[llvm-dev] [SCEV] getMulExpr could be extremely slow when creating SCEVs for a long chain of add/mul instructions

[Sanjoy Das via llvm-dev]

Hi Kevin,

Kevin Choi via llvm-dev wrote:
 > Correct me if I'm wrong, is the code trying to expand geometric series
 > into polynomial expression? (that doesn't sound like a very good thing
 > to do, it would spend so much time computing coefficients)
 >
 > Wouldn't this be better to transform as below:
 > mult *= n++   // 30 times
 >
 > ; into
 > mult = mult (n)...(n+29)
 >         = mult 1...(n+29)/((1)...(n-1))
 >         = mult * geo_sum(n+29) / geo_sum(n-1)  ; is this more expensive
 > than leaving as geo series?

Not sure what you mean by geo_sum -- is geo_sum(n) == 1 * 2 * ... * n?
If so, SCEV does not have a node for that.

-- Sanjoy