[PATCH] D12719: ScalarEvolution assume hanging bugfix

[Richard Smith via llvm-commits]

rsmith added a comment.

In http://reviews.llvm.org/D12719#242218, @sanjoy wrote:

> I think @rsmith is right -- in this case the complexity is O(n^2).  I thought I had an example where it was O(n!), but I cannot come with anything concrete right now.


Our initial analysis of the 'assume' problem appeared to be O(n!), but we don't have a reduced testcase for that. The cycle there was `isLoopBackedgeGuardedByCond` -> `isImpliedCond` -> `getZeroExtendExpr` -> `isLoopBackedgeGuardedByCond`. In the testcase in this patch, `getZeroExtendExpr` memoizes its result, but there are paths through it that do not appear to do so (in particular, the `isLoopBackedgeGuardedByCond` test in the `isKnownPositive` / `isKnownNegative` cases can lead to a `return` with no memoization of the `getZeroExtendExpr` result).

I could easily believe there are testcases for both loops that lead to O(n!) performance. (If not, we are emitting /vastly/ too many assumes...)


http://reviews.llvm.org/D12719