[llvm] r294181 - [SCEV] limit recursion depth and operands number in getAddExpr

[Chandler Carruth via llvm-commits]

On Mon, Feb 6, 2017 at 11:22 AM Chandler Carruth <chandlerc at gmail.com>
wrote:

> This adds a unittest that *with optimizations* takes over 20 seconds to
> run. That seems excessive.
>

Specifically...


> On Mon, Feb 6, 2017 at 4:49 AM Daniil Fukalov via llvm-commits <
> llvm-commits at lists.llvm.org> wrote:
>
> +static cl::opt<unsigned>
> +    MaxAddExprDepth("scalar-evolution-max-addexpr-depth", cl::Hidden,
> +                    cl::desc("Maximum depth of recursive AddExpr"),
> +                    cl::init(32));
>
>
The recursion limit is 32...



> +  for (int i = 0; i < 1000; i++) {
> +    Mul1 = BinaryOperator::CreateMul(Add2, Add1, "", EntryBB);
> +    Add1 = Add2;
> +    Add2 = BinaryOperator::CreateAdd(Mul1, Add1, "", EntryBB);
> +  }
>
>
And the test builds IR with 1000 adds and 1000 muls. It isn't clear at all
that this is a reasonable way to unittest something.

What's worse, when I make this 1000 be 10 instead, nothing fails despite
transitioning from above to below the recursion limit above. So I'm not
really sure what this is testing at all?

Anyways, this is making each of my test runs over 50% slower (18s -> 26s)
and I suspect several build bots are similarly impacted.
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