %4 = mul i32 %3, 123
>   -->  (123 * ((123 * (%0 /u 123)) /u 123)) U: [0,-36) S: [0,-36)
>
> Is there a specific reason for that, or can I add a folding rule?
>
> Maybe the problem is that division can round off some lsb, and
> multiplication can wrap around some msb, and therefore we can't easily
> simplify those. But even adding "exact" on udiv and "nuw nsw" on mul does

SCEV does not have an internal representation for exact UDiv, so it
can only apply local rules, like in ScalarEvolution::getUDivExactExpr.
To fix this you could either add a bit on SCEVUDivExpr that tracks
exactness and add another peephole rule to getMulExpr.

-- Sanjoy

> not help.
>
> Best regard.
>
> --
> Alexandre Isoard
>
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