Gracefully handle overflows

[upsj]

There are many places where integer computations might overflow in Ginkgo. I think the most important one is the prefix_sum kernel. So I would like to suggest that we add an overflow guard to it, by slightly modifying the addition operation to return -1 on overflow or if one of the operands is also -1. Then we can check the resulting value after the kernel has finished and throw an appropriate exception.
This gives a nicer error message than the cuda: failed to allocate memory block of 18446744066997193496B we usually get.

[yhmtsai]

it will give wrong result when input is indeed -1?

[upsj]

Can you think of a place where we use prefix_sum and negative inputs make sense? None come to mind for me. If we need it, we can add a kernel without the overflow check later.

[yhmtsai]

From prefix sum name, I would expect it allows the negative input if the type is signed type.
I would say adding the new postive_prefix_sum for the purpose, not changing the prefix_sum and adding another for the original meaning.
Is the overflow more than the GPU memory or the integer range?

[upsj]

Good point - either add a parameter bool catch_overflows or rename it. Though I don't agree that the original should remain, unless we really need it. This is related to arithmetic overflows, not out-of-memory issues