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

[Chris Lattner via llvm-dev]

> On Sep 6, 2017, at 7:44 AM, Alexandre Isoard via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hello,
> 
> 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 not help.

Hi Alexandre,

The general problem is that mul and div are defined to have 2’s complement semantics (unless they are marked as being undefined on overflow). As such, "x*123/123” is not a noop.    Similarly, "x/123*123” is not a noop for many cases like x = 12.

-Chris