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

[Alexandre Isoard via llvm-dev]

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.

Best regard.

-- 
*Alexandre Isoard*
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